Circuit Breaker Characteristics in Medium Voltage Equipment

Morten Lerche, s031889

Circuit Breaker Characteristics inMedium Voltage Equipment underVarious Network Configurations

MSc project, June 2009

Morten Lerche, s031889

Circuit Breaker Characteristics inMedium Voltage Equipment underVarious Network Configurations

MSc project, June 2009

Circuit Breaker Characteristics in Medium Voltage Equipment under Various NetworkConfigurations,

This report was prepared byMorten Lerche, s031889

SupervisorLektor Joachim Holbøll

Release date: 30. June 2009Category: 1 (public)

Edition: First

Comments: This report is part of the requirements to achieve the Masterof Science in Engineering (MSc) at the Technical University ofDenmark. This report represents 35 ECTS points.

Rights: c©Morten Lerche, 2009

Technical University of DenmarkDepartment of Electrical EngineeringCentre for Electric Technology (CET)Elektrovej 325building 325DK-2800 Kgs. LyngbyDenmark

www.elektro.dtu.dk/cetTel: (+45) 45 25 35 00Fax: (+45) 45 88 61 11E-mail: [email protected]

Preface

This Master’s thesis was prepared at the Technical University of Denmark(DTU).

The work is conducted at the Department of Electrical Engineering, Centrefor Electric Technology at DTU.

The work done in this project continues the work performed by Orn I.Bjorgvinssonin in his MSs project in 2006. My supervisor has been lectorJoachim Holbøll from Department of Electrical Engineering, which I wouldlike to thank for guidance, support and inspiration throughout my project.A thanks should also be directed to engineer assistant Freddie Fahnøe andoperation techinan Flemming Juul Petersen from Department of ElectricalEngineering who have both contributed with great help during the labora-tory tests.

I would also like thank my fellow studens and specially PhD student IvanArana, for theoretical discussions during the project.

Kgs. Lyngby 2009-06-30

——————————————–

Morten Lerche, [email protected]

Summary

This Master’s thesis presents an investigation of the transient overvoltagesgenerated by a vacuum circuit breaker. A theoretical model of vacuum cir-cuit breakers is investigated and the parameters used to describe the vacuumcircuit breaker in a simulation model is described.

A series of laboratory test were made in order to examine the transientscreated by the breaker and to calculate the parameters which are used todescribe the vacuum circuit breaker in the simulation model. The labora-tory setup consists of a transformer, a cable, a vacuum circuit breaker anda capacitive load. During the tests the cable lenght was varied to study hownetwork changes effects the transient overvoltages. The load was varied inorder to determine some parameters for the simulation model. The testsshows that the transient recovery voltage created by a switching operationis highly dependent on the system configuration. Further more it is shownhow the transient recovery voltage and the breaking angle effects the num-ber reignitions of the vacuum arc.

A simulation model of the laboratory setup is designed and used to test thevacuum breaker model and the parameters found. The simulations shows theslower transients and the prestrikes, but the lack of detail in the simulationmodel makes the results less precise than desired.

Dansk Resume

Dette kandidatspeciale præsenterer en undersøgelse af de transiente over-spændinger som genereres af vakuumbrydere. En teoretisk model af vaku-umbryderen bliver undersøgt og de parametre som bruges til at beskrivevakuumbryderen i en simuleringsmodel beskrives.

En serie af laboratorietest blev udført for at kunne undersøge de transientersom bryderen danner og udregne de parametre som bruges til at beskrivebryderen i simuleringsmodellen. Laboratorieopstillingen bestar af en trans-former, et kabel, en vakuumbryder og en kapacitiv belastning. Under forsø-gene blev længden af kablet varieret for at undersøge hvordan ændringer inetværket pavirker de transiente overspændinger. Belastningen blev vari-eret for at kunne bestemme nogle af de parametre til simuleringsmodellen.Forsøgene viser at ”the transient recovery voltage” som dannes ved en skifteoperation er afhænging af netwærks konfigurationen. Derudover vises dethvordan ”the transient recovery voltage” og brydevinklen pavirker antalletaf gentændinger af vakuum lysbuen.

En simuleringsmodel af laboratorieopstillingen bliver designet og denne modelbruges til at teste vakuumbryder modellen samt de fundne parametre. Simu-leringerne viser de langsomme transienter og antændinger af vakuum lysbuenunder en lukke operation, men pa grund af manglende detaljeringsgrad isimuleringsmodellen bliver resultaterne ikke sa præsise som ønsket.

List of Acronyms

Acronym MeaningVCB Vacuum Circuit BreakerTRV Transient Recovery VoltageRRDS Rate of Recovery of Dielectric StrengthRDDS Rate of Decay of Dielectric StrengthHF High FrequencyAC Alternating CurrentDC Direct Current

List of Symbols

Symbol Unit MeaningR Ω ResistanceL H InductanceC F CapacitanceG S ConductanceI A CurrentV V VoltageVm V Voltage amplitudet t timeφ rad Phase angleω rad/s Angular frequencyθ rad Phase angle at breaker closing timeτ s Time constantfTRV Hz Frequency of transient recovery voltagefTRV 10 Hz Frequency of transient recovery voltage

in the system using the 10m cablefTRV 100 Hz Frequency of transient recovery voltage

in the system using the 100m cableIch A The chopping current level|i| A Amplitude of currentα s Contact material constantβ – Contact material constantU V Dielectric withstandA V/µs Rate of rise of dielectric strengthB V Breaker transient recovery voltage

just before current zerot0 s Time of contact seperationCc

Aµs2

Breaker constantDd

Aµs Breaker constant

topen s Time of a opening operationof the VCB

tclose s Time of a closing operation of the VCB

Symbol Unit MeaningRclosed Ω Resistance over VCB contacts

in closed positionRopen Ω Resistance over VCB contacts

in open positionVLoad V Voltage measured on the

load side of the VCBVTrans V Voltage measured on the

transformer side of the VCBVopen V Dielectric withstand of the

VCB in open positionVvacuum V/mm Dielectric withstand of vacuumfopen Hz Frequency of the oscillating transient

caused by a VCB openingfopen10 Hz Frequency of the oscillating transient

caused by a VCB opening when 10mcable is used

fopen100 Hz Frequency of the oscillating transientcaused by a VCB opening when 100mcable is used

Z Ω Impeadance of the HTT transformerex – Short circuit impeadance of the

transformerVm V Rated voltage level of transformerSm V A Rated load of transformer

Contents

Preface vii

Summary ix

Dansk Resume xi

List of Acronyms xiii

List of Symbols xv

1 Introduction 1

1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Methods and restrictions . . . . . . . . . . . . . . . . . . . . . 3

1.3 Outline of Report . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Switching Transients 5

2.1 Closing Circuit Transient . . . . . . . . . . . . . . . . . . . . 5

2.2 Opening Circuit Transient . . . . . . . . . . . . . . . . . . . . 9

3 Vacuum Circuit Breakers 13

3.1 Construction of Vacuum Circuit Breakers . . . . . . . . . . . 14

3.2 Modelling of Vacuum Circuit Breakers . . . . . . . . . . . . . 16

4 Laboratory Setup 25

4.1 The Existing Setup . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2 Improvements to the Existing Setup . . . . . . . . . . . . . . 30

xviii

5 Laboratory Tests and Results 335.1 Preparatory tests . . . . . . . . . . . . . . . . . . . . . . . . . 365.2 Transient Recovery Voltage . . . . . . . . . . . . . . . . . . . 375.3 Chopping Current . . . . . . . . . . . . . . . . . . . . . . . . 415.4 Reignitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.5 High Frequency Quenching Capability . . . . . . . . . . . . . 475.6 Closing the circuit . . . . . . . . . . . . . . . . . . . . . . . . 51

6 Simulations 576.1 Opening the Vacuum Circuit Breaker . . . . . . . . . . . . . . 586.2 Closing the Vacuum Circuit Breaker . . . . . . . . . . . . . . 64

7 Discussion 677.1 Voltage Circuit Breaker Model Parameters . . . . . . . . . . . 677.2 Opening the Vacuum Circuit Breaker . . . . . . . . . . . . . . 687.3 Closing the Vacuum Circuit Breaker . . . . . . . . . . . . . . 707.4 Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

8 Conclusion 75

References 77

Appendix 79

List of Figures 79

List of Tables 82

A Plotting results 85

B Results of the TRV Calculations 89

C Results of Current Chopping Calculations 91

D Results of the Rate of Rise of Dielectric Strength Calcula-tions 93

E Results of HF Current Quenching Capability Calculations 97

F Results of Rate of Decay of Dielectric Srength 99

Chapter 1

Introduction

In the last part of the 19th century the demand of electric power startedincreasing rapidly because of new technical inventions. The increased useof electrical lighting, the introduction of the DC-motor and railway systemswere just some of the inventions that called for a power system. In 1882Tomas Edison opened the worlds first power station in New York City, thisis referred to as the beginning of the electric utility industry. From thisstarting point and until 1872 the electric utility industry grew at a remark-able pace [3]. In 1885 William Stanley developed the first commercial andpractical transformer and overcame the limitation of maximum distance andload in the exciting network. The year after the first AC distribution systemwas installed in Massachusetts. Nikola Tesla presented the first ideas of apolyphase AC system in 1888 introducing induction and synchronous mo-tors, and the first transmission of tree-phase alternating current took placeduring the international electricity exhibition in Frankfurt in 1891, trans-mitting power at 12kV over 175km [3].

The industrialization during the 20th century made the electrical infrastruc-ture a critical point. Interconnection of local distribution networks and theconstruction of large power plants were some of the main demands createdby the industrialization. These demands have led us to the power networkswe have today, which are designed to transport energy as efficient and re-liably, from the producer to the consumer, as possible. To deal with thechallenges of this task many network components have been developed andsome of the main components of today’s power networks are overhead lines,cables, transformers, circuit breakers and switches.

For many years lightning were the only phenomenon that could create steepfront pulses in the power system and thereby produce high overvoltages [1].When lightning strokes terminates on or near a power line they create a

2

path between the cloud and the power line or the adjacent earth and herebychanges the circuit conditions and creates a transient overvoltage. In orderto protect the insulation of the equipment against the lightning overvoltagessurge arresters were used, they kept the voltage on a level that was notharmful for the protected equipment. The research in this area was stoppeduntil an increased number of failures were detected on the insulation of theequipment, even at low voltage levels. It was discovered that these failureswere caused by some of the equipment that had been implemented in thepower network. One of the components that led to failure of insulation wasthe breakers used in the electrical grid.

Many types of breakers have been used in the power grid during the years.In the beginning of the 20th century oil circuit breakers were mainly used. In1959, SF6 circuit breakers came to the marked, this type of circuit breakerhad several advantages such as long life time and high reliability [15]. Thefirst vacuum circuit breaker (VCB) was constructed in the 1960s. VCBshave low maintenance costs, good durability and provide the best breakersolution for medium voltage below 24kV [1]. But the use of VCBs resulted inworldwide reports on transformer insulation failures possibly due to switch-ing operations of VCBs, also transformers that had previously passed allthe standard tests and complied to all quality requirements suffered failures[2]. It has still not been finally proved that the high frequency transientshave a negative influence on the transformer insulation. Some studies givea description of the phenomenon that produces the high overvoltages inter-nally in the transformer winding, which are potentially responsible for thetransformer insulation failure during the high frequency transients [10]. Aproblem of the transformer insulation failure also occured in the wind parks(WP) Middelgrunden and later at Hornsrev where almost all transformershad to be replaced with new ones due to the insulation failure [14]. Thisproblem is suspected to be caused by fast switching breakers as e.g. VCBs.

VCBs are the most used breaker type in the medium voltage area, due toits excellent breaking abilities and economic advantages. But as mentionedthe VCB also seems to cause some faults in the power network. The physi-cal phenomena in the VCB during a switching operation are very complex,and therefore the models of VCBs are also very complex. When performinga switching operation a conducting plasma channel is created between thebreaker contacts, this channel is called the vacuum arc. When the arc is ex-tinguished a transient recovery voltage appears across the terminals and thisvoltage can give rise to another breakdown in the vacuum and create a newconducting plasma channel between the breaker contacts. The arc formed bythe plasma can become unstable and create high frequency currents, whichthe breaker must be able to interrupt. The advanced and unstable nature ofthe conducting plasma channels does that there is no universal precise vac-

Introduction 3

uum arc model. The models that exist all take into account the stochasticproperties of the phenomena that take place in the breaking process [7].

1.1 Purpose

The purpose of this thesis is to test a VCB and to observe the physicalphenomena that occur in the VCB during the interruption process, especiallythe phenomena that cause high frequency transients. Based on laboratorytests the parameters, that are used to model the VCB, will be determined.In order to study the laboratory setup it is desirable to make a precisesimulation model of the setup, this simulation model can be used to testthe behaviour of the VCB model and compare it with the tests made on theVCB.

1.2 Methods and restrictions

The VCB model that is used in this project takes into account the followingstochastic properties of the VCB:

• Current chopping ability.

• Recovery of dielectric strength.

• High frequency current quenching.

In the test setup, used in order to determine these properties of the VCB,only one phase of the VCB is connected and measurements are performedon this phase. In order to supply the VCB with high voltages a transformeris used, the measurements are performed on the high voltage side on thetransformer. In order to change the network configurations, two similar ca-bles with different length are used to connect the VCB and the transformer.Two capacitive loads are used to load the system, these loads are both 0.5µFand the maximum load in the system is therefore 1.0µF . This means thatthe current running through the VCB during the tests will be rather limited.

When analysing the results of the VCB tests, the main focus will be ondetermining the parameters for the VCB model. As these parameters aremainly determined by the opening process of the VCB most analysis will beon VCB open operations. As the parameters are mainly described by thevery fast transients created by the VCB the main work will be put in thisarea.

The simulations are performed in PSCAD, a model of the laboratory setup

4 1.3 Outline of Report

is created using lumped circuit elements for the transformer and the cable.This will make the results of the simulations less accurate, but lumped cir-cuit elements are used to be able to finish the simulations within the timelimitations of the project.

1.3 Outline of Report

The structure of the thesis is as follows:

• Chapter 2 : Switching TransientsThis chapter will introduce two examples of switching transients ocur-ring due to an opening and a closing action of an ideal switch. For oneexample the full result of the transient current, caused by opening theswitch, will be calculated, where the other example will explain theareas of interest.

• Chapter 3 : Vacuum Circuit BreakersIn this chapter the design principles of VCBs are described and thetheory behind the vacuum arc will be explained. A model of the VCBwill be introduced and the different parameters of the model will beexplained according to the physical phenomena occurring in the VCB.

• Chapter 4 : Laboratory SetupA description of the laboratory setup and its components is given inthis chapter. The improvements and adjustment made on the setupare also described.

• Chapter 5 : Laboratory Tests and ResultsThis chapter concerns with the performed tests and the treatment ofthe test results. In this chapter the parameters of the VCB modelwill be calculated and the accuracy of the results will be discussed.Methods for achieving more accurate results will be discussed.

• Chapter 6 : SimulationsIn this chapter the simulation model will be described. The resultsfrom the PSCAD simulation of an opening and a closing operation ofthe VCB is analysed.

• Chapter 7 : DiscussionIn this chapter a discussion of the achieved results will be made, thediscussion will mainly focus on comparing the simulation results withthe measured results. A description of the further work that is neededin order to make a fully working model of the VCB will also be given.

• Chapter 8 : ConclusionThis section gives the conclusion.

Chapter 2

Switching Transients

An electrical transient is caused by a sudden change in the circuit condi-tions [4]. This change could be when a lightning hits the ground near a highvoltage line or when lightning strikes a substation directly. But the mostcommon transients in the power systems occur as a result of a switching ac-tion. This could be when circuit breakers, fuses, disconnectors etc, open andclose in order to switch off parts of the network, interrupt higher currentsand clear faults in the network and hereby secure the network. These switch-ing actions give rise to switching transients. The transient time is usuallyvery short, in the range of microseconds to milliseconds, but the transientsperiods are very important as it is in this period the network componentsare subject to the greatest stress. The transients may shorten the lifetimeof the components in the network or in worst case cause a breakdown of thepower system. In this chapter, two switching examples will be examined,both examples will use an ideal swich to represent a network switch as e.g.a VCB. An ideal switch acts as a disconnection when open and as a shortcircuit when closed and it switches between the two stages instantly. Usingan ideal switch gives a good idea of what happens when a VCB is opened orclosed even though it does not have the same characteristic. An ideal switchdoes not have the influence of reignitions and high frequency currents, whichexcist in a VCB due to arc instability.

2.1 Closing Circuit Transient

In this example a sinusoidal voltage is switched on to a series connection ofan inductance and a resistance. Figure 2.1 represent the simplest case of ahigh-voltage circuit breaker closing into a short-circuited transmission lineor a short-circuited underground cable. The voltage source V representsthe electromotive force from the connected generators [15]. The inductanceL represents the synchronous inductance from the generators, the leakage

6 2.1 Closing Circuit Transient

Vmsin(ωt+θ)

R

L

S

Figure 2.1: An sinusoidal voltage is switched on an RL-circuit.

inductance in the transformers and the inductance of bus bar, cables andtransmissions lines. The resistance R represents the resistive losses of thenetwork. Since the network consist of linear elements only, the current flow-ing in the network after closing the switch can be seen as the superpositionof a transient current and a steady-state current. Applying Kirchhoff’s volt-age law on the circuit in figure 2.1 gives us the nonhomogeneous differentialequation that represents the circuit after the switch has been closed [15]

R · I + L · dIdt

= V, (2.1)

where V represents the sinusoidal voltage of the source and I is the currentin the circuit.

