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Cahiertechnique

no. 181Directional protection equipment

P. Bertrand

Technical collection

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Cahiers Techniques are a collection of documents intended for engineersand technicians people in the industry who are looking for information ingreater depth in order to complement that given in display productcatalogues.

These Cahiers Techniques go beyond this stage and constitute praticaltraining tools.They contain data allowing to design and implement electrical equipement,industrial electronics and electrical transmission and distribution.Each Cahier Technique provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

The latest publications can be downloaded on Internet from theSchneider server.code: http://www.schneiderelectric.comsection: mastering electrical power

Please contact your Schneider representative if you want either a CahierTechnique or the list of available titles.

The Cahiers Techniques collection is part of the Groupe Schneiders Collection Technique .

ForewordThe author disclaims all responsibility further to incorrect use of informationor diagrams reproduced in this document, and cannot be held responsiblefor any errors or oversights, or for the consequences of using informationand diagrams contained in this document.

Reproduction of all or part of a Cahier Technique is authorised with the

prior consent of the Scientific and Technical Division. The statement Extracted from Schneider Cahier Technique no..... (please specify) iscompulsory.

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Pierre BERTRAND

An INPG Engineer (Institute National Polytechnique de Grenoble)graduating in 1979, he joined Merlin Gerin in 1983 and was involved inresearch on the operation and disturbances on electrical networks upto 1986. He then joined the Protection and Control activity in which heheld various marketing and technical positions. He is currentlyleading an electrotechnical excellence group within this activity'stechnical department.

n 181

Directional protectionequipment

E/CT 181 (e) first issued, march 1998

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Cahier Technique Schneider n 181/ p.2

Lexicon

ANSI code: digital code assigned to a protectionfunction, defined in the ANSI C37-2 standard.

Characteristic angle (in a directionalprotection equipment): angle between thepolarisation quantity of relay and the normal to

the tripping zone boundary line (see fig. 10 ).

Differential protection: zone protection which

detects a fault by measuring and comparingcurrents at the input and output of the protected

zone or equipment.

Directional protection: protection equipment

capable of detecting a fault upstream ordownstream (in a given direction) of its position.

Earth fault (e/f) protection: protection in whichthe residual variable (current and/or voltage) is

monitored to detect phase-to-earth fault.Phase protection: protection in which the phasecurrent and/or voltage variables are monitored.

Phase-to phase-voltage (annotation):U32 = V2-V3.

Polarisation quantity (in a directionalprotection equipment): the variable used as

the phase reference.Protection plan: the protection equipmentincorporated in an electrical network in order todetect faults and to disconnect the smallestpossible part of the faulty network.

(protection) Relay: equipment used to monitorone or more electrical variable (current orvoltage), generally to detect a fault and to controlthe opening of a circuit breaker.

Relay connection angle (in a phasedirectional protection equipment): the anglebetween the chosen polarisation variable and thephase to earth voltage of the monitored phasequalifies the polarisation variable.

Residual: (current or voltage in a three phasenetwork) : the vectorial sum of the values of allthree phases.

Zero sequence (current or voltage, in a threephase network): 1/3 of the residual variable.

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Cahier Technique Schneider n 181 / p.3

Directional protection equipment

This "Cahier Technique" aims to give the reader a better understanding of

so called "directional" protection: a very useful technology for HV networksand machines.Advances in digital technology and the integration of logical selectivity, hasenabled it to make great progress in terms of reliability, simplicity ofincorporation and even costs.

Its use allows to implement network configuration and selectivity systemimproving the dependability of power supplies.After reviewing their operating principle, the author goes on to present theirmany applications and gives some useful information on theirincorporation.

Contents

1. Introduction 1.1 The role of directional protection equipment p. 4

1.2 Applications p. 4

1.3 The codes and symbols of the various relay types p. 5

2. Description of directional relays 2.1 Earth fault directional protection p. 62.2 Phase directional protection p. 9

2.3 Power protection p. 11

3. Applications of directional protection 3.1 Protection of radial networks p. 12

3.2 Protection of closed rings p. 16

3.3 Protection of alternators p. 19

4. Use 4.1 Sizing of current and voltage transformers p. 21

4.2 Selection between two or three phase protection p. 22

4.3 Protecting parallel connected transformers p. 22

5. Developments and outlook 5.1 Developments in protection equipment technology p. 23

5.2 Developments in sensors p. 24

5.3 In conclusion p. 24

equipment

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1 Introduction

1.1 The role of directional protection equipment

Protection equipment has the basic role ofdetecting an electrical fault and disconnectingthat part of the network in which the fault occurslimiting the size of the disconnected section asfar as possible.