V = Vm · sin(ωt+ θ) ⇔V = Vm · [sin(ωt)cos(θ) + cos(ωt)sin(θ)] (2.2)

The angle θ is the phase angle at which the switch is closed. The termsin(ωt+θ) has been rewritten in order to make the solution of the differentialequation easier. The steady state power factor of the load in figure 2.1 isgiven by

cos(φ) =R

|L|=

R√(R2 + ω2 · L2)

. (2.3)

The differential equation is solved by the Laplace method. Inserting equation(2.2) in (2.1) gives

R · I + L · dIdt

= Vm · (sin(ωt)cos(θ) + cos(ωt)sin(θ)), (2.4)

Laplace transforming both sides yields

R · i(s) + L · s · i(s)− L · I(0) = Vm

(ω · cos(θ)s2 + ω2

+s · sin(θ)s2 + ω2

). (2.5)

Switching Transients 7

Setting I(0) = 0 in figure 2.1 makes it possible to find an expression for thecurrent

R · i(s) + L · s · i(s) = Vm

(ω · cos(θ)s2 + ω2

+s · sin(θ)s2 + ω2

)⇔

i(s) =Vm

L · s+R

(ω · cos(θ)s2 + ω2

+s · sin(θ)s2 + ω2

)⇔

i(s) =VmL· 1s+ R

L

(ω · cos(θ)s2 + ω2

+s · sin(θ)s2 + ω2

). (2.6)

In order to transform back into the time domain the equation is rewrittento the following form

i(s) =A

(s+ α)(s2 + ω2)+

B · s(s+ α)(s2 + ω2)

, (2.7)

where the constants

A =VmL· ω · cos(θ), B =

VmL· sin(θ), α =

R

L. (2.8)

Equation (2.7) can be transformed back into the time domain when thefollowing two inverse Laplace transforms are known

L −1

[A

(s+ α)(s2 + ω2)

]=

A

(s2 + ω2)· [e−α·t

− cos(ω · t) +α

ωsin(ω · t)] (2.9)

L −1

[B · s

(s+ α)(s2 + ω2)

]=

B

(s2 + ω2)· [−α · e−α·t

+ ω · sin(ω · t) + αcos(ω · t)] (2.10)

Using equation (2.9) and (2.10), equation (2.7) can be transformed into thetime domain

i(t) =L −1

[A

(s+ α)(s2 + ω2)+

B · s(s+ α)(s2 + ω2)

]

=A

(s2 + ω2)· [e−α·t − cos(ω · t) +

α

ωsin(ω · t)]

+B

(s2 + ω2)· [−α · e−α·t + ω · sin(ω · t) + αcos(ω · t)]. (2.11)

8 2.1 Closing Circuit Transient

0 20 40 60 80 100−1.2

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8Closing a RL−circuit at 90 degrees

Time[ms]

Cur

rent

[I]

Resultant current I(t)Steady−state currentTransient current

Figure 2.2: The sinusoidal voltage is switched on to the RL-circuit with a switch-ing angle of 90.

Incerting A and B from (2.8) into equation (2.11) yields

i(t) =Vm · ω · cos(θ)L · (s2 + ω2)

· [e−α·t − cos(ω · t) +α

ωsin(ω · t)]

+Vm · sin(θ)L · (s2 + ω2)

· [−α · e−α·t + ω · sin(ω · t) + αcos(ω · t)]. (2.12)

Equation (2.12) can be simplified by using the power factor described in(2.3) and inserting α from (2.8), the following expression for the current canbe found [4]

i(t) =Vm√

R2 + ω2 · L2[sin(ω · t+ θ − φ)− sin(ω − φ)e−

RL·t]. (2.13)

The first term is the steady-state term, it has an amplitude of Vm/|Z| andit has a phase angle of −φ with respect to the voltage. The second term isthe transient term, it includes an exponential function e−

RL·t. At t = 0 the

steady-state term and the transient term are the same but with differentsign, assuring that the current starts in zero when the breaker closes. Infigure 2.2 the transient current, the steady-state current and the resultantcurrent is shown for a switching angle of θ − φ = 90. As figure 2.2 showsthe transient term starts at its lowest possible value, which is equal to theamplitude of the current. Opening the breaker at θ − φ = 90 gives thelargest transient, on the other hand opening the θ − φ = 0 makes the


0 20 40 60 80 100−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8Closing a RL−circuit at 0 degrees

Time[ms]

Cur

rent

[I]

Resultant current I(t)Steady−state currentTransient current

Figure 2.3: The sinusoidal voltage is switched on to the RL-circuit with a switch-ing angle of 0.

transient term turn zero. This can be seen in figure 2.3. It is seen that thetransient term is zero, which causes the resultant current to be equal to thesteady state current from the moment of contact separation.

2.2 Opening Circuit Transient

When a switch opens in order to switch off parts of the network or clear faultsin the network it can cause high overvoltages in the network. This sectionwill investigate what happens when interrupting a capacitive current usingan ideal switch. A simple model of the laboratory setup used in this projectis seen in figure 2.4, the inductance of the circuit represents a transformer,the capacitance C1 represents a cable and the capacitance C2 is the loadof the network. When opening the switch in the system in figure 2.4 thecircuit will only consist of the inductance L and the capacitance C1. Afterthe switch has opened, a HF voltage appears across the switch contacts, thisvoltage is called the transient recovery voltage (TRV). This transient willhave the frequency

fTRV =1

2 · π√L · C1

. (2.14)

The TRV has no real influence on the switching when using a ideal switch,but it is of high importance of the switching in real switching devices. In real

10 2.2 Opening Circuit Transient

Vmsin(ωt+θ)

S

C2C1

L

Figure 2.4: An sinusoidal voltage is switched on an RL-circuit

switching devices the characteristics (amplitude and rate of rise) determinesif the current interruption is successful or fails (reignition of the arc betweenthe contacts). As seen in eqation (2.14) the frequency of the TRV dependson the circuit in which the ideal switch or circuit breaker is working. In theexample from figure 2.4, the TRV created by the switch when opening thecircuit could look like the graph in figure 2.5, this figure shows how the TRVeffects the transformer side of the circuit seen in figure 2.4. The TRV canbe harmful for network equipment since its high amplitude can exceed thevoltage level of the system.

As mentioned the circuit in figure 2.4 represents the setup used in thisproject. In [11] the capacitance of the cable used is found to be 157.57 ·10−12F/m and the in this project the value of L used to represent the trans-former is found to be 0.318H (see chapter 6). In this project a 10m and a100m cable is used, for the 10m cable the frequency of the TRV is expectedto be

fTRV 10 =1

2 · π√

0.318H · 157.57 · 10−12F/m · 10m= 7110Hz, (2.15)

and for the 100m cable a frequency of

fTRV 100 =1

2 · π√

0.318H · 157.57 · 10−12F/m · 100m= 2248Hz. (2.16)

These TRVs are the only effect of interrupting current with ideal switches,but as mentioned in the use of real switching devices, such as VCBs, theTRV can cause reignitions of the conducting arc between the switching con-tacts. These reignitions can lead to high overvoltages and HF currents inthe system. In order to describe the phenomena of reignitions in VCBs amore detailed study of the design and principles of the VCB must be made.This investigation is done in the following chapter.


50 55 60 65 70 75 80−15

−10

−5

0

5

10Simulation of VCB opening

Time[ms]

Tra

ns.

sid

e vo

ltag

e[kV

]

Figure 2.5: The figure shows the voltage on the transformer side of a VCB undera opening operation in a circuit with a capacitive load.

12 2.2 Opening Circuit Transient

Chapter 3

Vacuum Circuit Breakers

A circuit breaker is in principle an electrical switch that is designed to protectthe power system [9]. Circuit breakers play an important role in transmis-sion and distribution networks. They must clear faults and isolate faultednetwork sections fast and clearly and they are also used for normal loadswitching [7]. For a circuit breaker to fulfil its purposes the following isrequired [15]:

• It functions as a good conductor in closed position.

• It functions as a good insulator in open position.

• It is able to switch from open to closed in a short period of time.

• It does not cause overvoltages during switching.

• It is reliable in its operation.

When a circuit breaker interrupts a current, an electric arc is usually formedbetween the breaker contacts and the current continues to flow in this arc.The current interruption is performed by cooling the arc plasma so that theelectric arc disappears. Circuit breakers are classified according to the cool-ing and extinguishing medium used. There are four main types of circuitbreakers namely, oil, air blast, vacuum and SF6 circuit breakers. This thesisconcerns with the functions of a vacuum circuit breaker (VCB).

Vacuum is used as an extinguishing medium for medium voltage circuitbreakers. VCBs have excellent interruption and dielectric recovery charac-teristics and can interrupt the high frequency currents which results fromarc instability [16]. VCBs are primarily designed for switching operations incapacitive circuits [12]. The main advantages of the VCB are:

• It has excellent interruption capability.

14 3.1 Construction of Vacuum Circuit Breakers

• It can interrupt high frequency currents, created by arc instability.

• It is completely self-contained and does not need supply of gasses orliquids.

• It does not need maintenance.

• It is not flammable.

These advantages of the vacuum breaker technique have been the drivingforce of VCB development [1]. Due to the fact that there is nothing toionize between the contacts in VCBs, the characteristics of the electric arcin VCBs are different than the electric arc in other types of breakers. VCBshave a very little arc and the arc extinguishes with small distance betweenthe breaker contacts [15].

3.1 Construction of Vacuum Circuit Breakers

A VCB consist, like other circuit breakers, of two contacts, a fixed contactand a moving contact. The moving contact has two positions, one whereit is touching the other contact and one where the two contacts are apart.When the contacts are touching the VCB is conducting current and when thecontacts are apart the VCB is not conducting current. The two contacts of aVCB are inside a vacuum chamber. When the moving contact starts to moveaway from the fixed contact, an arc is formed between the two contacts andthe VCB does not stop conducting current before this arc is extinguished. Infigure 3.1 the basic concept of the VCB design is shown. The moving contactis normally moved by a stored-energy operating mechanism, in most cases aclosing and an opening spring [5]. These springs stores the energy to openand close the VCB, when the closing spring gets released the VCB closes.During the closing of the VCB the opening spring is charged so that theVCB is ready to open immediately after the closing operation is over. Afterthe closing operation is over the closing spring recharges automatically.

3.1.1 Vacuum Arc

The vacuum arc is a key element when analysing the behaviour of a VCB.The name vacuum arc is not entirely accurate, because an electric arc cannotexist in vacuum [5]. The arc that appears between the contacts of a VCB isa result of metal-vapour, ion- and electron emission. After being establishedthe vacuum arc is relatively stable and will draw energy from the electricalsystem until the current reaches a zero crossing and thereby removes theenergy source. When conducting small currents the vacuum arc can becomeunstable and extinguish before current zero is reached, this phenomenon is

Vacuum Circuit Breakers 15

Figure 3.1: The design principle of a VCB, showing contacts, arching chamberand insulation, the picture is taken from [12] page 8.

called current chopping.

Depending on the current level and on the size and shape of the contactthe vacuum arc appears in different ways [5]. At lower currents small spotson the negative electrode (the cathode) appear. These cathode spots are inconstant movement over the cathode surface. Electrons and ions radiatesfrom the spots and contributes with around 50A to 150A depending on thecathode material [15]. The plasma channel formed by the emitted electronsand ions is called a vacuum arc, this arc connects the cathode and the an-ode (the positive electrode). After leaving the cathode the arc spreads outfilling almost the entire volume of the vacuum chamber before hitting theanode. The electrons and ions leave the arc and get collected all over the an-ode and for this reason the arc is said to be in diffuse mode at lower currents.

When the current is increased the arc takes a different form, the arc be-

16 3.2 Modelling of Vacuum Circuit Breakers

Figure 3.2: A vacuum interrupter with slits in the contacts to avoid uneven ero-sion of the contact surface, this picture is from [15] page 66.

comes focused on a small area of the anode. These spots are normallyformed around a sharp edge on the contact. Due to the high current densityin these anode spots the contact material evaporates and when the vapouris ionised it supplies positive ions to the arc. The cathode spots becomesgrouped together, giving the arc a much more defined and columnar appear-ance and the arc is said to be in constrict mode [5].

3.1.2 Construction of Vacuum Circuit Breaker Contacts

The constrict mode leads to erosion of both contacts, in diffuse mode thecathode spots leads to evaporation but in the constrict mode melting occursat both contacts especially at the anode [5]. To avoid uneven erosion of thesurface of the contacts the arc should be kept in motion or kept burning indiffused mode. The most common way of avoiding melting is to make slitsin the contacts, as showed in figure 3.2, by doing this the arc is being kept indiffuse mode. The contact in figure 3.2 provides a axial magnetic field andit is this field that keeps the arc in diffuse mode. This means that the stresson the disc shaped contact surfaces is uniform and local melting is avoided[12].

3.2 Modelling of Vacuum Circuit Breakers

In order to study the behaviour of VCBs it is desirable to describe theirphysical phenomena by a mathematical model that can be used for simula-tions. In this project a breaker model will be investigated and be appliedon the VCB tested in the project. The model used in this project describes


the VCB according to the follow parameters:

• The chopping current.

• The dielectric withstand.

• The high frequency quenching capability.

The model is developed and described in [7]. The parameters used, andtheir effect on the VCB will be described in the following sections.

3.2.1 Current Chopping

Current chopping is a phenomena that can lead to overvoltages, it occurswhen small capacitive and inductive currents are interrupted [1]. When thevacuum arc is conducting a small current it will become very unstable andnormally it will disappear and cause the current to be interrupted before itreaches its natural zero. This premature interruption of the current is calledcurrent chopping. The value of the current when the arc extinguishes iscalled the chopping current level and is referred to as Ich. Figure 3.3 showscurrent chopping during switching of a VCB.

The value of the chopping level depends mainly on the type of contact ma-terial used in the breaker but also on the level and form of the current thatis interrupted. The prediction of the actual current chopping value, consid-ering all its dependents is very complex. But in [13] an expression of themean chopping level has been estimated

Ich = (2 · π · f · |i| · α · β)(1−β)−1, (3.1)

where

• f = Power frequency.

• |i| = Amplitude of the load current.

• α,β = Contact material constants.

Equation (3.1) is used to calculate the current chopping level of the VCB.When simulating the VCB the values of α and β are normally consider tobe α = 6.2 · 10−16s and β = 14.2 [6].

If the current through the breaker is lower than the chopping level, thenthe current is chopped immediately after contact separation. During currentchopping the current declines with a very high di/dt (very steep slope) thisproduces very high overvoltages due to the inductances in the network. For


36 38 40 42 44 46 48 50−1.5

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1Current Chopping

Time[ms]

Cu

rren

t[A

]

(a) The current when the breaker opens

44.84 44.85 44.86 44.87 44.88 44.89 44.9 44.91 44.92

−15

−10

−5

0

5x 10

−3 Current Chopping

Time[ms]

Cu

rren

t[A

](b) Zoomed plot at the point of arc extin-

guish

Figure 3.3: The figure shows the current during an opening of the breaker. Asseen on figure b the current chops around the value 0,005 and jumpsto zero.

this reason current chopping is considered to be a major disadvantage ofthe VCB. The current chopping level for VCBs usually varies between 3Aand 8A [1]. When modelling the chopping current it is usually consideredto have a Gaussian distribution with a standard deviation 15% of the meanchopping current, that is calculated using equation (3.1) [7].

3.2.2 Reignitions

A reignition of the vacuum arc is a temporary electrical breakdown of thevacuum in the VCB. The dielectric withstand of the VCB is an importantsubject in the analysis of the switching transients that occurs due to reig-nitions in the VCB [1]. When the breaker contacts start to separate thewithstand voltage of the gap starts increasing. During the first millimetre ofseparation the withstand voltage increases linearly and here after it increasesproportionally to the square of the distance between the contacts [1]. In themodel that is used in this project a linearly relation between the withstandvoltage and the time after separation is assumed [7]. This relation is seen inequation (3.2)

U = A(t− t0) +B, (3.2)

where

• t0 = The moment of contact separation.

• A = Rate of rise of dielectric strength.


32 33 34 35 36 37 38−80

−60

−40

−20

0

20

40

60

80

Time[ms]

Vo

ltag

e[kV

]

Dielectric withstand

TRVBreaker withstand voltage

Figure 3.4: The figure shows 5 reignitions of the vacuum arc during contact seper-ation. When the reignitions occur the TRV jumps to zero. The redline shows the RDDS of the circuit breaker.

• B = Breaker transient recovery voltage (TRV) just before current zero.

The values of A and B vary from the different VCBs. The constant A de-scribes as mentioned the rate of rise of dielectric strength (RRDS) when thebreaker is opening. When the breaker is closing the constant A describesthe rate of decay of dielectric strength (RDDS). In [16] the value of the con-stant A is suggested to be between 2V/µS and 50V/µS when B is set tozero, which is quiet normal when determining the dielectric withstand of thebreaker. The value of the dielectric strength determined in equation (3.2) isalso following a Gaussian distribution with a standard deviation of 15% ofthe dielectric mean value [7].

When the contacts separate and the current is interrupted a TRV appearsacross the breaker contacts, as described in chapter 2. This TRV is deter-mined by the configuration of the network on both sides of the breaker. Ifthe value of the TRV exceeds the dielectric withstand of the gap between thecontacts, the arc will be re-established and the breaker will conduct currentagain. This causes a high frequency (HF) current to be superimposed on thepower frequency current. This HF current will be extinguished at currentzero and the race between the TRV and the dielectric withstand will beginagain. The relation between the reignitions and the dielectric withstand isillustrated in figure 3.4 and in figure 3.5 both the restrikes and the HF cur-rent is shown.


34.65 34.7 34.75 34.8 34.85 34.9 34.95

−40

−20

0

20V

olt

age[

kV]

High frequency quenching capability


34.65 34.7 34.75 34.8 34.85 34.9 34.95−0.5

0

0.5

Time[ms]

Cu

rren

t[A

]

Figure 3.5: The figure shows 3 reignitions of the vacuum arc during contact sep-aration. The figure also shows the high frequency currents caused bythe arc.

The simulation model simulates restrikes by sending a closing signal to thebreaker whenever the TRV exceeds the dielectric strength of the gab [7].This means that the resistance of the arc is expected to be the same as theresistance of the VCB in closed position.

3.2.3 High Frequency Quenching Capability

The HF currents that occur after a reignition of the arc are mainly deter-mined by the stray parameters of the VCB. The HF current will be super-imposed on the power frequency current and if the HF current has a largermagnitude than the power frequency current it can cause the current to passzeros. Most VCBs have the ability to quench the HF current at a zero cross-ing, and thereby extinguish the vacuum arc [7]. The VCB cannot extinguishthese HF currents if the di/dt value of the current is too high. Since themagnitude of the currents is damped quite quickly the di/dt of the currentis also decreasing. When di/dt is small enough the VCB quenches the HFcurrent at one of its zero crossings. Figure 3.5 shows how a HF current iscreated when the vacuum arc is established and how the arc is extinguishedwhen di/dt of the HF current becomes small enough. The critical value ofdi/dt represents the quenching capability of the VCB. A method of deter-mining the quenching capability of a VCB is to model it as a linear function


with respect to time

di/dt = Cc(t− t0) +Dd, (3.3)

where

• t0 = The moment of contact separation.

• Cc, Dd = Breaker constants.

Equation (3.3) gives the mean value of the quenching capability and onceagain it follows a gaussian distribution where the standard deviation is 15%of the mean value. The suggested values of the constant Cc is between−0.034A/µs2 and 1A/µs2. Some authors describes the HF quenching capa-bility di/dt to be constant, Cc = 0 and suggested values of Dd to be between100A/µs and 600A/µs [16].

3.2.4 Multiple Reignitions and Voltage Escalation

When the VCB breaks the HF current that has occurred due to a reignitionof the arc, the TRV of the breaker starts rising again. When the TRVreaches the dielectric withstand of the breaker gab the arc will ignite againand course another HF current to be superimposed on the power frequencycurrent. This phenomenon is called multiple reignitions. Figure 3.6 showsthe current of the breaker during multiple reignitions of the vacuum arc.The occurrence of multiple reignitions depends mainly on tree parameters.

• The arching time of the breaker.

• The RRDS and the dielectric withstand of the breaker.

• The HF current quenching capability.

The two last areas have been discussed in the previous sections, but thearching time has not been introduced yet. The time between contact sepa-ration and first arc extinguishing is called the arching time, in other wordsthe arching time is the time between contact separation and the time of cur-rent chopping. If the arcing time is short then the dielectric strength of thegap will not have time to reach a high value before the arc is extinguishedand the probability of reignitions is higher. In VCB with high RRDS thepossibly of restrikes will be smaller since the breaker regains its dielectricwithstand faster than breakers with low RRDS.

After some reignitions the VCB does not have a high enough HF quenchingcapability to break the HF current at a zero crossing in the last reignition,this is seen in figure 3.7. Due to this the power frequency takes over and


34.4 34.6 34.8 35 35.2 35.4−0.6

−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4Unsuccessful current interruption

Time[ms]

Cu

rren

t[A

]

Figure 3.6: The figure shows the HF currents caused by 5 reignitions. The lastcurrent cannot be quenched at a zero crossing and therefore the arcis maintained until the next zero crossing of the current.

32 34 36 38 40 42 44 46 48−100

−50

0

50

100Complete breaking operation

Vo

ltag

e[kV

]


32 34 36 38 40 42 44 46 48−1

−0.5

0

0.5

1

Time[ms]

Cu

rren

t[A

]

Figure 3.7: Multible reignitions lead to unsuccessful interruption of the currentat first current zero.


interruption is effected at the next current zero (around 45ms) as seen infigure 3.7. As seen in figure 3.7 successful interruption takes place afterthe contacts are fully apart and the dielectric withstand has reached its fi-nal value. The process of multiple restrikes can lead to voltage escalation,where every breakdown of the arc can lead to higher and higher voltage atthe load side of the VCB since the TRV is superimposed on the steady state50 Hz voltage.

3.2.5 Prestrikes

Prestrikes are like reignitions a temporary breakdown of the vacuum dielec-tric. Prestrikes occur during the closing operation of the breaker. Prestrikesnormally occur during energizing of capacitive loads and are caused by thesame phenomena that cause reignitions during opening operations.

When the VCB contacts starts to move towards each other the dielectricstrength of the gap starts to decrease. As soon as the dielectric withstandof the VCB becomes smaller than the voltage over the breaker an arc willignite and current will flow through this arc. This current consists of a HFcurrent and a current at power frequency. The arc will be extinguished at azero crossing, when di/dt of the HF current becomes lower than the quench-ing capability of the VCB [1]. The interruption of the HF current causes aTRV to build up over the breaker. When this voltage reaches the dielectricstrength of the gab another prestrike will occur and the TRV will go to zeroagain. Figure 3.8 shows how prestrikes create a HF current and how thesecurrents are quenched, causing the arc to be extinguished.

This process continues to produce prestrikes until the dielectric withstandof the VCB is no longer high enough to extinguish the arc. And when thelast HF current is damped the VCB only conducts current at the powerfrequency. The slope of the dielectric strength seen in picture 3.8 is calledthe rate of decay of dielectric strength (RDDS). RDDS is normally said tohave the same value as RRDS.


34.55 34.6 34.65 34.7 34.75 34.8−40

−20

0

20

40Prestrikes

Vo

ltag

e[kV

]


34.55 34.6 34.65 34.7 34.75 34.8−0.4

−0.2

0

0.2

0.4

Time[ms]

Cu

rren

t[A

]

Figure 3.8: The figure shows 4 reignitions of the vacuum arc during contact sep-aration. The figure also shows the high frequency currents caused bythe arc.

Chapter 4

Laboratory Setup

In 2006 a laboratory setup for investigation of switching transients in windturbine systems was made. The setup was completed and modified by OrnI. Bjorgvinssonin during his master’s project [11]. The modifications madeby Orn I. Bjorgvinssonin made it possible to remotely open and close theVCB and at the same time, measure the voltage over the VCB and thecurrent through the breaker using a LabVIEW interface. In a preparatoryproject [9] the laboratory setup was investigated and some improvementswere implemented on both the measurement system and the control system.A MATLAB program that processes and shows the measured data was alsoconstructed.

4.1 The Existing Setup

As mentioned the existing laboratory setup was made to represent a wind-mill system, in order to examine the switching transients that are createdin such a system [11]. In this project the setup will be used only to examinehow the VCB behaves, and affects the system. These studies will be usedto find the parameters of the VCB that are used in the simulation model.

The high voltage components used in the laboratory setup are:

• A AXA 3BT − 380/45 vario-transformer.

• A HTT 10/0.4kV , 100kV A transformer.

• A 100m and a 10m NKT ”1-conductor PEX-CU 17.5kV” cable.

• A 12kV Siemens vacuum circuit breaker.

• A 0.5µF and a 1.0µF capacitive load.

26 4.1 The Existing Setup

Rogowski Current

Transducer

LeCroy

OscilloscopePosition

Meter

Voltage

Probe

Voltage

Probe

LabVIEW

Switch unit

Cable

HTT Transformer

Vario-transformer

Net Voltage

VCB

Load

High Voltage

Measurement signal

Control Signal

Ch1

Ch2

Ch4

Ch3

Figure 4.1: The laboratory setup including high voltage components and the con-trol and measurement system

The 3 phases from the vario-transformer are connected to the low voltageside of the HTT transformer. From the high voltage side of the HTT trans-former only one phase is connected to one side of the Siemens VCB via the100m or 10m NKT cable. The other side of the breaker is loaded with thecapacitive load. In figure 4.1 the high voltage setup is shown. Figure 4.1also shows the control and measurement system. The main element in thissystem is a LabVIEW program that is used to control the VCB and to mea-sure the voltage, current and the position of the VCB moving contact. Thefull control and measurement system consist of:

• The labVIEW program.

• A four channel LeCroy LC334 oscilloscope.

• A Hewlett Packard 34970A data acquisition/switch unit.

• Two Tektronix P6015A voltages probes.

Laboratory Setup 27

• A Rogowski current transducer of type CWT03.

• A linear position meter.

A more detailed description of the high voltage components and the controland measurement components, will be given in the following two sections.

4.1.1 High Voltage Setup

The vario-transformer is a AXA 3BT-380/45 vario transformer with a volt-age rating of 3x380V/3x0V − 380V . The transformer has a nominal currentof 45A and is rated at 29.6kV A. The vario-transformer is on the primaryside connected to the power grid and therefore supplied with 380V . The 3phases from the secondary side of the vario-transformer is connected to theHTT transformer.

The HTT transformer is a 10/0.4kV wire-wound transformer rated at 100kV Aand has 1136 windings at the high voltage side. The transformer is star con-nected on both sides. The transformer is a dry type transformer. On thesecondary side of the transformer a cable is connected on one of the threephases while the other two are left open.

In the project two identical cables with different length are used in order tocreate various network characteristic. The cables are ”1-conductor PEX-CU17.5kV” cables from NKT, the copper conductor has a diameter of 25mmand the insulation used is polyethylene. Some tests have been made on thecables in order to determine its losses [11], the main results of these testsare shown in table 4.1. As seen in table 4.1 the losses in the cable increases

– 50Hz 1kHz 500kHz 1MHz

R[Ω/m] 727.00 · 10−6 727.00 · 10−6 738.17 · 10−6 770.11 · 10−6

L[H/m] 239.79 · 10−9 239.79 · 10−9 239.79 · 10−9 239.79 · 10−9

C[F/m] 157.57 · 10−12 157.57 · 10−12 157.57 · 10−12 157.57 · 10−12

G[S/m] 14.85 · 10−12 297.01 · 10−12 148.51 · 10−9 297.01 · 10−9

Table 4.1: Cable parameters calculated at different frequencies [11]

when the frequency exceeds 1kHz. The two cables are used to connect theHTT transformer and the VCB.

The VCB used in this project is a 12kV Siemens ”3AH1 115-2” vacuumcircuit breaker. The breaker has a rated short circuit current of 31.5kVand a rated normal current of 1250A. In the tests done for this projectthe current will not come close to the rated current. The breaker is a 3phase VCB with a distance of 210mm between the centre of the 3 sets of

28 4.1 The Existing Setup

breaker contacts. The distance between the two VCB contacts in each phaseis in open position 9mm [12]. The operating drive of the VCB is using astored-energy mechanism, an opening spring and a closing spring. The clos-ing spring can be charged either electrically, by a motor, or mechanically,using a handle. It can also be unlatched either electrically by means of theremote control or mechanically using the local ”CLOSE” pushbutton [12].When the closing spring unlatches the opening spring automatically charges.

The loads chosen for the setup is a 0.5µF and a 1.0µF load. The rea-son for this is that the loads should represent a cable network under no-loadconditions, where a very small current flows in the network [11]. The loadsare installed on the frame of the VCB in order to avoid long connectionsthat can cause undesired transients.

4.1.2 Measurement and Control System

As mentioned the measurement and control system is build up around aLabVIEW program. This program concerns with controlling the vario-transformer, opening and closing the breaker, defining the measurementsettings and saving the measured data. As seen in figure 4.2 the LabVIEWprogram communicates with almost all parts of the laboratory setup. Infigure 4.2, a screen shot of the program GUI is shown. The program has 4graphs that show the measurements done on the high voltage system. In themiddle of the GUI there are 6 control boxes, 4 that control the oscilloscope,1 that controls the breaker and 1 that controls the vario transformer. Onthe right side of the GUI there is a button called Enable which is used tosave the measured data to a .lvm file. A description on how to plot the datafrom the saved .lvm file is seen in appendix A.

In order to control the vario-transformer and the VCB the LabVIEW pro-gram sends a signal to the ”Hewlett-Packard 34970A data acquisition/switchunit” via the GPIB interface. The switch unit is equipped with a I/O cardwhich switches 26V on 4 different channels. This unit is used to controltwo relays that sends a 170V dc signal to the VCB, these signal energizesthe two coils which are used for opening and closing the VCB. When thesprings for opening or closing the breaker is unlatches they automaticallylatches again using a motor supplied with 230V ac. The ”Hewlett Pacard34970A data acquisition/switch unit” is also used to send control signals tothe vario-transformer in order to increase or decrease the ratio of the trans-former or to bring the secondary side voltage to zero. The fact that it ispossible to increase and decrease the ratio of the vario-transformer enablesthe user to control the voltage level in the system.

Laboratory Setup 29

Figure 4.2: Screenshot of the LabVIEW program.

The data measurements in the system are collected in an oscilloscope andare sent to the LabVIEW program using a GPIB interface. The oscilloscopeused is a ”LeCroy LC334”, which can sample with a frequency of up to500MS/s. When the LabVIEW program is running it controls the oscillo-scope, it can setup the measurement range, it takes care of the trigger modeand setup, and the program enables the user to choose which measurementsto show on the oscilloscope display.

The voltage measurements are performed by two Tektronix P6015A volt-age probes. The probes are set to have a scaling of 1000:1 and they cantolerate up to 20kV and can measure frequencies up to 75MHz. The twoprobes are placed on each side of the VCB, the probe on the load side ofthe VCB is connected to channel 1 on the oscilloscope and the probe on thetransformer side of the VCB is connected to channel 2.

A Rogowski current transducer of type CWT03 is placed to measure thecurrent that runs through the high voltage system. The current transduceris placed after the load, meaning that the connection from load to groundruns through the coil. The current transducer can measure currents from300mA to 600A and can measure frequencies up to 16MHz. The output ofthe Rogowski current transducer is connected to channel 3 on the oscillo-

30 4.2 Improvements to the Existing Setup

scope.

To measure the distance between the contacts in the VCB a position meteris used. The position meter is connected to a fibreglass rod which is fastenedto the moving contact of the VCB [11]. When the contact moves, it movesthe fibreglass rod and thereby changes the output from the position meter.The position meter, is in fact just a variable resistance, and is supplied by a9V battery, which means that the output from the position meter is between0V and 9V . The output is connected to channel 4 on the oscilloscope.

4.2 Improvements to the Existing Setup

The first measurements showed the need for some improvements of the setup.The main improvements made to the setup were adjusting the probe con-nections, installing a discharging resistance to the load and changing thesetup of the Rogowski current transducer. Some minor adjustments werealso made, e.g. moving the loads closer to the circuit breaker and rewiringthe ground connection from the cable and the load making the connectionsas short as possible.

4.2.1 Improving probe connections

The first measurements made on the system gave rise to some strange oscil-lations. These oscillations were a result of a too movable connection betweenthe voltage probes and the VCB, when the VCB switches, it does so withlarge mechanical forces causing both the VCB and its frame to move. Dur-ing these movements the voltage probes lost the connections with the VCBin small time intervals, this causes the voltage oscillations seen in the mea-surements. This was avoided by fastening the connections as seen in figure4.3. The result of the new setup can be seen in figure 4.4, where the twoplots show the results of the measurements before and after the new setupwas used. Figure 4.4 also shows the TRV caused be the switching opera-tion, and as seen the shape of the TRV also changes when the probes arefastened. Before fastening the probes the shape of the TRV were effectedby disturbances and after the improvement the TRV obtains the expectedshape.

4.2.2 Installing discharging resistance

When a test of opening the breaker is made, the voltage on the capacitiveload has to go to zero before the next measurement is taken. For thisreason a discharge resistance is installed in parallel with the capacitor inorder to make the discharging of the capacitor faster. It was estimated that

Laboratory Setup 31

(a) Transformer side (b) Load side

Figure 4.3: The two pictures show how the probes are fastened to the setup.Before this was done the probes were connected loosely to the setupby the hooks on the tip of the probes.

0 10 20 30 40 50−2

−1.5

−1

−0.5

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1x 10

4HTTP1L100C1x05Dt1−6V5,75kOUT.lvm

Time[ms]

Vol

tage

on

tran

s. s

ide

of th

e V

CB

[V]

(a) Before improving the setup

0 10 20 30 40 50−1

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s. s

ide

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e V

CB

[V]

(b) After improving the setup

Figure 4.4: The plots show the voltage measured on the transformer side probe,before and after fastening the probe. At around 30ms oscillations canbe seen on figure a.

a discharging time of approximately 10 seconds would be suitable, due tothe time used on saving the measurements to the .lvm file. The followingcalculations show how the size of the discharging resistance, R, is found

τ = R · C,

setting τ = 5s and C = 0, 5µF gives

R =5s

0.5µF= 10MΩ.

As seen from the calculations a 10MΩ resistance gives a time constant of 5seconds. This means that after 5 seconds the voltage will have decreased to

32 4.2 Improvements to the Existing Setup

Figure 4.5: The picture shows the Rogowski current transducer. In order toimprove the current-to-noise ratio the current measurement is ledthrough the Rogowski coil 4 times as seen on the picture

37% of the initial voltage, which means that after 10 seconds the capacitorshould be discharged, therefore a 10MΩ resistance is chosen as dischargeresistance.

4.2.3 Improving the Rogwski current transducer setup

The first tests of the laboratory setup showed a low frequency disturbance onthe current measurement. Where this disturbance comes from is unknown,but since it had quiet a big influence on the current it was decided to improvethe current-to-noise ratio so that the low frequency fault current had lessinfluence. Therefore the wire conducting the current through the Rogowskitransducer was twisted several times so that it runs through the Rogowskicoil 4 times, increasing the output current by a factor 4, meaning that theratio of the Rogowski transducer is changed from 10mV/A to 40mV/A.In figure 4.5 it can be seen how the new setup of the Rogowski currenttransducer looks. A change in the LabVIEW program was made in order tofit the program to the new voltage-current ratio of the Rogowski transducer.

Chapter 5

Laboratory Tests and Results

The main purpose of the tests is to determine the paremeters that are usedto describe the VCB in the simulation model. All the measurements madein this project have been made on one phase. The tests have been per-formed with two different setups, one using the 10m cable to connect theHTT transformer and the VCB and one using the 100m cable. This is donein order to observe how the VCB reacts on different configurations of thenetwork it is operating in.

At both cable lengths tests were made at different voltage levels. The volt-ages levels were chosen based on the knowledge that the HTT transformerhas a nominal voltage of 5.75kV on the secondary side. This voltage levelwas chosen to be the base of the measurements and test series were madeon voltage levels of 20%, 40%, 60%, 80%, 100% and 120% of the 5.75kV .The reason why the system was tested at different voltage levels was to seehow the voltage level effects the generated transients. In order to calculatethe current chopping level of the breaker, the tests at the low voltage levelsare very useful, since they do not create any significant transients. Dur-ing the work with the current chopping level it was chosen to make use ofthe extra load capacitor in order to increase the current in the system. Asthis project mainly concentrates with the very fast transients caused by thebreaking operation the measurement time has been set to 10ms. Duringthis 10ms, 50000 data measurements are taken, which means that the timebetween each measurement, ∆t, is 0.2µs. This gives a good and precisepicture of the fast transients. When analysing the chopping current of theVCB tests with a measuring time of 50ms is used, since the amplitude ofthe current is a parameter in the chopping current calculations and cannotbe read on the 10ms measuremets. A few more measurements, with a mea-surement time of 50ms, were made to illustrate the breaking process andmeasure the opening and closing time of the VCB.