Directional protection enables betterdiscrimination of the faulty part of the networkthan with overcurrent protection.

It is necessary to use it in the followingconditions:

cin a system with several sources,

c in closed loop or parallel-cabled systems,

c in isolated neutral systems for the feedback ofcapacitive currents,

c and to detect an abnormal direction of flow ofactive or reactive power (generators).

Figure 1 illustrates a situation in which bothpower sources would be tripped if overcurrentprotections were used.

Directional current protection equipment iscapable of only tripping the faulty incomer.

The direction in which the fault occurs isdetected by measuring the direction of current

flow, or in other words the phase displacementbetween the current and voltage.

fig. 1: lIlustration of an application of directional

protection equipment.

Direction of detection of the protection equipmentDirection of fault current flow

1 2

fig. 2: The directional protection equipment (1) is

tripped since the direction of current flow is abnormal.

Power protection equipment measures either theactive or the reactive power flowing through theconnection in which the current sensors are placed.The protection equipment operates if the poweris greater than a set threshold and if it isflowing in a given direction.

Directional power and current protection requiresthe current and the voltage to be measured.

1.2 Applications

Directional protection equipment is useful for allnetwork components in which the direction offlow of power is likely to change, notably in theinstance of a short circuit between phases or ofan earthing fault (single phase fault).

c phase directional protection is installed toprotect two connections operated in parallel, a

loop or a network component connected to twopower sources (see fig 2 ).

c earth fault (e/f) directional protection issensitive to the direction of flow of the current toearth. It is necessary to install this type ofprotection equipment whenever the phase toearth fault current is divided between several

earthing systems.

However, this current flow is not only due to theearthing of the network's neutral, but also due to

the phase to earth capacitance of the lines and

cables (1 km of 20 kV cable causes a capacitive

current flow of around 3 to 4 amps).

Residual directional overcurrent protection, aswell as zero sequence active power protection

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Cahier Technique Schneider n181/ p.5

are used to protect feeders with a capacitivecurrent of the same order of magnitude as theearthing fault current.On these feeders, the phase to earthcapacitance is sufficiently high for a zerosequence current to be detected by a non-directional e/f protection as soon as a phase toearth short circuit occurs, wherever it may be onthe network (see fig. 3 ).

Directional protection is thereforecomplementary to overcurrent protection,enabling good discrimination of the faultynetwork section to be achieved in the abovementioned situations.

Active or reactive power protection equipment isused to detect abnormal power flow other thanthe one due to a short circuit; e.g.: in the event ofthe failure of the prime mover, a generator willcontinue to run as a synchronous motor, drawing

power from the system.

1.3 Codes and symbols of the various relay types

For each of the ANSI codes, the table in figure 4groups one or more types of protection

1 2

fig. 3: The directional residual current protection

equipment (2) does not trip since the current is flowing in

the opposite direction.

fig. 4:ANSI codes, symbols and applications.

equipment and also gives the usual names andfields of application.

Graphical symbol ANSI code (C37-2) Usual names Applicational fields

67 c directional overcurrent, directional detection of shortc phase directional circuits between phases

67 N c directional residual overcurrent directional detection of phasec directional earth fault to earth faults

c zero sequence active power

32 P c active overpowerc active reverse power

32 Q c reactive overpower protection of generators andc reactive reverse power synchronous motors

32 P c active underpower detection of abnormal power flow

32 Q c reactive underpower

I >

Ir >

Q >

P

0.4 s or more

0.1 s

Ih >

Ih >0.4 s or more

0.1 s

Direction of

detectionIh >

a

fig. 21: Directly earthed neutral network: detection of earthing faults.

fig. 22: Earthing fault protection on a network earthed at several points.

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protection is less costly and easier toincorporate. Note that the detection of earthingfaults is performed whatever the neutral pointarrangement of the installation, whereasdifferential line protection equipment has limitedsensitivity.

Phase to phase fault protection

Directional phase protection equipment is used

on a radial network for substations supplied

simultaneously by several sources.

In order to obtain good continuity of service, it is

important that a fault affecting one of the sourcesdoes not cause all the sources to trip. The

required selectivity is achieved by installing

phase directional protection equipment on the

incomer of each of the sources.

Figure 23 shows a typical layout for phase to

phase fault protection equipment.

In this figure, the arrow shows the direction of

detection of each directional phase protection

equipment.

Directional phase protection equipment isgenerally two phase. Cases requiring threephase protection are described in 4.