34

In order to observe the effect the arching time has on the VCB and theTRV it was decided to make several tests on each voltage levels to observemost possible breaking angles and thereby different arching times. Since it isnot possible to control the breaking angle or the breaking time of the VCB,random tests were made and for every test the angle was registered. Thevoltage sine curve was divide in 8 sections and the testing was continueduntil a breaking angle in each section was obtained. The breaking angle wasread on the voltage measurements, this means that with a breaking angle of0, the VCB opens when the power frequency voltage curve is at the risingzero crossing. A breaking of 90 would mean that the VCB opens when thevoltage is at its maximum. When the breaker closes, the angle at which itstarts conducting current is called the closing angle and is found in the sameway as the breaking angle.

To distinguish between the different test series it was decided to give eachfile containing the measurements a name referring to the test setup. Anexample of the file name could be:

HTTP1L100C1x05Dt2− 7V 5, 75kIN1

where the meaning of the different parts of the name are:

• HTT : HTT transformer is used.

• P1 : One phase is connected between the HTT transformer and theVCB.

• L100 : The length of the cable between the HTT transformer and theVCB is 100m.

• C1x05 : The load on the VCB is capacitive with a size of 0.5µF .

• Dt2-7 : The time step between measurements is 2 · 10−7s.

• V5,75k : The voltage on the secondary side of the HTT transformeris 5.57kV .

• IN1: The measurements are taken when the VCB closes.

The HTT transformer and number of phases connected are included in thefilename, even though they are not changed throughout the project. This isdone so that it is easy to make more tests with different setups and comparethe new tests with the ones made in this project in future work on the VCB.

As described in chapter 4, the measurement system takes 4 measurements,the position of the moving VCB contact, the current, the voltage on the

Laboratory Tests and Results 35

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Figure 5.1: Closing the VCB at voltage level 6.9kV , the setup is using the 100mcable and the load with a capacitance of 0.5µF . The time betweenthe measurements ∆t is 1 · 10−6s, and the closing angle is 0

load side of the VCB and the voltage on the transformer side of the VCB. Infigure 5.1 the 4 measurements from a closing process of the VCB are shown.As figure 5.1a shows, the moving VCB contact starts to move towards thefixed VCB contact after around 20ms. The picture shows a distance betweenthe contacts is only 7.8mm and not 9mm, this is due to a small calibrationerror in the position meter. After around 30ms the distance between thecontacts is 0mm and the VCB starts conducting current, as seen in figure5.1b. Figure 5.1c and 5.1d show the voltage on both sides of the VCB, thevoltage on the load side is zero until the contacts are together and after thatit follows the transformer side voltage since the VCB forms a short circuitbetween the two voltage probes. This measurement is taken at aclosing angleof about 0. As mentioned this angle can easiest be seen on the transformerside voltage curve, where a small switching transient appears in the area of

36 5.1 Preparatory tests

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Figure 5.2: The figure shows two plots of the distance between the VCB contacts,when the VCB is opening and closing.

the rising zero crossing or around 0. As this shows the measurement of theangle is not very precise and the closing and opening angles decribed will beapproximate values.

5.1 Preparatory tests

5.1.1 Breaker position

The opening time of this type of VCB should be less than 15ms [12] andprevious projects have measured the opening and closing time of the specificVCB to be around topen = 7ms and tclose = 12ms [9]. In figure 5.2 ameasurement of the distance between the VCB contacts during a closingand an opening process is seen. In figure 5.2 two data markers are set, thesedata markers shows the time and the distance between the contacts and thisdata can be used to find the closing and opening time of the VCB.

tclose = 32.001ms− 20.391ms = 11.61mstopen = 27.602ms− 19.889ms = 7.713ms

The values used for the calculations are more exact than the values seen onthe data markers on the figures. The exact values of the data markers canbe exported to a file in MATLAB and the values from this file are used tomake the calculations, this method was used throughout the project. Theresults of the closing and opening time corresponds to the values calculatedin previous projects. As seen of figure 5.2b some vibrations occurs when themoving breaker contact reach the open position. These vibrations are causedby the mechanical impact the contact gets when reaching the open position


and it stabilise after some time. The used breaker model does not offer anoption to change the opening and the closing time of the VCB. Instead themodel uses a fixed time of 0.55ms as both the opening time and the closingtime of the VCB.

5.1.2 Resistance of the Voltage Circuit Breaker

The simulation model of the VCB uses the resistance of the VCB in openand closed position as parameters and for this reason these values have tobe measured. Since the resistance in open position is very high and theresistance in closed position is very small, a normal ohmmeter cannot beused in the measurement. In order to measure the resistance of the VCBin open position an insulation tester is used, the tester is a ”Fluke 1520MegOhmMeter” that can measure a maximum of 4000MΩ. The test of theopen resistance measurement gave

Ropen > 4000MΩ.

In order to measure the resistance of the VCB in closed position a Wheat-stone bridge is used. This device is a bridge circuit which consists of 2known resistors, one adjustable resistor and the last part of the bridge is theresistor being measured. A 4-point measurement technique, applying botha current and a voltage over the measured resistance, is used in order to getthe most precise measurement. The measurement of the closed resistancegave the following result

Rclosed = 200µΩ,

The value of Ropen will be set to 1MΩ in the simulations, since it is thehighest possible value in the simulation model, and Rclosed will be set to themeasured 200µΩ.

5.2 Transient Recovery Voltage

When the VCB contact separates and the vacuum arc extinguishes, a TRVwill arise across the two contacts. This TRV is a critical parameter in theinterruption process, the TRV can either cause the arc to be reestablishedor it can lead to successful interruption. As described in chapter 2, the TRVis dependent on the network configuration.

The TRV is calculated by using the voltage measurement from the loadside of the VCB and the voltage measurement from the transformer side ofthe VCB, like this

TRV = VLoad − VTrans. (5.1)

38 5.2 Transient Recovery Voltage

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Figure 5.3: The TRV across the breaker contacts using a 10m and a 100m cable.The breaking angle is in both cases 0.

5.2.1 Frequency of the Transient Recovery Voltage

In chapter 2 it is described how the frequency of the TRV depends on thenetwork configuration, and should be the same for all breaking angles. Testshave been performed with two network configurations in order to observedifferent frequencies of the TRV. Figure 5.3 shows the TRV in the two net-work configurations, in both cases the breaking angle is 0. As figure 5.3clearly shows the tests made with the different cable lengths gives differentfrequencies of the TRV. To find the two frequencies a number of measure-ments were made on the curves. In figure 5.4 the measurement from one testis shown. As seen in figure 5.4, 7 markers have been placed on the curve,one marker is placed to show at what time the TRV is damped, and the last6 markers are placed to find the wavelength of the TRV at different places.If using the two first markers at the top tips of the TRV it can be foundthat the wavelength of the TRV in this area is

λ = 4.2908ms− 4.1808ms = 0.11ms,

and by using the wavelength the frequency can be found

fTRV =1λ⇒ 1

0.11ms= 9090.9Hz

In order to minimise inaccuracy caused by the data measurements, moredata markers are set. The wavelength and the resulting frequencies arefound between the other data markers as well and the mean of the fourfrequencies is found. In appendix B the results of all the tests made at5, 75kV , for both 10m and 100m, is shown. In the full appendix, which isfound on the cd, the measurements and data points used for the calcula-tions are found. The results in tables B.1 and B.2 show the breaking angle,


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Figure 5.4: The measurements for calculating the frequency of the TRV whenusing a 10m cable in the test. The breaking angle is again 0.

the amplitude, the frequency and the damping time of all the measurements.

The two tables show that the frequency is independent of the breaking angleand the average value of the frequencies are:

fTRV 10 = 8708, 92HzfTRV 100 = 2953, 87Hz.

The results show that when changing from a 100m cable to a 10m cable, thefrequency of the TRV becomes almost 3 times higher. This is a result of thedifference in capacitance and inductance in the two cables. As the cablesused are of same type, the 100m cable has a capacitance and inductancethat is 10 times higher than in the 10m cable, the specific parameters of thecable can be seen in table 4.1.

5.2.2 Amplitude of the Transient Recovery Voltage

The amplitude of the TRV is a very important parameter as this voltagecan reach values that are higher than the normal peak voltage and therebyapply a high eletrical stress to components. The amplitude of the TRV ismeasured as the first and undamped maximum of the TRV, e.g. in the testshown in figure 5.4 the amplitude of the TRV is 5125V . In contrast to thefrequency of the TRV, the amplitude of the TRV is dependent of the break-ing angle. This dependency can be seen in tables B.1 and B.2. In table 5.1

40 5.2 Transient Recovery Voltage

some of the results from the tests using the 10m cable have been taken outand sorted by the breaking angle.

From table 5.1 the relation between the breaking angle and the ampli-

Test nr. 12 4 3 9 2 8 7 10Brk. angle[] 0 22.5 45 90 180 202.5 225 247.5Amplitude[V ] -5500 -5375 -4875 0 5250 4500 4250 1625

Table 5.1: The table shows the relation between the breaking angle and the am-plitude of the TRV. The results are from the tests made at 5.75kVusing the 10m cable.

tude can be seen. When the VCB breaks the voltage close at a maximumvalue of the voltage, 90 and 270, the amplitude of the TRV is low andwhen the VCB breaks the voltage close to a zero crossing, 0 and 180, theamplitude is high. The reason for this is that the VCB conducts a capac-itive current, causing the current to lag the voltage with 90. This meansthat when the VCB breaks at a high voltage the current that is interruptedis low and will cause low amplitudes of the TRV. In the best case, if theinterruption happens at a current zero crossing, no TRV will be generated.When the voltage is around zero at the time of interruption, the current willbe interrupted around its maximum and cause a TRV with high amplitude.

The tables B.1 and B.2 also show that for breaking angles from 90 to 270

the TRV will start by rising, resulting in a positive first amplitude. Simi-larly for breaking angles from 270 to 90 the TRV will start by falling, thereason for this relation is the direction of the current, which will be oppositein the two intervals. Table B.1 and B.2 show that there is a clear connectionbetween breaking angles that are 180 apart. They have the same amplitudebut with different sign, this is because measurements that are 180 apart areon the same place of both the voltage and current curve, except from thefact that one is on the negative part and one is on the positive part. Thissimilarity has been observed throughout the project.

The tests have shown that the setup with the short cable gave larger TRVamplitudes. As the short cables also have a larger frequency the rate of riseof the TRV will be higher when using the short cable. This will lead to morereignitions, since a circuit where the TRV has a high rate of rise will reachthe dielectric withstand level of the VCB faster than in a setup where therate of rise of the TRV is lower.

The tables B.1 and B.2 also show the time it takes for the TRV to becompletely damped. The TRV generated when using the short cable needsless than half the time to be damped than the TRV generated when using


the long cable. The mean value of the damping time is 0.97ms when usingthe 10m cable and 2.3ms when using the 100m cable.

5.3 Chopping Current

The current chopping is an undesirable effect of the VCB, since the steepslope of the current produces the TRV that can cause overvoltages in thenetwork. As described the current chopping is a result of arc instabilitywhich causes the vacuum arc to be extinguished before reaching a currentzero. The value of the current chopping level is found using equation (3.1).In order to find the contact parameters α and β, it is necessary to know theamplitude of the current through the breaker and the current chopping level.Figure 5.5 shows a measurement of the current when opening the VCB atvoltage level 5.75kV with breaking angle 180 using the 100m cable.

As seen in figure 5.5a there is a high spike on the current around 20ms,this current spike is a result of the HF currents, which occur after arc ex-tinguises. In order to read the amplitude of the current and the currentchopping level a zoomed plot of the current around the breaking time isneeded. In figure 5.5b a plot zoomed around the power frequency currentis shown. As seen the zoomed plot is very blurred and it is hard to make aprecise measurement of the current chopping level and the current magni-tude.

This problem occurs when recording the measurements in LabVIEW. Inorder to get the full picture the settings of the oscilloscope have to be in away so that all data points fit in the plot. For this reason the sensitivity ofthe oscilloscope has to be set so that the maximum point of the HF currentis within the measuring range. This means that the measurements get lessaccurate and the picture gets blurred. Therefore it is convenient to look atmeasurements taken at lower voltage levels, where smaller or almost no HFcurrents are produced. In figure 5.6 a plot of the current during a breakingprocess is shown. As seen the current is interrupted at around 22ms, be-fore this time the power frequency current (with disturbance) is conducted.At 22ms the current chopping level is apparently reached and the currentis chopped. After this point the only current comes from discharging thecapacitor and after a short time the current measured in the Rogowski coilis zero.

The full appendix C on the CD contains the measurements for calculat-ing the values α and β. From the graphs in the full appendix C, it can beobserved that the current chopping occurs at the same time in all measure-ments. This indicates that the current level is under the level of the chopping

42 5.3 Chopping Current

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Figure 5.5: The plots shows the current through the VCB at 5, 75kV using a100m cable. The interruption is made at a breaking angle of 180.

current and therefore the current is chopped instantaneously after contactseparation. In table C.1 the value of α has been calculated when β has thevalue 14.3, this is expected to result in a value of α around 6.2 · 10−16s.As seen in table C.1 the calculated values of α are very different from theexpected value, and the calculated values vary a lot.

The results from table C.1 indicate that the current is under the currentchopping level of the breaker, which means that the VCB breaks the currentas soon as the contacts separate. Therefore another test series with the extracapacitor, increasing the load to 1.0µF , was made. These tests can also beseen in the full appendix C on the CD. The constant α has been calculatedagain using the new measurements and the results can be seen in table C.2the values of α are still very different from the expected value and they stillvary a lot. And since it has not been possible to find a reasonable value ofα no attempts at finding β has been made. The plots of the measurementsand the calculations of the constant α strongly indicates that the currentlevel in the tests is lower than the current chopping level of the VCB. Thiscorresponds with the fact that the normal current chopping level for VCBsusually varies between 3A and 8A. In order to find the parameters of α andβ for the breaker, different types of loads must be used in order to conducta current that is larger than the current chopping level. But since only thesmall capacitive loads were available standard values of α and β will be usedin the simulation model:

α = 6.2 · 10−16s

β = 14.2.


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Figure 5.6: The plots shows the current through the VCB at 1, 15kV using a100m cable. The interruption is made at a breaking angle of 45.

5.4 Reignitions

To find the dielectric withstand of the VCB, the reignitions that occur whenopening the breaker have been studied. After the vacuum arc has been ex-tinguished, the race between the TRV and the dielectric withstand of theVCB begins. When the TRV exceeds the dielectric withstand of the VCB abreakdown of the vacuum occurs and creates a conducting path between thetwo VCB contacts. When the conducting path is created the TRV jumpsback to zero and does not start to rise again before the arc is extinguished.In figure 5.7 it is seen how reignitions appears after contact seperation.

Figure 5.7 shows there is a difference in the number of reignitions betweenthe tests made with the 100m cable and the 10m cable. In figure 5.7 thereis 3 reignitions when the 100m cable is used and around 11 reignitions whenusing the 10m cable. It is also seen that the conducting time of the vacuumarc is a lot shorter when using the 10m cable, with this setup the arc isextinguished almost instantaneous. The reignitions when using the 100mcable appear for a longer time, around 0.2ms. The difference is a result ofthe different shape of the TRV, as mentioned the TRV has a large rate ofrise in the system when the 10m cable is used. Therefore the TRV will reachthe dielectric withstand of the VCB much faster and create more reignitionsas seen in figure 5.7.

44 5.4 Reignitions

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Figure 5.7: The plots shows the voltage across the breaker contacts during anopening of the VCB. Both tests have a breaking angle of 225.

5.4.1 Rate of rise of Dielectric Strength

In order to model the VCB it is necessary to know its dielectric withstand.The dielectric withstand of the breaker can be found by using equation (3.2)and in order to simplify the calculations the value of the TRV just beforecurrent zero is set to be zero. This means that the dielectric withstand ofthe breaker is proportional to the RRDS or the value A used in the simu-lation model. Figure 5.8 shows how the RRDS of the VCB is found. Thered line on figure 5.8 illustrates how the dielectric withstand is increasedwith respect to time. The time t0 is set to zero in the calculations, thisis done because the time of contact separation is not know. This will givecorrect results when making the linear regression to find the RRDS, sincethe progress of t − t0 is the same as long as t0 is set constant. The datameasurements that are showed in figure 5.8d are used to perform a linearregression, finding the RRDS of the VCB. In the example on figure 5.8d thevalue of the RRDS becomes 24.37V/µS. The full appendix D on the CDshows the data measurements that have been used to calculate the value ofthe RRDS and the tables D.1, D.2, D.3, D.4, D.5 and D.6 shows the resultsof the calculations. As the tables show an average value of the RRDS isfound in each of the 6 test series, and these values can be seen in table 5.2.As it is seen in table 5.2 the calculated values of the RRDS is much smallerwhen using the 100m cable than the ones calculated for the 10m cable. Thetable also shows that the value of the RRDS seems to be dependent of thevoltage when using the 100m cable, where the calculations of the RRDS aremore constant when the 10m cable is used. The results found in appendixD also shows a much larger variance of the results using the 100m cablewith many large and small values of the RRDS. This is particularly the case


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Figure 5.8: The figure illustrates how the RRDS is calculated from the laboratorymeasurements.

Cable lengthVoltage level 10m 100m

4.6kV 38.24V /µS 18,52V /µS5.75kV 39.39V /µS 21,94V /µS6.9kV 36.02V /µS 24,50V /µS

Average RRDS 37.88V /µS 21.65V /µS

Table 5.2: The table shows the average RRDS, for the 6 analysed test series andthe average value of the RRDS found for the two cable lengths.

at voltage level 4.6kV and is probably due to the fact that less reignitionsoccurs at this voltage level and thereby makes the results vulnerable to mea-surement mistakes. In the tests using the 10m cable the results are moreclose to the average value and only a few results are very different from the

46 5.4 Reignitions

average values.

With basis in these consideration the value of the RRDS that is used inthe simulations, has been chosen. It is decided to choose the value of theRRDS that was calculated in the tests where the 10m cable is used. Thisdecision was made because of the lack of stability in the results from the100m cable tests. Specially the fact that the value of the RRDS increaseswhen the voltage level (number of reignitions) increases indicate that thereis a lot of inaccuracy in the calculations, and therefore the results using the10m cable, which creates a lot of reignitions, are used. The value of theRRDS that will be used when simulating the VCB will therefore be the av-erage of the value found in the 10m cable tests and the value of B will bezero:

A = 37.88V/µsB = 0.

This means that the dielectric withstand of the VCB when it is fully open(after 7.23ms) will be:

Vopen = 37.88V/µS · 7.23ms · 1000µsms

= 273.87kV.

This means that the withstand of the vacuum between the breaker contactsis:

Vvacuum =273.87kV

9mm= 30.43kV/mm.

As seen from the calculations the dielectric withstand of vacuum in the VCBis approximately 10 times larger that the withstand in air.

5.4.2 Effect of breaking angle

As seen in table 5.1 the breaking angle has an influence in the amplitudeof the TRV. Therefore it is also expected that the breaking angle has aninfluence in the number of reignitions that occurs after separating the VCBcontacts. As figure 5.7 shows there is a difference in reignitions in the testsdone with different cables. A closer look at tables D.1, D.2, D.3, D.4, D.5and D.6 show that the number of reignitions are dependent on the breakingangle. To illustrate this the results from the test made at 5.75kV with10m cable have been sorted by the breaking angle and are shown in table5.3. Table 5.3 shows that the relation between number of reignitions andbreaking angle follows the same pattern as the relation between amplitudeof the TRV and the breaking angle. When the VCB breaks the voltage closea maximum value of the voltage at 90 and 270, the number of reignitions


Test nr. 6 5 3 9 1 8 7 10Brk. angle[] 0 22.5 45 90 180 202.5 225 270Reignitions 20 18 15 0 15 15 10 1

Table 5.3: The table shows the relation between the breaking angle and the num-ber of reignitions of the vacuum arc. The results are from the testsmade at 5.75kV using the 10m cable.

is low and close to zero and when the VCB breaks the voltage close to a zerocrossing, 0 and 180, the number of reignitions is quiet high. The reason forthis is that the current is capacitive and therefore zero at voltage maximumand maximum at voltage zero. It is clear that the number of reignitionsmust be zero if the VCB opens at a current zero crossing since no current isinterrupted and no vacuum arc is formed. During a current maximum a largecurrent will be interrupted this creates a TRV with high amplitude whichleads to many reignitions. Tabel 5.3 shows that the number of reignitionsis almost the same at the positive part and the negative part of the curve,this can again be related to the TRV which had the same amplitude at thepositive part and the negative part.

5.5 High Frequency Quenching Capability

When a reignition of the vacuum arc occurs it will cause a HF current tobe superimposed on the power frequency current. This HF current may bequenched at one of its zero crossings, if it has a low enough di/dt. If thecurrent is quenched the TRV will again start rising over the VCB gab anddepending if it reaches the dielectric withstand a new reignition and a newHF current will be created. In figure 5.9 the TRV and the current throughthe VCB is seen. Figure 5.9 is a measurement taken when the 100m cable isused, in this measurement it is easy to see that when the two first reignitionoccur (TRV jumps to 0), the HF current is formed. The third reignition(after 2940µs) gives only a short appearance of the HF current, which isquenched after only a half period. It is also seen that the first HF current(13 periods) is longer than the second HF current (11

2 periods). This phe-nomenon of shorter lifetime of the reignitions has been observed in almostall measurements on the 100m cable, but in most of the measurements thesecond reignition creates a HF current with a length of 3-7 periods.

As figure 5.9 shows, the magnitude of each HF current is small at the firstreignition and rises from reignition to reignition. In this case the magnitudestarts at around 10A on the first HF current and at the second HF currentthe magnitude is around 30A ending at a magnitude of around 80A for thelast HF current. This phenomenon is also observed in all the measurements.

48 5.5 High Frequency Quenching Capability

2700 2750 2800 2850 2900 2950 3000−6000

−4000

−2000

0


TR

V[V

]

2700 2750 2800 2850 2900 2950 3000−100

−50

0

50

100

Time[us]

Cur

rent

[A]

Figure 5.9: The plot shows how the reignitions of the VCB create a HF currentthat is superimposed on the power frequency current. The test is fromthe system with 100m cable and the breaking angle is 292.5.

The frequency of the HF currents is around 400kHz, which means that thewavelength is only 2.2µs. With this short wavelenght and a ∆t of 0.2µs theplots of the HF currents do not get as precise as desired. The plots are goodenough to determine the HF current quenching capability, but in order toget a more precise result the measuring time, when examining HF currents,should be set down in comming projects.

When the 10m cable is used in the measurement the shape of the HF cur-rents become very different from the ones observed in figure 5.9. The HFcurrents that appear in the system using a 10m cable can be seen in figure5.10. The HF currents seem to be quenched instantly after they appear andcan just be seen as small spikes on the current curve, which occur when theTRV jumps to zero. Some of the HF currents are quenched even before theyreach their first maximum, and therefore it is not possible to see the increasein magnitude in the HF currents.

5.5.1 Determining the High Frequency Quenching Capabil-ity

Since the HF currents from the tests with the 10m cable have a very shortlife time and are often quenched even before the first maximum is reached, itis not possible to use them when calculating the HF quenching capability of


3850 3900 3950 4000 4050−5000

0


TR

V[V

]

3850 3900 3950 4000 4050−50

0

50

Time[us]

Cur

rent

[A]

Figure 5.10: The plot shows how the reignitions of the VCB create a HF currentthat is superimposed on the power frequency current. The test isfrom the system with 10m cable and the breaking angle is 225.

the VCB. Before determining the HF quenching capability of the VCB manyconsiderations on the approach were made. The two main considerationswere how to determine the constant Cc that appears in equation (3.3) andthe second was how to set the opening time of the VCB, t0. The value of Cccan be described as the change in di/dt with respect to time. The value istherefore found by finding the slopes of the HF current between a maximumand a minimum point and describe the slopes as a function of time. Thetime used to find Cc is in equation (3.3) given as t− t0. The time t0 shouldbe the opening time of the breaker, but since this time is not known it, wasdecided to set t0 as the time when the HF current starts appearing. Whencalculating the value of RRDS the time t0 was set to zero, but in this casethe beginning time of the HF current was chosen, since this time have to beused anyway when finding the value of Dd. The value of the constant Cccan now be calculated from the measurements of the HF currents. In figure5.11, 2 plots are seen, plot 5.11a shows 3 occurrences of HF currents duringa breaking operation and plot 5.11b is a zoomed plot showing the values usedto calculate the HF quenching capability of the VCB. As described the valueof Cc is found by making a linear regression, using the di/dt, found betweenthe data markers, and the difference between the time of a the zero crossingand the start time of the HF current. The value of Dd is found simply by

50 5.5 High Frequency Quenching Capability

2700 2750 2800 2850 2900 2950 3000−100

−80

−60

−40

−20

0

20

40

60

80


Time[us]

Cur

rent

[A]

(a) Plot showing the reignitions

2770 2780 2790 2800 2810 2820 2830

−25

−20

−15

−10

−5

0

5

10

15

20

25

X: 2783Y: −1.562


Time[us]

Cur

rent

[A]

X: 2784Y: −17.19

X: 2785Y: 14.06

X: 2787Y: −15.62

X: 2787Y: 12.5

X: 2789Y: −14.06

X: 2790Y: 12.5

X: 2792Y: −12.5

X: 2793Y: 10.94

X: 2794Y: −12.5

X: 2795Y: 10.94

X: 2797Y: −12.5

X: 2798Y: 7.812

X: 2799Y: −10.94

X: 2801Y: 7.812

X: 2802Y: −10.94

X: 2803Y: 7.812

X: 2804Y: −9.375

X: 2806Y: 7.812

X: 2807Y: −9.375

X: 2808Y: 6.25

X: 2809Y: −7.812

X: 2811Y: 6.25

X: 2812Y: −9.375

X: 2813Y: 6.25

X: 2815Y: −7.812

X: 2816Y: 1.562

(b) Zomed plot showing the first reignition

Figure 5.11: The plots shows the data markers used to calculate HF currentquenching capacity of the VCB.

finding the slope between the first two data markers seen on figure 5.11b.

Dd =−1.562− (−17, 1875)

2783, 2− 2784, 2= −15.626

A

µs.

In the full appendix E on the CD the measurements and the data used forcalculation of the HF quenching capability of the VCB are found. The resultsof the calculations is seen in table E.1, in the printed version, and as seenthe calculations of Cc and Dd are made separately for each reignition. Theresults in the table are only results from HF currents that have a appearancetime, which is long enough to give a realistic result (more that 1 period).The average value of Cc and Dd can be seen in table 5.4. As table 5.4, shows

Arc nr. Arc 1 Arc 1 Arc 2 Arc 2 Arc 3 Arc 3C[ A

µs2] D[ Aµs ] C[ A

µs2] D[ Aµs ] C[ A

µs2] D [ Aµs ]

Average -0.591 25.670 -1.190 48.572 -1.912 75.703

Table 5.4: The average results of the calculations of C and D

the values of Cc and Dd both increase depending on the number of reignitionit is calculated for. And as seen in table E.1 in the appendix, the value of theconstants vary a lot even for the calculation done on the same arc number.When comparing the results to the suggested values presented in chapter 3,it shows that the lowest value of Cc is pretty far from the suggested negativevalue, −0.034A/µs2 and the value of Dd is also far from the suggested values.

Due to the found results, another approach of finding the HF quenchingcapability was tried. Instead of looking at the value of di/dt according to


equation (3.3) the value was set to be constant. Using this approach thevalue of di/dt can be seen as the slope between the last two data pointersin figure 5.11b, the example from the figure gives

Dd =−7.8125− 1.5622814.8− 2815.6

= 11.718A

µs.

This approach leads to the results seen in table E.2 in appendix E. Theaverage of the results is shown in table 5.5. As the table shows the results

Arc nr. Arc 1 Arc 2 Arc 3D[ Aµs ] D[ Aµs ] D[ Aµs ]

Average 13.255 34.03 55.742

Table 5.5: The average results of D when considering di/dt to be constant.

calculated with this approach is again not close to the expected values pre-sented in chapter 3, (100 A

µs - 600 Aµs). Since none of the two approaches gives

results close to the expected values of the HF quenching capability param-eters, it could indicate that the quenching capability of the VCB cannot becalculated when the current through the VCB is very low. Because of theresults the values of the HF current quenching constants, Cc and Dd are setto the suggested values in [1]

Cc = 0A

µs2µS

Dd = 350A

µs.

5.6 Closing the circuit

When the contacts in the VCB start moving together the dielectric withstandof the gab between them starts to get smaller. At one point the dielectricwithstand of the gab will become smaller than the voltage across the contactand a breakdown of the vacuum will occur.

5.6.1 Prestrikes

This conducting vacuum arc formed by the breakdown will cause the volt-age between the breaker contacts to go to zero. In figure 5.12 a plot of thevoltage across the VCB channels and the current during 3 prestrikes of thebreaker is seen. As seen in figure 5.12 the prestrike creates an oscillatingHF current. This current is interrupted at one of its current zeros, whendi/dt is low enough and the voltage over the VCB channels reappears. Thearc appears again the next time the voltage over the contacts reaches the

52 5.6 Closing the circuit

6180 6190 6200 6210 6220 6230 6240 6250 6260−10000

−5000

0


Vol

tage

[V]

6180 6190 6200 6210 6220 6230 6240 6250 6260−200

−100

0

100

200

Time[us]

Cur

rent

[A]

Figure 5.12: The plot shows the voltage across the breaker channels and the cur-rent through the breaker when during 3 prestrikes. The test is madewith 100m cable and at a closing angle of 22.5.

decaying dielectric withstand of the VCB.

Prestrikes are also observed in the system using a 10m cable and in thesame way as the reignitions, the prestrikes created in this setup seems tohave a much shorter lifetime. In figure 5.13, 4 prestrikes from measurementswith the 10m cable are seen. In the same way as when the 100m cable isused the prestrike forces the voltage across the contact to go to zero. Afterthe restrikes in figure 5.13 the voltage is going more smoothly back to theprevious voltage level than when using the 100m cable. Just after it reachesthe previous voltage level another restrike is created whereas in the casewith 100m cable the voltage across the cannels kept were steady for a whilebefore another restrike was created.

In the VCB model used in the simulations the rate of decay of dielectricwithstand (RDDS) is set to have the same value as the RRDS. In the de-scription of the VCB model [7] prestrikes are not treated, and therefore itwas decided to examine the RDDS to see if it has the same value as theRRDS. In appendix F the RDDS in the VCB have been calculated. On theCD the fulle appendix F is found, this contains the pictures and data usedin the calculation, the result of the calculations is seen in appendix F in thereport. The calculations have only been done at voltage level 5.75kV for thesystem using the 10m cable, due to the experiences made when calculating


5500 5510 5520 5530 5540 5550−5000

0

5000


Vol

tage

[V]

5500 5510 5520 5530 5540 5550−100

−50

0

50

100

Time[us]

Cur

rent

[A]

Figure 5.13: The plot shows the voltage across the breaker channels and the cur-rent through the breaker when during 4 prestrikes. The test is madewith 10m cable and at a breaking angle of 202.5.

the RRDS. Table F.1 show the results of the calculations. As the resultsshow the average value of the RDDS is found to be 147.1V/µs, this value isalmost 4 times higher than the result of the RRDS which was found to be37.88V/µs.

This result points out a weakness of the used VCB model and shows a needof a VCB model where the RDDS is investigated and treated as a individualparameter.

The results in table F.1 also show how the closing angle affects the num-ber of prestrikes. This can be seen more clearly from table 5.6 where someresults from table F.1 have been picked out and arranged according to theclosing angle. As seen from table 5.6 the closing angle has the opposite effect

Test nr. 9 8 7 3 4 5 10 6Clos. angle[] 0 67.5 90 112.5 180 202.5 270 316

Prestrikes 0 3 6 4 2 4 6 2

Table 5.6: The table shows the relation between the opening angle and the numberof prestrikes of the vacuum arc. The results are from the tests madeat 5.75kV using the 10m cable.

on the number of prestrikes than the breaking angle has on the number of


0 10 20 30 40 50−10

−8

−6

−4

−2

0

2

4

6


Time[ms]

Cur

rent

[A]

(a) Breaking angle 0

0 10 20 30 40 50−50

−40

−30

−20

−10

0


Time[ms]C

urre

nt[A

](b) Breaking angle 270

Figure 5.14: The plots shows the current through the breaker when closing thecircuit at 0 and at 270, both measurements are made with 10mcable.

reignitions. Table 5.6 shows that when closing the breaker at low voltageand high current the number of prestrikes is low and when closing the VCBat high voltage and low current the number of prestrikes is high.

5.6.2 Current During Closing

The current during the closing operation of the breaker is expected to be ofthe same shape as found in the example in chapter 2. In figure 5.14 the cur-rent transients generated by an closing operation, with closing angles of 0

and 290 is seen. As seen from figure 5.14a the exponential transient termbecomes almost zero and the current follows the power frequency currentimmediate after separation, when the closing angle is 0. In figure 5.14bthe closing angle is 270 this means that the exponential transient term willobtain its maximum starting value and after separation lead the current to-wards the steady state current.

As figure 5.14 shows the current after the closing operation consist of theexponential transient term and the steady state term, as explained in chap-ter 2. But as seen in figure 5.14, a higher frequency term is also affecting thecurrent just after contact separation. This term is caused by the same phe-nomena that causes the TRV and the oscillating term will have a constantfrequency dependent of the inductances and capacitances in the system. Thefrequency of the oscillating transient caused by the closing operation can befound by using equation 2.14, which is also used to find the frequency ofthe TRV. The inductance of the system is the same as in the opening case,but now the load capacitance (0.5µF ) becomes the dominant capacitance.


0 10 20 30 40 50−10

−8

−6

−4

−2

0

2

4

6

8

X: 13.47Y: −1.953

X: 10.36Y: −4.297


Time[ms]

Cur

rent

[A]

(a) 10m cable

0 10 20 30 40 50−180

−160

−140

−120

−100

−80

−60

−40

−20

0

20

X: 31.63Y: −143.8

X: 34.52Y: −125


Time[ms]

Cur

rent

[A]

(b) 100m cable

Figure 5.15: The plots shows the current through the breaker when closing thecircuit at 0 and at 290. Data measurements are set in order tocalculate the frequency of the oscillating transient.

This means that the frequency of the oscillating transient should be almostsimular for the two cable lengths. In this case we will say that the load ca-pacitance is the only capacitance in the system and the frequency is thereforeexpected to be

fopen =1

2 · π√

0.318H · 0.5µF= 399.14Hz. (5.2)

In figure 5.15 data markers have been placed on the current curves andfrom the value of the data markers the frequency of the oscillating transientcan be found. The frequency is only found in these two measurements, asthe results from section 5.2 showed that the frequency of the oscillations isconstant. The 2 frequencies is found to

fopen10 =1

12.84ms− 10.28ms= 390.63Hz

fopen100 =1

34.66ms− 31.45ms= 311.53Hz.

The results shows that the frequency, for both cable lengths, is almost thesame as the calculated frequency 399.14Hz. For the system using the 100mcable the frequency is more inaccurate. This is because the capacitance ofthe cable is larger in this system and thereby has a larger influence on thesystem.


Chapter 6

Simulations

To simulate the system it was decided to set up a simulation model inPSCAD. The VCB model that is used in the simulation of the system isdescribed in [7]. The model uses the parameters found in chapter 5 todescribe the behaviour of the VCB. In figure 6.1 the setup of the simulationmodel for the system is seen.

Figure 6.1: Setup of the system representing the laboratory setup when using the10m cable.

The ideal voltage generator in the left side of the circuit represents the netvoltage supplying the vario-transformer, the vario-transformer and the cableleading from the vario transformer to the HTT transformer. The inductanceof 0.318H represents the HTT transformer, the value of the inductance isfound by looking at the nominal load of the transformer. The impedance ofthe transformer is calculated to be

Z = ex ·V 2m

Sm= 0.1 · 10kV 2

100kV A= 100Ω (6.1)

The factor ex, is the short-circuit impedance of the transformer, this valueis set to be purely resistive and its value is set to 0.1. The inductance of the

58 6.1 Opening the Vacuum Circuit Breaker

transformer can now be found.