The time delays of the protection equipment areshown. The characteristic angles are set to takeaccount of the chosen angle of connection. For anangle of connection of 90, the most universallyused setting of the characteristic angle is 45.

It should be noted that if the generator set's shortcircuit power is low compared with that of thenetwork, the directional protection equipmentinstalled on the generator set's incomer can bereplaced by simple overcurrent protectionequipment with a threshold set to be both greaterthan the generator set's short circuit current andless than that of the network.

0.1 s

I >

I >

0.4 s or more

0.4 s or more

0.1 s

I >

a

I> >

I>

fig. 23: Short circuit protection on a network with several sources.

3.2 Protection of closed rings

c In such networks, one on more rings areclosed when in normal operation (see fig. 24 ).

The advantage of such a network structure isthat is ensures excellent dependability of powerto all consumers situated on the ring ; it enablesa faulty connection to be disconnected from the

network without interrupting the supply to theconsumers.The disadvantage of this solution is its cost: itrequires a circuit breaker to be installed at theend of each connection in addition to complexprotection equipment.

c Two protection principles may be usedv differential protectionv directional protectionThe later functions if, on the ring, a singlesubstation has one or more sources and alsoearths the neutral. In practice, the selectivity ofdirectional protection equipment is ensured bylogical selectivity systems.

Compared with differential protection, which hasthe advantage of being quick, directional

fig. 24: CLosed ring layout diagram.

a

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Parallel connected lines

Two parallel connected lines are the simplest

and most frequently encountered example of a

closed ring.The protection system must bedesigned in such a way that a fault on one line

does not cause the other line to trip.A typical protection system is shown in

figure 25 . In this figure, the arrow shows the

direction of detection of each directional

protection equipment.

Directional phase protection equipment is of two

phase type. Its characteristic angle is set to take

account of the chosen angle of connection

(45 for an angle of connection of 90).

The characteristic angle of directional earthing

protection equipment is set according to the

neutral point arrangement as explained in the

previous paragraphs.

The protection equipment time delays are shown

in the figure. Non directional protectionequipment used on upstream substation feeders

are time delayed so as to grade with the

directional protection equipment of the

downstream substation incomers.

In the instance of a short circuit on one of the

lines, the current is divided according to the

impedance in each circuit : part flows directly

from the upstream substation in the faulty line,

the rest passes through the downstreamsubstation.

The protection equipment is activated in the

following order :

c A1, D1 and D2 detect the fault,c A1 trips (time delay : 0.1 s),

c D2 resets before its time delay has elapsed,

c D1 trips (time delay : 0.4 s).

When a short circuit occurs near to the upstream

substation's busbars, the proportion of current

passing through the downstream substation is

very low, less than the directional phase

protection threshold value.

This is the case when the position x of the

fault is between 0 and twice the ratio of Is/Isc

(between the directional protection relay's

threshold and the short circuit current). In this

case, the faulty line feeder's overcurrentprotection equipment (D1) trips first (time delay:

0.4 s) with A1 tripping next.The total time to elimination of the fault is

therefore prolonged. This disadvantage can be

overcome by installing a second overcurrent

relay on feeders D1 and D2 with a high threshold

(tripping for a Isc corresponding to less than

90% of the length of the line) with a time delay

of 0.1 s.

0.4 s

0.1 s

I >

Ih >

I>

Ih>

A2

D2

0.4 s

0.1 s

I >

Ih >

I>

Ih>

A1

D1

Fault's position

0

x %

100 %

fig. 25: Protection of parallel connected lines.

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Closed loop

Each circuit breaker is equipped with two directio-nal protection systems, each detecting the fault inopposite directions (apart from the circuit breakersat the start of the loop, which are equipped with asingle non-directional type protection system).

This protection equipment arrangement is shownin figure 26 . Each protection system comprisestwo phase directional protections and two earthfault directional protection equipments. Thedirection of detection of each protection system

is shown by an arrow. Two selectivity series areformulated, one for each direction that the faultcurrent can flow in :v A > B > C > D > E,v F > E > D > C > B.If the selectivity is purely time-based, the tripping

times rapidly become prohibitive. In practice, thissolution is implemented with logical selectivity(see fig. 27 ), which enables very short trippingtimes (0.1 s) to be obtained by using line linksbetween each substation.

fig. 26: Protection of a closed loop using directional relays and time-based selectivity.