Z = 2 · π · f · L⇒ L = 0.318 (6.2)

The capacitance of 1575.7pF represents the 10m cable, the value of thecapacitance is found according to table 4.1. The box called BRK is themodel of the VCB where the values found in chapter 5 are inserted in orderto represent the VCB used in this project. The box over the VCB calledTimedBreakerLogicClosed@t0 is the control of the VCB, the box enablesthe user to open or close the VCB at a specific time. The capacitance of0.5µF and the resistance of 10MΩ is the load and the discharging resistance.The signals Eload, Etrans and Iout correspond to the voltage and currentmeasurements from the laboratory setup and the small boxes over the systemhandles the plotting of the simulation results. The measurement Ea is thevoltage across the VCB contacts.

6.1 Opening the Vacuum Circuit Breaker

In the first simulations made with the described system, a breaker openingis simulated. The breaking time is set to be 50ms, which should give abreaking angle of 180 (measured on the voltage) and should result in somereignitions of the breaker and a large amplitude of the TRV, as observed inthe laboratory measurements. In figure 6.2 the simulation is seen. As seenfrom the simulation the current does not break at 50ms as expected, insteadthe vacuum arc remains and the current is not interrupted before it crosseszero at 55ms. It was discovered that this problem arises due to the currentchopping level. The parameters used to find the current chopping level inthe model of the VCB are α = 6.2 · 10−16s and β = 14.2. These parametersshould give a chopping level that is more than enough to chop the currentas soon as the VCB contacts separate.

It was tried to change the time of contact separation, to see if the arch-ing time had any influence on the chopping level in the simulation model. Itwas discovered that if the VCB opens at current maximum or minimum thechopping level is set to zero. Therefore the time of contact separation wasset to 0.49ms, the results of running the simulations with the new openingtime of the VCB is seen in figure 6.3. As figure 6.3 shows the VCB has acurrent chopping level of 0.0005A for this current, and a oscillating TRVis created when the current is interrupted, when the oscillation is damped,the voltage between the VCB contacts follows the transformer side voltageas expected, with opposite sign due to the orientation of the measurementof Ea. The damping time of the oscillation is around 10ms, in the testswith the 10m cable the damping time was only 1ms. The reason for thisdifference is probably the lack of detail in the simulation model.

Simulations 59

52 54 56 58 60 62 64 66 68−20

−10

0


TR

V[k

V]

52 54 56 58 60 62 64 66 68−15

−10

−5

0

5x 10

−4

Time[ms]

Cu

rren

t[A

]

Figure 6.2: Simulation of opening the VCB with the constants found and de-scribed in chapter 5. The breaker opens at angle of 180, but currentis not interrupted before 270. The simulation uses the parametersfor the 10m cable.

52 54 56 58 60 62 64 66 68−20

−10

0


TR

V[k

V]

52 54 56 58 60 62 64 66 68−15

−10

−5

0

5x 10

−4

Time[ms]

Cu

rren

t[A

]

Figure 6.3: This simulation shows the TRV created by an opening operation, thecurrent is interrupted just before 270. The simulation is for thesystem using 10m cable


It was expected to see some reignitions of the vacuum arc, but this is notseen in the simulation model. The reason why no reignitions occur is thatthe TRV never exceeds the dielectric withstand of the VCB. As mentionedbefore the lack of detail in the simulation models of the different componentsresults in a TRV that is not the same as the measured TRV. Therefore itwas decided to use the pi-equivalent circuit model for the cable, to try andimprove the model. This model includes also the inductance and the resis-tance of the cable from table 4.1. The setup of the new simulation model isseen in figure 6.4.

Figure 6.4: Setup of the system representing the laboratory setup when using thepi-equivalent circuit model and the parameters for the 10m cable.

Using the pi-equivalent circuit to model the cable does not make any changesto the simulation results, the result is still the same as in figure 6.3, witha damping time of around 10ms. In figure 6.5 a graph of the TRV underthe opening process is shown together with the dielectric withstand of theVCB. The parameters used to describe the dielectric withstand is set toA = 37.88V/µs and B = 0 according to the calculations from section 5.4. Itcan be seen from figure 6.5, that the problem of the simulations is found inthe shape of the TRV created by the breaking operation. To try and force areignition of the VCB, the opening time of the VCB was moved very close tothe time of current chopping, but the amplitude of the TRV does not get avalue that is high enough to make the TRV exceed the dielectric withstandof the VCB. The difference between the tests and the simulation model isthat, as soon as the VCBs contact seperate in the real tests the current ischopped and the TRV starts rising. This causes reignitions of the vacuumarc. In the simulation the current is not chopped at the time of seperationand the dielectric withstand becomes too high for reignitions to occur.

The simulation model of the VCB is a closed model and its source codecannot be seen. Therefore it is not possible to investigate the problem ofthe current chopping level further. The results of the simulations suggestthat the VCB model always sets the current chopping level under the cur-rent level. This would explain why the current is not chopped immediately

Simulations 61

53.8 53.9 54 54.1 54.2 54.3 54.4 54.5 54.6−5

0

5

10

15

20


Time[ms]

TR

V[k

V]


Figure 6.5: The simulation shows the TRV and the dielectric withstand of theVCB. As seen the time of seperation has been moved to 54ms (270)to try and force reignitions.

after contact seperation. In order to analyse the problem, the VCB modelshould be tested at a higher current level.

Another step of improving the simulation model would be to use the PSCADcable model to describe the cable and to transform an already made Simulinkmodel of the HTT transformer to a PSCAD model in order to get a moreprecise model of the system. These opportunities were investigated, butthey were found to be too time consuming to be included in this project.The VCB model should also be tested at other current levels to see if theproblem with the chopping level is a fault in the model or a result of the lowcurrent level of the VCB.

Instead it was decided to continue using the circuit in figure 6.4 and onlyanalyse the frequency of the TRV and effect of breaking angle during theopening operation. As mentioned, figure 6.3 shows the simulated TRV inthe system using a 10m cable. In figure 6.6 the result of a simulation, usingthe parameters for the 100m cable is seen. The simulation opens the VCBcontacts after 49ms. Again it is seen that the current does not get choppedat contact seperation but a vacuum arc conducts the current and it is notextinguished before the current chopping level, set by the simulation model,is reached at 55ms. Figure 6.6 shows that the damping problem in thesimulation model becomes more significant when simulating the 100m ca-


52 54 56 58 60 62 64 66 68−20

−10

0


TR

V[k

V]

52 54 56 58 60 62 64 66 68−15

−10

−5

0

5x 10

−4

Time[ms]

Cu

rren

t[A

]

Figure 6.6: Simulation of opening the VCB in the system with 100m cable, thecurrent is interrupted at 270.

ble. The TRV created by the breaking process continues to oscillate withoutany damping, where the test result showed that the oscillation in this setupwould be completely damped after around 2.3ms. These simulation resultsagain show the need for a better simulation model of the circuit components.

6.1.1 Frequency analysis

The frequency of the performed simulations was measured in order to makea comparison of the calculated frequencies and the measured frequencies. Infigure 6.7 the simulations and data points for the frequency calculations areseen. Using this data the frequencies of the TRV for the two cable lengthswere found to

fTRV 10 =1

54.4ms− 54.25ms= 7149Hz

fTRV 100 =1

54.92ms− 54.48ms= 2272Hz

These results correspond to the data found in chapter 2 where a circuit repre-senting a transformer and a cable where disconnected from a capacitive load.The frequencies found in the measurements where fTRV 10 = 8708, 92Hz andfTRV 100 = 2953, 87Hz. The difference in simulated and measured frequencycan be seen as a result of the lack of detail in the simulation model, asalready described.

Simulations 63

54 54.5 55 55.5 56−5

−4

−3

−2

−1

0

1

2

3

4

5 X: 54.4Y: 4.223

X: 54.26Y: 4.362

Simulation of VCB opening

Time[ms]

TR

V[k

V]

(a) 10m cable

54 54.5 55 55.5 56−1.5

−1

−0.5

0

0.5

1

1.5

X: 54.48Y: 1.245

X: 54.92Y: 1.332

Simulation of VCB opening

Time[ms]

TR

V[k

V]

(b) 100m cable

Figure 6.7: The two plots shows the TRV generated in the system using 10m and100m cable. For both simulations the breaking angle is 270.

44 44.5 45 45.5 46−5

−4

−3

−2

−1

0

1

2

3

4


Time[ms]

TR

V[k

V]

(a) VCB contacts separates at 40ms

54 54.5 55 55.5 56−5

−4

−3

−2

−1

0

1

2

3

4


Time[ms]

TR

V[k

V]

(b) VCB contacts separates at 50ms

Figure 6.8: The simulation shows the TRV when the VCB is set to open at 39msand 49ms which results in breaking angles of almost 90 and almost270.

6.1.2 Analysis of breaking angle

As the current chopping level in the simulations cannot be changed, thebreaking angle can only obtain two values. The two possible breaking anglesof the voltage lay just before 90 or just before 270. In the simulationsalready made the opening time of the VCB has been set to 49ms whichgives a breaking angle of almost 270 and to obtain the breaking angle ofalmost 90 the opening time of the VCB was set to 39ms. Figure 6.8 showsthe result of the simulations performed with the two opening times. Thetwo simulations in figure 6.8, results in a TRV with the same amplitude but

64 6.2 Closing the Vacuum Circuit Breaker

52 54 56 58 60 62 64 66 68−0.015

−0.01

−0.005

0

0.005

0.01

0.015

X: 55.9Y: −0.009806

X: 58.37Y: −0.008888

Simulation of VCB closing

Cu

rren

t[A

]

Time[ms]

(a) 10m cable

42 44 46 48 50 52 54 56 58−4

−3

−2

−1

0

1

2

3x 10

−3

X: 51.23Y: −0.003063 X: 53.74

Y: −0.002395

Simulation of VCB closing

Cu

rren

t[A

]Time[ms]

(b) 100m cable

Figure 6.9: The two plots shows simulations of closing the VCB with both cablelenghts. The data markers are used to find the frequency of the oscil-lating transients. The closing angle of the VCB is 270 for plot a and180 for plot b.

with different sign. This is similar to the results obtained in the laboratorytests, where breaking angles that are 180 apart have the same amplitudebut with different sign.

6.2 Closing the Vacuum Circuit Breaker

When closing the VCB model, the slow transients, described in chapter2, the fast oscillating transients, described in section 5.6.2 and prestrikesof the vacuum arc, that were seen in section 5.6.2, can be seen from thesimulation results. In this section the frequency of the oscillating transientswill be calculated and an analyse of the prestrikes will be made. During thesimulations of the closing operation it was discovered that the prestrikes inthe VCB only occurs when the pi-equivalent model of the cable was used.This emphasises the importance of the level of detail in the simulation model.

6.2.1 Frequency of Oscillating Transient

The frequency of the oscillation transient is expected to be around 400Hz.Figure 6.9 shows a plot of the oscillating transient for both cable lenghts,the frequency of the two setups can be found using the data markers seenon figure 6.9

Simulations 65

55.2 55.3 55.4 55.5 55.6 55.7 55.8−5

0

5

10

15Simulation of VCB closing

Vo

ltag

e[kV

]

Voltage between VCB contactsBreaker withstand voltage

55.2 55.3 55.4 55.5 55.6 55.7 55.8−10

−5

0

5x 10

−3

Time[ms]

Cu

rren

t[A

]

Figure 6.10: The plot shows the voltage between the VCB contacts and the cur-rent through the VCB just before the VCB contacts meet. As theplot shows, prestrikes of the VCB occur.

fTRV 10 =1

58.372msb− 55.941ms= 411.35Hz

fTRV 100 =1

53.74ms− 51.235ms= 399.2Hz

As these results show the two frequencies are both very close the expectedfrequency of 400Hz. The calculations shows that the frequencies are notthe same for both cable lenghs, this is as mentioned earlier because thecapacitance and inductance in the system changes a bit when changing thecable. But since the load capacitance 0.5µF is the dominant capacitancein the system, the change of cable will not have a very large effect on thefrequency of the oscillations.

6.2.2 Prestrike simulation

A look at what happens very close to the time where the two VCB contactsmeet, shows that some prestrikes occur in the simulation results. These pre-strikes take the same shape as expected, and force the voltage betweeen theVCB contacts to go to zero and after the vacuum arc extinguishes. In figure6.10 simulated prestrikes from the system using a 10m cable are seen. Thesimulation shown in figure 6.10 is made with a cable length of 10m and ata closing angle of 270. When the voltage between the VCB contacts goesto zero, because of a restrike, the current starts oscillating. These oscilla-


50.6 50.7 50.8 50.9 51 51.1 51.2 51.3 51.4−0.5

0

0.5

1

1.5

2

2.5


Vo

ltag

e[kV

]

Time[ms]

(a) Closing angle 180

55 55.1 55.2 55.3 55.4 55.5 55.6 55.7 55.8−1

0

1

2

3

4

5

6

7

8


Vo

ltag

e[kV

]Time[ms]

(b) Closing angle 270e

Figure 6.11: The two plots shows simulations of closing the VCB at differenttimes, iin the system using the 10m cable. As seen the two differentclosing angles causes a different number of prestrikes.

tions are quenched very fast and the voltage between the contacts are ledsmoothly back to the previous level. This process repeats it self when moreprestrikes occur.

The voltage plot on figure 6.10 also shows how the dielectric withstandof the VCB decays and as expected the prestrikes occur when the voltagebetween the VCB contacts exceeds the dielectric withstand. The RDDS canbe calculated from the plot and is found to be A = 37.88V/µs, the samevalue as the inserted value of RRDS. The VCB contacts start moving to-gether at 55ms and are fully together at 55.55ms, this gives a closing timeof the VCB of 0.55ms. This time is not adjusteble in the model and cantherefore not be set to the value found in the laboratory tests.

In order to investigate if the number of prestrikes were dependent on theclosing angle of the VCB, a number of simulations were made. The relationbetween the number of prestrikes and the closing angle was found to be thesame as in the laboratory tests and in figure 6.11 two simulations of a closingoperation are seen. As seen from the plot in figure 6.11, a closing angle of180 on the voltage gives few prestrikes, in this case 1, and a closing angleof 270 gives more prestrikes, in this case 4. So as found in the laboratorytests, high voltage between the VCB contacts when a closing operation isstarted gives many prestrikes and a little voltage between the contacts givesfew prestrikes.

Chapter 7

Discussion

The results obtained in the project will be discussed and compared in thischapter. First a description of the measurements and calculations, made inorder to find the VCB model parameters will be given. Then a descriptionand comparison of the calculated, measured and simulated results whenopening and closing the VCB is given. Finally a description of the furtherwork needed, in order to determine the model parameters of the VCB andthe further work on the simulation model, will be given.

7.1 Voltage Circuit Breaker Model Parameters

The parameters used to model the VCB in the PSCAD simulation model,were found based on a series of tests. The result of these tests were analysedand from these analyses the parameters were calculated.

The chopping current was the first parameter of the VCB model that wastreated. The current chopping phenomena were hard to observe at highvoltage levels because of the high current transients created at these levels.At lower voltage levels the chopping effect was seen quiet clearly and an at-tempt on calculating the current chopping level of the VCB was made. Butsince the laboratory setup conducts a current which is under the currentchopping level of the breaker the value of the parameters α and β could notbe determined for this breaker. The parameters of α and β were thereforechosen to standard values, which should give a current chopping level of3A-8A in the simulation model.

An analyse of the reignitions of the vacuum arc was made in order to de-termine the dielectric withstand of the VCB. As expected the circuit con-figuration changed the number of restrikes of the vacuum arc, when usingthe 100m cable few reignitions were created and when using the 10m cable


many reignitions were created. The analyse of the reignitions showed thatthe VCB has a RRDS of 37.88V/µs. This value corresponds to the suggestedvalue range of RRDS when testing VCBs. The value of the RRDS gives amaximum dielectric withstand of the vacuum between the VCB contactsof 30.43kV/mm. In [8] the dielectric withstand of vacuum is stated to bebetween 20kVrms/mm and 30kVrms/mm. The rms value in this project is21.52kVrms/mm, therefore found value of the RRDS of the VCB seems tobe acceptable.

The HF quenching capability of the VCB was also examined and two meth-ods of determening the simulation parameters were used. The methods gavevery different and varying results. When the HF quenching capability is con-sidered to be constant its values should lay between 100A/µs and 600A/s,which indicate that the result found in this project is wrong. The differ-ence between the calculated value and the expected value, is most likely tobe caused by the current level in the system, and it is expected that testsconducting larger currents will give better results of the HF quenching ca-pability of the VCB. Due to this, the value of the HF quenching capabilitywas set to be 350A/s in the simulation model.

The prestrikes during a closing operation of the VCB were also observedand the RDDS of the VCB was calculated. The value of RDDS is calcu-lated in the same way as the value of RRDS and the RDDS of the VCB wasfound to be 147.1V/µs. However it is not possible to set the RDDS in thesimulation model, the value of the RDDS will be set to the the value of theRRDS, 37.88V/µs.

When inserting the found and decided parameters in the VCB model andinserting the model in the simulation model of the laboratory setup it wasexpected to observe the phenomena, that is described by the VCB theoryand is also seen from the test results.

7.2 Opening the Vacuum Circuit Breaker

The TRV that arises in the system, after a breaking operation, was observedin both the laboratory tests and in the simulations of the system. In chapter2 the expected frequency of the TRV was calculated based on the capacitanceof the cable and the inductance of the HTT transformer. The results of thesecalculations were fTRV 10 = 7110Hz and fTRV 100 = 2248Hz. A comparisonof the TRV found in the laboratory and the TRV found in the simulationsis seen in figure 7.1. As figure 7.1 shows the measured frequency and thesimulated frequency for the system using 10m cable are almost similar, andhave been calculated to 8708, 92Hz for the measurements and 7149Hz for

Discussion 69

54 54.5 55 55.5 56 56.5−5

0


TR

V[k

V]

3.5 4 4.5 5 5.5 6−2

−1

0

1

2

TR

V[k

V]

Measurement of VCB opening

Time[ms]

Figure 7.1: Comparison of the simulated and the measured results of at VCBopening, when the breaking angle is 270 and the 10m cable is used.

the simulations. When using the 100m cable the frequency of the TRV havebeen calculated to 2953, 87Hz for the measurements and 2272Hz for thesimulations. As the results shows the simulated value is almost equal tothe value calculated, whereas the measured result is a bit different. Thisdifference between the measured result and the simulated/calculated resultcomes from the lack of detail in the simulation/calculation model. In orderto improve this a more detailed model of both transformer and cable, shouldbe used in further work with the simulation model.

As figure 7.1 shows the simulation does not apply the right damping tothe system, the measured damping time of the TRV, using the 10m cable,is 1ms whereas the simulation gives a damping time of 10ms. This is alsocaused by the lack of detail in the simulation model. The damping problemof the simulation model gets worse when switching from the 10m cable tothe 100m cable, where almost no damping is applied to the TRV.

When simulating a opening operation of the VCB it was expected to ob-serve some reignitions of the vacuum arc, but no reignitions were seen. Itwas found that these problems were caused by the current chopping levelof the VCB. In the laboratory tests the current is chopped immediately af-ter the contact seperation, since the current level in the tests is under thecurrent chopping level of the VCB. This was also expected in the simula-tion, but in the simulation the current chopping level is set to be 0.0005A


with the used values of α and β, where a current chopping level of 3A-8Ais expected. Since it is not possible to see the source code of the simulationmodel a investigation of the problem could not be made. But the resultssuggest that the current chopping level in the VCB model is always set tobe under the current level.

As explained in the report the rate of rise of the TRV is a very importantparameter in the generation of reignitions of the vacuum arc. Therefore itis important that the simulation model gives a correct TRV according tothe laboratory measurement, when studying reignitions. When the prob-lems with the current chopping level is solved the simulation model mightbe good enough to make a study of reignitions of the vacuum arc. But inorder to get the best results, the problems with the simulation of the TRVshould be solved, this is done by using more accurate models of the trans-foremr and the cable. PSCAD has a cable model that could be applied inthe simulation setup, a detailed model of the HTT transformer is alreadymade in Simulink, this model should be transformed to a PSCAD modeland used in the simulation.

7.3 Closing the Vacuum Circuit Breaker

The measurements taken during a VCB closing operation show that the ex-ponential transient term is affecting the current through the VCB in theway described in section 2.1. The result of two tests is shown in figure 5.14,one where the VCB closes at 270 and one where the VCB closes at 0.When the VCB closes at a angle of 90 or 270 the exponential transientwill have maximum effect and when the VCB closes at angles of 0 or 180

the exponential transient will have minimum effect.

When closing the VCB some fast oscillations occure due to the capacitancesand the inductances of the system. The frequency of these oscillations isfound in the same way as the frequency of the TRV. But when the circuitis closing the dominant capacitance of the system becomes the load andthe frequency of the oscillations have been calculated considering the loadcapaticance to be the only capacitance of the system. The frequency of theoscillations have been calculated to 399.14Hz for both cable lenghts. Whenexamining the system using the 10m cable a frequency of 390.63Hz is foundfrom the measurements and a frequency of 411.35Hz is found in the simu-lations. And when using the 100m cable a frequency of 311.53Hz is foundfrom the measurements and a frequency of 399.2Hz is found in the simula-tions. As seen the measured frequencies and the simulated frequencies arevery similar, only the measurement using the 100m cable is a bit different.The reason for this is that the capacitance and inductance of the 100m cable

Discussion 71

55 55.2 55.4 55.6 55.8 56−5

0

5


Vo

ltag

e[kV

]

5.2 5.25 5.3 5.35 5.4−5

0

5

10

Vo

ltag

e[kV

]

Measurement of VCB closing

Time[ms]

Figure 7.2: Comparison of prestrikes in simulation and measurement, both plots,shows the voltage between the VCB contacts. The system uses the10m cable and have a closing angle of 270.

affects the system and the load capacitance is not as dominating as whenthe 10m cable is used.

When simulating a VCB closing operation it was possible to observe theprestrikes that occur in the VCB. When a prestrike occurs in the simula-tion it forces the voltage between the contacts to jump to zero and causes aoscillating HF current. Figure 7.2 shows a plot of the voltage between theVCB contacts from simulation and from the measurement, during a closingoperation of the VCB. Figure 7.2 shows a clear connection between the mea-sured prestrikes and the simulated pretrikes. In both cases the voltage dropsto zero when a restrike occurs, the created vacuum arc extinguishes almostimmediately and the voltage between the contacts is smootly led back to itsprevious level.

The opening time of the VCB and the RDDS are not variable parameters inthe VCB model, this means that the simulation of restrikes is not as preciseas desired. The simulation has a fixed closing time of 0.55ms where the ac-tual closing of the VCB was measured to be 12ms. The RDDS of the VCBmodel is set to have the same value as the RRDS, and in this case 37.88V/µswhere the RDDS measured on the VCB was found to be 147.1V/µs. Thismeans that the voltage in the simulations, does not reach the right levelwhen recovering after a prestrike, since the slope of the dielectric withstand

72 7.4 Further Work

is too low.

The simulation of the prestrikes showed the same dependency of closingangle as found in the measurements. Closing angles at high voltages givesmany prestrikes and closing angles at low voltages gives few prestrikes. Infigure 6.11 this is shown by simulating a closing angle of 180 (voltage zerocrossing) and 270 (voltage maximum) these simulations give 1 and 4 pre-strikes respectively. This also shows that 4 is the maximum number of pre-strikes in the simulation, the measured results showed a maximum numberof 6 prestrikes (at angle 270). The difference in the maximum number ofprestrikes for the measurements and for the simulations can be seen in figure7.2. This difference comes from the difference in closing time and the dif-ferene in RDDS between the real VCB and the simulation model of the VCB.

The prestrikes of the VCB can only be simulated in the simulation modelthat is using the pi-equivalent model of the cable. This fact shows the im-portance of a precise simulation model and it suggestes to improve the modelof the transformer and the cable as already described.

The fact that it is possible to simulate the prestrikes in the VCB model,support the theory that the current chopping level is the reason why therestrikes of the VCB cannot be simulated.

7.4 Further Work

The results of this project shows the need of some further work, both on thelaboratory setup and on the simulation model. In order to determine thecurrent chopping level and the HF quenching capability of the VCB, sometest at a higher current level than used in this project are needed. The idealscenario would be tests conducting the nominal current of the VCB 1250A.If this is not possible, tests with a current of at least 8A − 10A (expectedcurrent chopping level) should be applied. Tests done with these currentsshould make it possible to find the values of α and β and with the rightvalues of these parameters the simulation model should be able to producerestrikes, HF currents etc. From tests with higher currents it is also ex-pected that a more precise value of the quenching capability of the VCB canbe found.

The simulation model also requires some further work in order to fully rep-resent the laboratory setup. In this project the laboratory setup has beenrepresented by lumped circuit element, in order to improve the model thisshould be improved. The HTT transformer has already been modelled inSimulink in a previous project, so in order to improve the model used in this

Discussion 73

project the Simulink model could be transformed to a PSCAD model andused. In order to improve the representation of the cable, a PSCAD cablemodel should be used.

The above improvements should make it possible to fully test the VCBmodel. These tests will determine if the model is fulfilling its purpose orif changes in the model are required. Changes to the VCB model could beintroducing the RDDS, the opening time and the closing time as variableparameters.

74 7.4 Further Work

Chapter 8

Conclusion

This project has investigated the generation of high voltage transients froma vacuum circuit breaker (VCB). The investigation has concerned with the-oretical research, expermential tests and simulations studies.

The different physical phenomena of a VCB were investigated, the cur-rent chopping level, the dielectric strength and the HF quenching capability.These phenomena are all described using a mathematical model and themodel includes parameters which determine the behaviour of the specificVCB. Methods for finding the parameters from test results wer determinedand a series of tests were performed on the VCB.

The result of the tests showed that a transient recovery voltage (TRV) iscreated when the VCB is opened. The frequency of the TRV was found tobe dependent of the network setup and when using a 100m cable the fre-quency was found to be 2953, 87Hz where the 10m cable gave a frequencyof 8708, 92Hz. It was shown that the amplitude of the TRV is dependentof the VCB breaking angle. The rate of rise and the amplitude of the TRVwere found to have a big influence on the behaviour of the VCB duringcurrent interruption. The two parameters play a big role in the creationof restrikes in the VCB, the reignitions in the VCB are created wheneverthe dielectric strength is exceeded by the TRV. The rate of rise of dielectricstrength (RRDS) during an opening operation was found to be 37.88V/µs.The parameters for determening the current chopping level could not befound, as the current in the test setup was under the current chopping levelof the VCB. An attempt of finding the HF quenching capability of the VCBwas made, but the result was very varying, which is also expected to bebecause of the low currents level in the tests. The parameters of the currentchopping and the HF quenching capability were therefore set to standartvalues in the simulations.

76

Measurements were also taken during closing operations of the VCB, thesemeasurements showed an exponential current transient and a faster oscil-lating transient, as expected. The frequency of the fast oscillating transientwas expected to be 399.14Hz for both cable lenghts and was found to be390.63Hz when the 10m cable was used and 311.53Hz when 100m cable isused. On the measurement of the closing operation, prestrikes of the vacuumarc was observed and the RDDS of the VCB was calculated to be 147.1V/µs.

The parameters of the VCB model were inserted in a PSCAD model ofa VCB. The model of the VCB is used in a simulation model that repre-sents the complete laboratory setup. The simulation results in a TRV witha frequency of 7149Hz for the system with 10m cable and 2272Hz for thesystem using 100m cable. During the simulations it was discovered that thelevel of the current chopping is always set to be smaller than the current insystem, this meant that the simulations could not reproduce the restrikes ofthe vacuum arc seen in the VCB tests. The simulation of a closing action ofthe VCB, shows the exponential transient, the oscillating transients and theprestrikes of the VCB. The RDDS of the VCB found from the tests resultscannot be inserted in the PSCAD model of the VCB, the PSCAD modeluses the value of the RRDS as the value of the RDDS.

In order determine the parameters for modelling the current chopping leveland the HF quenching capability, new tests at higher current level shouldbe made. In order to get more precise simulation results an improvement ofthe simulation model is also required.

Bibliography

[1] Tarik Abdulahovic. Analysis of high-frequency electrical transients inoffshore wind parks. Master’s thesis, Chalmers University of Technol-ogy, Department of Energy and Environment Division of Electric PowerEngineering, 2009.

[2] D.J.Clare. Failures of encapsulated transformers for converter windersat oryx mine. Electron Magazine, March 1991.

[3] J. Duncan Glover and Mulukutla S. Sarma. Power System Analysisand Design. The Wadsworth Group, 3th edition, 2002.

[4] Allan Greenwood. Electrical Transients in Power Systems. John Wileyand Sons, inc, 2th edition, 1991.

[5] Allan Greenwood. Vacuum Switchgear. The institution of ElectricalEngineers, 2th edition, 1997.

[6] J. Helmer and M. Lindmayer. Mathematical modeling of the highfiequency behavior of vacuum interrupters and comparison with mea-sured transients in power systems. XVIIth International Symposium onDischarges and Electrical Insulation in Vacuum,, July. 1996.

[7] Rao Kondala and Gajjar Gopal. Development and application of vac-uum circuit breaker model in electromagnetic transient simulation.IEEE Power India Confrence, 2006.

[8] F. H. Kreuger. Industrial High Voltage: Fields/Dielectrics/Construc-tions. Ios Pr Inc, 3th edition, 1991.

[9] Morten Lerche. Optimization of laboratory setup for determination ofbreaker characteristics. Msc prepatory project, Technical University ofDenmark, 2008.

78 BIBLIOGRAPHY

[10] A. Mazur, I. Kerszenbaum, and J. Frank. Maximum insulation stressesunder transient voltages in the hv barrel-type winding of distributionand power transformers. IEEE Transactions on Industry Applications,1988.

[11] Orn I. Bjorgvinsson. Theoretic and experimental investigations ofswitching transients in wind turbines. Msc project, Technical Universityof Denmark, 2006.

[12] Siemens. 3ah vacuum circuit-breakers. Medium-Voltage Equipment,Catalog HG 11.11, 1999.

[13] Rene Peter Paul Smeets. ow-current behaviour and current choppingof vacuum arcs. Proefschrift, Technische Universiteit Eindhoven, 1987.

[14] W. Sweet. Danish wind turbines take unfortunate turn. Spectrum,IEEE, November 2004.

[15] Lou van der Sluis. Transients in Power Systems. John Wiley and Sons,inc, 1th edition, 2002.

[16] S.M. Wong, L.A. Snider, and E.W.C. Lo. Overvoltages and reigni-tion behaviour of vacuum circuit breaker. International Conference onPower System Transients, 2003.

List of Figures

2.1 An sinusoidal voltage is switched on an RL-circuit. . . . . . . 6

2.2 The sinusoidal voltage is switched on to the RL-circuit witha switching angle of 90. . . . . . . . . . . . . . . . . . . . . . 8

2.3 The sinusoidal voltage is switched on to the RL-circuit witha switching angle of 0. . . . . . . . . . . . . . . . . . . . . . 9

2.4 An sinusoidal voltage is switched on an RL-circuit . . . . . . 10

2.5 The figure shows the voltage on the transformer side of a VCBunder a opening operation in a circuit with a capacitive load. 11

3.1 The design principle of a VCB, showing contacts, archingchamber and insulation, the picture is taken from [12] page 8. 15

3.2 A vacuum interrupter with slits in the contacts to avoid un-even erosion of the contact surface, this picture is from [15]page 66. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3 The figure shows the current during an opening of the breaker.As seen on figure b the current chops around the value 0,005and jumps to zero. . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4 The figure shows 5 reignitions of the vacuum arc during con-tact seperation. When the reignitions occur the TRV jumpsto zero. The red line shows the RDDS of the circuit breaker. 19

3.5 The figure shows 3 reignitions of the vacuum arc during con-tact separation. The figure also shows the high frequencycurrents caused by the arc. . . . . . . . . . . . . . . . . . . . 20

3.6 The figure shows the HF currents caused by 5 reignitions.The last current cannot be quenched at a zero crossing andtherefore the arc is maintained until the next zero crossing ofthe current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

80 LIST OF FIGURES

3.7 Multible reignitions lead to unsuccessful interruption of thecurrent at first current zero. . . . . . . . . . . . . . . . . . . . 22

3.8 The figure shows 4 reignitions of the vacuum arc during con-tact separation. The figure also shows the high frequencycurrents caused by the arc. . . . . . . . . . . . . . . . . . . . 24

4.1 The laboratory setup including high voltage components andthe control and measurement system . . . . . . . . . . . . . . 26

4.2 Screenshot of the LabVIEW program. . . . . . . . . . . . . . 29

4.3 The two pictures show how the probes are fastened to thesetup. Before this was done the probes were connected looselyto the setup by the hooks on the tip of the probes. . . . . . . 31

4.4 The plots show the voltage measured on the transformer sideprobe, before and after fastening the probe. At around 30msoscillations can be seen on figure a. . . . . . . . . . . . . . . . 31

4.5 The picture shows the Rogowski current transducer. In orderto improve the current-to-noise ratio the current measurementis led through the Rogowski coil 4 times as seen on the picture 32

5.1 Closing the VCB at voltage level 6.9kV , the setup is usingthe 100m cable and the load with a capacitance of 0.5µF .The time between the measurements ∆t is 1 · 10−6s, and theclosing angle is 0 . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.2 The figure shows two plots of the distance between the VCBcontacts, when the VCB is opening and closing. . . . . . . . . 36

5.3 The TRV across the breaker contacts using a 10m and a 100mcable. The breaking angle is in both cases 0. . . . . . . . . . 38

5.4 The measurements for calculating the frequency of the TRVwhen using a 10m cable in the test. The breaking angle isagain 0. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.5 The plots shows the current through the VCB at 5, 75kVusing a 100m cable. The interruption is made at a breakingangle of 180. . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.6 The plots shows the current through the VCB at 1, 15kVusing a 100m cable. The interruption is made at a breakingangle of 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.7 The plots shows the voltage across the breaker contacts duringan opening of the VCB. Both tests have a breaking angle of225. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.8 The figure illustrates how the RRDS is calculated from thelaboratory measurements. . . . . . . . . . . . . . . . . . . . . 45

LIST OF FIGURES 81

5.9 The plot shows how the reignitions of the VCB create a HFcurrent that is superimposed on the power frequency current.The test is from the system with 100m cable and the breakingangle is 292.5. . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.10 The plot shows how the reignitions of the VCB create a HFcurrent that is superimposed on the power frequency current.The test is from the system with 10m cable and the breakingangle is 225. . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.11 The plots shows the data markers used to calculate HF cur-rent quenching capacity of the VCB. . . . . . . . . . . . . . . 50

5.12 The plot shows the voltage across the breaker channels andthe current through the breaker when during 3 prestrikes.The test is made with 100m cable and at a closing angle of22.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.13 The plot shows the voltage across the breaker channels andthe current through the breaker when during 4 prestrikes.The test is made with 10m cable and at a breaking angle of202.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.14 The plots shows the current through the breaker when closingthe circuit at 0 and at 270, both measurements are madewith 10m cable. . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.15 The plots shows the current through the breaker when closingthe circuit at 0 and at 290. Data measurements are set inorder to calculate the frequency of the oscillating transient. . 55

6.1 Setup of the system representing the laboratory setup whenusing the 10m cable. . . . . . . . . . . . . . . . . . . . . . . . 57

6.2 Simulation of opening the VCB with the constants found anddescribed in chapter 5. The breaker opens at angle of 180,but current is not interrupted before 270. The simulationuses the parameters for the 10m cable. . . . . . . . . . . . . . 59

6.3 This simulation shows the TRV created by an opening oper-ation, the current is interrupted just before 270. The simu-lation is for the system using 10m cable . . . . . . . . . . . . 59

6.4 Setup of the system representing the laboratory setup whenusing the pi-equivalent circuit model and the parameters forthe 10m cable. . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.5 The simulation shows the TRV and the dielectric withstandof the VCB. As seen the time of seperation has been movedto 54ms (270) to try and force reignitions. . . . . . . . . . . 61

82 LIST OF FIGURES

6.6 Simulation of opening the VCB in the system with 100m ca-ble, the current is interrupted at 270. . . . . . . . . . . . . . 62

6.7 The two plots shows the TRV generated in the system using10m and 100m cable. For both simulations the breaking angleis 270. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.8 The simulation shows the TRV when the VCB is set to openat 39ms and 49ms which results in breaking angles of almost90 and almost 270. . . . . . . . . . . . . . . . . . . . . . . . 63

6.9 The two plots shows simulations of closing the VCB withboth cable lenghts. The data markers are used to find thefrequency of the oscillating transients. The closing angle ofthe VCB is 270 for plot a and 180 for plot b. . . . . . . . . 64