I>

Ih>

I>

Ih>

A F

B

C

E

D

1.3 s

I>

Ih>

1.3 s

1 s 0.1 s

0.7 s

I>

Ih>

0.4 s

I> I>

Ih> Ih>

I>

Ih>

0.7 s

I>

Ih >

0.4 s

1 s 0.1 s

I> I>

Ih> Ih>

I>

Ih>

I>

Ih>

A F

B

C

E

D

I>

Ih>

I>

Ih>

Ih> Ih>

I> I>

I>

Ih>

I>

Ih>

Ih> Ih>

I> I>

Logical wait

fig. 27: Protection of a closed loop by directional relays and logical selectivity.

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3.3 Protection of alternators

Detection of the loss of excitation

The break or the short circuiting of the excitationcoil of an alternator is a serious fault. It either

causes the alternator to function as anasynchronous generator, or it stops theconversion of energy and causes an increase inspeed. The first case occurs if the excitationcircuit is short circuited or if the rotor has damperwindings; the situation is stable, but the machineis not designed to accept it for very long. In thesecond case, the situation is unstable and thedrive machine must be stopped as quickly aspossible.It is therefore necessary to monitor the excitationcircuit. Unfortunately, it is often inaccessible,located totally within the rotor (alternator withoutrings or brushes). We then use the measurement

of the reactive power absorbed by the machineor the measurement of the impedance at itsterminals (see fig. 28 ).The reactive power measurement is most oftenused to protect low and medium powermachines. It detects any absorption of reactivepower and therefore any time the alternator isfunctioning as an asynchronous generator. Itmust be possible to set the threshold to a value

Q >

a

Positive direction

of P and Q

Neutral point

of the machine

Q

P

lower than Sn (the machine's rated apparentpower) ; typically 0.4 Sn.

Detection of motor operation

A generator set connected to a power networkcontinues to turn synchronously even if theprime mover (diesel or turbine) is no longerenergy supplied. The alternator then functions asa synchronous motor. Operating in such a waymay be detrimental to the prime mover.In order to detect such operation, a directionalactive power relay must be used (see fig. 29 ).The threshold of this protection equipment is setto a low value compared with the alternator'srated apparent power, typically between 5 and20%, sometimes less for turbo-alternators.Special attention must be given to the design ofthis very sensitive relay in order to ensurestability under all normal operatingcircumstances of the alternator.

Protection for parallel operation

When an industrial installation has one orseveral generation equipments designed tooperate in parallel with the supply authoritynetwork, it is advisable to provide a specificprotection system.

P >

a

Q

PPositive direction

of P and Q

fig. 29: Detection of motor operation, by an active reverse power relay.

fig. 28: Protection against excitation losses by a reactive reverse power relay.

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This protection equipment has two objectives:c ensuring the safety of the production unit'spower station,c ensuring the safety of the main network, whichcan be supplied from the industrial premise'spower station.

This protection equipment is generally installedon the industrial network incomer circuit breaker,controlling its opening. It can also control theopening of a coupling circuit breaker betweentwo parts of the installation.

One of the roles of the decoupling protectionequipment is illustrated in figure 30 : it involvesdetecting a fault situated upstream of theindustrial installation, with the dual aim of:c ensuring the safety of the network: cutting thesupply to the fault,c ensuring the safety of the alternator : avoidingthe reclosing of the feeder to the source

substation, performed without taking anyaccount of the conditions of synchronisation,causing a dangerous coupling.

Fault detection is ensured by phase directionaland earth fault protection equipment:c earth fault directional protection detects theresidual current created by phase to earthcapacitance in the installation and / or thatgenerated by the earthing of the power station,c directional phase protection detects anupstream fault between phases.

Because it is directional, this protectionequipment is insensitive to a fault situated withinthe industrial installation.

c Apart from directional protection equipment, aprotection for parallel operation often comprises

fig. 30: Example of decoupling protection equipment.

I>

Ih>

Source

substation

a

Power

station

a rate of change of frequency relay (df/dt): thespurious increase in power demand at the powerstation, in the case of a loss of mains, causes avariation in generator frequency.Voltage and frequency protection equipment canbe requested by the supply authority in order toguarantee the quality of energy supplied by theprivate generation equipement.

Lastly, an active overpower protection systemcan also be installed in order to indicate anabnormal direction of power flow.

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4 Use

4.1 Sizing of current and voltage transformers

The choice of VT's (voltage transformers) doesnot pose any particular problem.VT's normally installed on distribution networksare either of class 0.5 or 1; they are perfectlysuitable for the supply of directional protectionequipment as long as the sum of the loadsconnected to them is neither greater than theirrated burden, nor too low, in order to avoid risksof ferro-resonance.

CT's (current transformers) are more tricky todesign for this purpose. Should they be under-designed and in the instance of a short circuitcurrent having an aperiodic component with ahigh time constant, the CT's become saturated.