6.10 The plot shows the voltage between the VCB contacts andthe current through the VCB just before the VCB contactsmeet. As the plot shows, prestrikes of the VCB occur. . . . . 65

6.11 The two plots shows simulations of closing the VCB at dif-ferent times, iin the system using the 10m cable. As seenthe two different closing angles causes a different number ofprestrikes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.1 Comparison of the simulated and the measured results of atVCB opening, when the breaking angle is 270 and the 10mcable is used. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

7.2 Comparison of prestrikes in simulation and measurement, bothplots, shows the voltage between the VCB contacts. The sys-tem uses the 10m cable and have a closing angle of 270. . . . 71

List of Tables

4.1 Cable parameters calculated at different frequencies [11] . . . 27

5.1 The table shows the relation between the breaking angle andthe amplitude of the TRV. The results are from the tests madeat 5.75kV using the 10m cable. . . . . . . . . . . . . . . . . . 40

5.2 The table shows the average RRDS, for the 6 analysed testseries and the average value of the RRDS found for the twocable lengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.3 The table shows the relation between the breaking angle andthe number of reignitions of the vacuum arc. The results arefrom the tests made at 5.75kV using the 10m cable. . . . . . 47

5.4 The average results of the calculations of C and D . . . . . . 50

5.5 The average results of D when considering di/dt to be con-stant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.6 The table shows the relation between the opening angle andthe number of prestrikes of the vacuum arc. The results arefrom the tests made at 5.75kV using the 10m cable. . . . . . 53

B.1 Results of the long TRV. The calculations have been madefor the test where the 10m cable is used and at a voltage levelof 5,75kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

B.2 Results of the long TRV. The calculations have been madefor the test where the 100m cable is used and at a voltagelevel of 5,75kV . . . . . . . . . . . . . . . . . . . . . . . . . . 90

C.1 Measurements and results of the current chopping analysis.The data is based on measurements with a load of 0.5µF ,100m cable and a voltage level of 1.15kV . . . . . . . . . . . 92

84 LIST OF TABLES

C.2 Measurements and results of the current chopping analysis.The data is based on measurements with a load of 1.0µF ,100m cable and a voltage level of 1.15kV . . . . . . . . . . . 92

D.1 The table shows the calculation of the RRDS for the systemusing with 100m cable and have a voltage level of 4.6kV . . . 94



D.4 The table shows the calculation of the RRDS for the systemusing with 10m cable and have a voltage level of 4.6kV . . . . 95



E.1 The results of the parameters C and D when expressing theHF current quenching capability as a linear function. Alltests are made at voltage level 5.75kV in the system usingthe 100m cable. . . . . . . . . . . . . . . . . . . . . . . . . . . 98

E.2 The results of D when considering the HF current quenchingcapability to be constant. Again all tests are made with 100mcable and at voltage level 5.75kV . . . . . . . . . . . . . . . . 98

F.1 The results of the Rate of decay of Dielectric Strength, thecalculations are made on test results from the system usingthe 10m cable and voltage level 5.75kV . . . . . . . . . . . . 100

Appendix A

Plotting results

The measurements in the LabVIEW program are stored in a .lvm file, thisfile concists of six columns. The data stored in each column is

• Column 1, contains the measurement number.

• Column 2, the measurement from channel 4 on the oscilloscope (breakerposition).

• Column 3, the measurement from channel 1 on the oscilloscope (volt-age, transformer side).

• Column 4, row 1 the time of the trigging moment.

• Column 5, the measurement from channel 2 on the oscilloscope (volt-age, load side).

• Column 6, the measurement from channel 3 on the oscilloscope (cur-rent).

MATLAB is used to take the data out of the columns, process it and plot itin the desired way. Later in this appendix an example of how the results areplotted is shown, this file is also found on the CD. The concept of the MAT-LAB file is that it loads a directory containing a number of measurementresults files, .lvm files. The .m file then processes all the measurement filesand saves a picture of the plotted results. The .m file is modified in orderto get the desired plot, these modifications can be a zoom of the x − axis,plotting the difference between the load side- and transformer side voltagein order to plot the TRV, etc.

In order to use the matlab file Plot res.m to plot the measured resultsit is important that the file is placed in the right directory. The file must bein the same directory as the folders containing the .lvm files. The file plot.m

86

has two parameters that the user must change to get the wanted plots. Thefirst one is step time, delta t, this number has to be set in order to get theright time on the x-axis. Delta t is calculated by the labview program whenthe measurements are taken and can also be seen in the filename of the .lvmfile. The second parameter that can be changed by the user is the directoryname. The name must be the name of the directory containing the .lvmfiles with the data for which a plot is wanted.

If you have any problem plotting the results or have questions on how tomodify the Matlab file you can send me an email and i’ll try and help you.The code of the matlab file Plot res.m is seen here

1 clc ;2 clear a l l ;3

4

5 %Values that has to be s e t by the user6 %−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−7 d e l t a t=2e−7; %The step time Delta t8 cd ChoopingCurrent ; % Name o f the d i r e c t o r y with the ...

r e s u l t s9 %−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

10

11 %Loads a l l the measurements f i l e s from the d i r e c t o r y12 d=dir ( ’./*.lvm’ ) ;13

14

15

16 %Generates a for−loop that run through a l l the f i l e s17 for k=1: length (d) ;18 fname=d( k ) . name ;19

20 %Loads the measurement r e s u l t s to a matrix21 x = csvread ( fname , 1 , 0) ;22

23 %Loads the vo l tage from the p o s s i t i o n meter24 vo l t age pos=x ( : , 2 ) ;25 %Converts the vo l tage to the d i s t ance between the breaker ...

c ontac t s26 d i s t =(9−vo l t age pos ) ∗9/9 ;27

28 %Loads the vo l tage o f the load s i d e o f the breaker29 v o l t a g e l o a d s i d e=x ( : , 5 ) ;30

31 %Loads the vo l tage o f the t rans fo rmer s i d e o f the breaker32 v o l t a g e t r a n s s i d e=x ( : , 3 ) ;33

34 %Loads the trough the breaker35 cur rent=x ( : , 6 ) ;

Plotting results 87

36

37 %Loads and s e t s the time acord ing to the step time38 time=d e l t a t ∗x ( : , 1 ) ∗10ˆ3 ;39

40

41 %Opens a new f i g u r e42 F=f igure ;43 %Sets the f i l ename o f the p l o t to the name o f the measurement ...

f i l e f o l l owed44 %by Result45 f i l ename =[fname ( 1 : length ( fname )−4) ’Breakerpos’ ] ;46 T i t l e=fname ;47 %Plots the d i s t ance between the breaker contac t s as a func t i on ...

o f time48 plot ( time , d i s t ) ;49 %Sets the x−a x i s to the l ength o f the time vec to r50 xlim ( [ 0 time (end) ] )51 %Adding a t i t l e to the f i g u r e52 t i t l e ( T i t l e , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;53 %Adds l a b e l s to the a x i s54 xlabel ( ’Time[ms]’ , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;55 ylabel ( ’Distance between VCB contacts[mm]’ , ’FontWeight’ , ’bold’ , ...

’Fontsize’ , 16) ;56 %Saves the p l o t as a png f i l e under the f i l e name57 print (F , ’-dpng’ , f i l ename ) ;58 saveas ( gcf , [ fname ( 1 : length ( fname )−4) ’Breakerpos.fig’ ] )59

60

61 %New f i g u r e f o r p l o t t i n g the t rans fo rmer s i d e vo l tage62 F=f igure ;63 %Plots the load s i d e vo l tage as a func t i on o f time64 plot ( time , v o l t a g e t r a n s s i d e ) ;65 %Sets the f i l ename o f the p l o t to the name o f the measurement ...

f i l e f o l l owed66 %by Result67 f i l ename =[fname ( 1 : length ( fname )−4) ’Voltagetransside’ ] ;68 T i t l e=fname ;69 %Sets the x−a x i s to the l ength o f the time vec to r70 xlim ( [ 0 time (end) ] )71 %Adding a t i t l e to the f i g u r e72 t i t l e ( T i t l e , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;73 %Adds l a b e l s to the a x i s74 xlabel ( ’Time[ms]’ , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;75 ylabel ( ’Trans. side voltage[V]’ , ’FontWeight’ , ’bold’ , ’Fontsize’...

, 16) ;76 %Saves the p l o t as a png f i l e under the f i l e name77 print (F , ’-dpng’ , f i l ename ) ;78 saveas ( gcf , [ fname ( 1 : length ( fname )−4) ’Voltagetransside.fig’ ] )79

80 %New f i g u r e f o r p l o t t i n g the load s i d e vo l tage81 F=f igure ;82 %Plots the load s i d e vo l tage as a func t i on o f time83 plot ( time , v o l t a g e l o a d s i d e ) ;

88

84 %Sets the f i l ename o f the p l o t to the name o f the measurement ...f i l e f o l l owed

85 %by Result86 f i l ename =[fname ( 1 : length ( fname )−4) ’Voltageloadside’ ] ;87 T i t l e=fname ;88 %Sets the x−a x i s to the l ength o f the time vec to r89 xlim ( [ 0 time (end) ] )90 %Adding a t i t l e to the f i g u r e91 t i t l e ( T i t l e , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;92 %Adds l a b e l s to the a x i s93 xlabel ( ’Time[ms]’ , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;94 ylabel ( ’Load side voltage[V]’ , ’FontWeight’ , ’bold’ , ’Fontsize’...

, 16) ;95 %Saves the p l o t as a png f i l e under the f i l e name96 print (F , ’-dpng’ , f i l ename ) ;97 saveas ( gcf , [ fname ( 1 : length ( fname )−4) ’Voltageloadside.fig’ ] )98

99 %New f i g u r e f o r p l o t t i n g the load s i d e vo l tage100 F=f igure ;101 %Plots the load s i d e vo l tage as a func t i on o f time102 plot ( time , cur r ent ) ;103 %Sets the f i l ename o f the p l o t to the name o f the measurement ...

f i l e f o l l owed104 %by Result105 f i l ename =[fname ( 1 : length ( fname )−4) ’current’ ] ;106 T i t l e=fname ;107 %Sets the x−a x i s to the l ength o f the time vecto r108 xlim ( [ 0 time (end) ] )109 %Adding a t i t l e to the f i g u r e110 t i t l e ( T i t l e , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;111 %Adds l a b e l s to the a x i s112 xlabel ( ’Time[ms]’ , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;113 ylabel ( ’Current[A]’ , ’FontWeight’ , ’bold’ , ’Fontsize’ , 16) ;114 %Saves the p l o t as a png f i l e under the f i l e name115 print (F , ’-dpng’ , f i l ename ) ;116 saveas ( gcf , [ fname ( 1 : length ( fname )−4) ’Current.fig’ ] )117 end118

119

120

121 %Returns to the top d i r e c t o r y with the measurement f o l d e r s122 cd . .

Appendix B

Results of the TRVCalculations

The pictures contaning the data points used for the calculations is foundon the cd. The PDF file named Full Appendix.pdf contains the picturesshowing the data and the results of the calculation. The appendix seen inthe printed version only shows tabels contaning the calculation results. Toget the full appendix, just send me an email and i will send the file.

90

Test nr. Breaking angle[] Amplitude[V ] Frequency[Hz] Time to damp[ms]1 180 5000 8739.824137 0.98762 180 5250 8827.700477 1.15863 45 -4875 8502.21631 1.06524 22.5 -5375 8753.602989 1.07425 22.5 -5500 8750.663754 0.94266 0 -5125 8729.068075 1.1197 225 4250 8717.853469 1.08848 202.5 4500 8941.000094 1.02789 90 0 – –10 247.5 1625 8473.056716 0.993211 247.5 1625 8587.166059 1.011412 0 -5500 8775.923838 1.19

Average – – 8708.915993 0.9700

Table B.1: Results of the long TRV. The calculations have been made for the testwhere the 10m cable is used and at a voltage level of 5,75kV

Test nr. Breaking angle[] Amplitude[V ] Frequency[Hz] Time to damp[ms]1 22.5 -4218.75 2914.948534 2.49642 0 -4500 2978.72869 2.59163 67.5 -3000 2948.610075 2.75944 202.5 3843.75 2881.742751 2.64225 315 -1875 2951.113172 1.9026 135 3468.75 2866.342232 2.4227 180 4500 3047.715332 2.4728 225 3468.75 2945.719274 2.4939 90 468.8 3053.748933 –10 337.5 -3937.5 2950.057835 2.4624

Average – – 2953.872683 2.30162

Table B.2: Results of the long TRV. The calculations have been made for the testwhere the 100m cable is used and at a voltage level of 5,75kV

Appendix C

Results of Current ChoppingCalculations

The pictures contaning the data points used for the calculations is foundon the cd. The PDF file named Full Appendix.pdf contains the picturesshowing the data and the results of the calculation. The appendix seen inthe printed version only shows tables contaning the calculation results.

92

Test nr. i Ich α(when β = 14, 3)

1 0,28905 0,1797 6.3249·106

2 0,3086 0,1484 75.517·106

3 0,2734 0,2266 306.03·106

4 0,28905 0,1875 3.59·106

5 0,2969 0,1641 20.60·106

6 0,2818 0,1016 12.76·109

Table C.1: Measurements and results of the current chopping analysis. The datais based on measurements with a load of 0.5µF , 100m cable and avoltage level of 1.15kV

Test nr. i Ich α(when β = 14, 3)

1 0,54295 0,3828 144.292 0,5547 0,4688 9.543 0,53905 0,1953 1120844.084 0,5547 0,4141 49.655 0,5742 0,2734 11995.846 0,5625 0,4062 63.26s

Table C.2: Measurements and results of the current chopping analysis. The datais based on measurements with a load of 1.0µF , 100m cable and avoltage level of 1.15kV

Appendix D

Results of the Rate of Rise ofDielectric Strength Calculations


94

Test nr. Breaking angle[] Number of reignitions A [V/µ S]1 0 2 26.462 225 2 20.833 247.5 2 21.944 337.5 3 19.175 22.5 3 23.856 202.5 3 19.447 22.5 2 12.238 315 0 –9 0 3 10.46410 45 2 9.9711 0 2 32.7112 292.5 0 –13 315 2 6.58

Average A – – 18.52

Table D.1: The table shows the calculation of the RRDS for the system usingwith 100m cable and have a voltage level of 4.6kV .

Test nr. Breaking angle[] Number of reignitions A [V/µ S]1 0 3 21.832 0 2 –3 67.5 2 16.544 225 3 24.375 315 2 24.726 135 2 9.627 0 2 34.108 202.5 3 24.069 90 0 –10 337.5 2 20.27



Results of the Rate of Rise of Dielectric Strength Calculations 95

Test nr. Breaking angle[] Number of reignitions A [V/µ S]1 135 5 28.822 180 6 25.663 247.5 2 10.204 315 3 22.435 67.5 3 24.136 22.5 4 31.737 157.5 4 20.688 0 4 21.459 0 5 26.6010 0 4 27.2311 45 3 31.64



Test nr. Breaking angle[] Number of reignitions A [V/µ S]1 22.5 15 44.292 292.5 8 47.943 225 8 29.714 90 4 23.545 90 0 –6 225 6 37.907 22.5 11 54.278 135 5 34.789 67.5 6 51.7810 225 9 58.20



96

Test nr. Breaking angle[] Number of reignitions A [V/µ S]1 180 15 31.002 180 13 42.823 45 15 35.164 22.5 10 38.905 22.5 18 34.686 0 20 42.697 225 10 39.858 202.5 15 40.359 90 0 –10 270 1 –11 270 1 –12 0 17 49.02



Test nr. Breaking angle[] Number of reignitions A [V/µ S]1 247.5 8 36.882 45 15 34.613 112.5 12 43.454 202.5 17 42.915 0 23 39.106 247.5 7 37.367 202.5 23 35.918 0 19 39.199 315 17 34.8210 45 19 44.9311 0 5 24.1712 225 5 18.57



Appendix E

Results of HF CurrentQuenching Capability

Calculations


98

Arc numberArc 1 Arc 1 Arc 2 Arc 2 Arc 3 Arc 3

Test nr. C[ Aµs2

] D[ Aµs ] C[ Aµs2

] D[ Aµs ] C[ Aµs2

] D [ Aµs ]

1 -0.294 9.375 -0.668 33.2 -0.705 62.52 -0.489 31.25 -1.436 50.78 -3.272 103.5163 -0.52524 27.34375 – – – –4 -0.48529 36.45 – – – –5 -1.684 29.296 -2.182 52.734 – –6 -0.316 5.580 -1.774 85.937 – –7 -0.705 21.484 -0.673 14.5 1.898 51.5638 -0.429 15.626 – – – –9 -0.437 17.188 – 51.563 -2.174 12510 -0.114 9.766 -0.588 19.531 -1.512 35.93811 -0.87 117.188 -0.609 85.938 – –12 -0.74513 85.938 -1.597 42.969 – –

Average -0.59125 25.67046 -1.19084 48.57241 -1.9121 75.70312

Table E.1: The results of the parameters C and D when expressing the HF currentquenching capability as a linear function. All tests are made at voltagelevel 5.75kV in the system using the 100m cable.

Arc numberArc 1 Arc 2 Arc 3

Test nr. D[ Aµs ] D[ Aµs ] D[ Aµs ]

1 7.812 28.125 60.9372 19.531 15.625 107.1423 11.718 – –4 11.718 – –5 24.553 33.854 –6 6.696 29.017 –7 12.695 26.041 30.2738 11.718 29.5139 12.695 93.75 47.99110 6.696 12.276 32.36611 21.875 45.312 –12 13.021 26.785

Average 13.255 34.03 55.742

Table E.2: The results of D when considering the HF current quenching capabilityto be constant. Again all tests are made with 100m cable and atvoltage level 5.75kV

Appendix F

Results of Rate of Decay ofDielectric Srength


100

Test nr. Breaking angle[] Number of Prestrikes A [V/µ S]1 112.5 3 125.972 90 4 149.233 112.5 4 136.354 180 2 240.385 202.5 4 103.866 315 2 174.547 90 6 147.378 67.5 3 130.799 0 0 –10 270 6 115.37


Table F.1: The results of the Rate of decay of Dielectric Strength, the calculationsare made on test results from the system using the 10m cable andvoltage level 5.75kV

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Circuit Breaker Characteristics in Medium Voltage Equipment