This phenomenon causes an error in the phasecurrent measurement during the transient, as

shown in figure 31 .The current measured on the CT's secondarywinding always leads the primary current.

Incorrect design of CT's can have twoconsequences:

c it may cause spurious tripping - a risk thatdecreases the longer the protection equipment'stime delay,

c it may cause delayed tripping - a risk that isindependent of the selected time delay.The main factor influencing the protectionequipment's behaviour is the phasedisplacement between the short circuit current

and the protection's tripping zone boundary line,as defined in figure 32 .

In practice, if this angle is greater than 45(which is very often the case with the recommen-ded settings), the design requirements for the CTare not so strict : choose the accuracy limit factorfor the CT (as defined in Cahier Techniquen 164) to be greater than or equal to 0.3 times thevalue of the maximum short circuit currentobserved by the directional protection equipment.

1

p.u.

Degrees

00.04

15

0

45

30

Time (s)

2

- 1

- 2

Angle error

0.02 0.06 0.08 0.10

Secondary CT current

I

V polarisation

Tripping zone

Characteristic axis

Characteristic angle

fig. 31: Angle error calculated under the following conditions:

cthe fault comprises an aperiodic component of 100% and a time constant of 40 ms;

c the CT's saturation current is twice the short circuit current.

fig. 32: Definition of the angle.

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4.2 Selection between two or three phase protection

With analog technology, directional phaseprotection equipment is often single phase: itmeasures the current in one single phase. It is

possible to equip one, two or all three phases witha relay.With digital technology, several protectionfunctions are integrated within one piece ofequipment: directional phase protectionequipment is most often two phase andsometimes three phase.As a general rule, when detecting an abnormaltransfer of power (protection of machines), the

phenomenon is balanced over the three phasesand therefore a single phase relay is sufficient.When wanting to detect a short circuit between

two phases, two phase directional protectionequipment is sufficient : at least one of the twophases will be involved in the fault.To detect a phase to earth fault, either threephase directional phase protection equipment orearthing protection equipment will be required. Ifthe installation's neutral is directly earthed, the firstsolution is often best suited. For all other neutralpoint arrangements, choose the second one.

4.3 Protection of parallel connected transformers

Directional phase protection equipment may be

preferred to differential protection equipment inorder to protect two parallel connectedtransformers, especially if the two busbars arequite far apart (in practice it is impossible to wirethe secondary circuits of CT's over more than ahundred or so metres).The protection system to use in this case isshown in figure 33 , taking care to follow thefollowing setting recommendations:

c the instantaneous overcurrent protectionequipment threshold is set to only detect faultson the transformer's primary circuit;c primary-secondary inter-tripping;

cdirectional phase protection equipment set to

only detect faults on the transformer's secondarycircuit.

Depending on the neutral point on the transfor-mer's secondary circuit, two variants appear:c if the secondary neutral point is located on thebusbars, earth fault directional protection isreplaced by simple residual overcurrentprotection;cif each transformer has its own neutral point andif the secondary circuit busbars and the transfor-mers are located within the same substation,restricted earth fault protection can be used inplace of the earth fault directional protection.

fig. 33: Protection layout for two parallel connected transformers.

0.2 s

0.6 s

0.1 s

0.2 s

I>

I>>

Ih> 0.2 s

0.6 sI>

I>>

Ih>

Ih>

I> 0.1 s

0.2 sIh>

I> 0.1 s

0.2 sIh>

I>

Variant

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5. developments and outlook

5.1 Developments in protection equipment technology

The generalisation of integrated and digitalprotection equipment makes directionalprotection equipment simple and relativelyinexpensive to use. This type of protectionequipment is therefore now seen as presentingan excellent opportunity to improve both thepower transmitted through the network and thequality of service.For example, two lines, one to provide powerand the other to provide back-up, may now beoperated in parallel using directional protection

equipment.

Combining logical selectivity (ref.Cahier Technique n2) and directionalprotection equipment enables systems to bedesigned that improve the dependability of theelectrical power.The appearance on the market of multiple-function relays (in other words ones combiningall the protection functions plus the requiredcontrol logic) that are dedicated to eachapplication, simplifies the design andincorporation of the protection system (see

fig. 34 ).

fig. 34: The SEPAM 2000, a multiple function digital relay enabling directional protection equipment to be usedcombined with logical selectivity.

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5.2 Developments in sensors

The advent of digital protection equipment,requiring very little power for measurements, hasenabled new sorts of sensors to be used.

Rogowski coils (CT's without a core), becausethey do not become saturated, enable directionalphase protection equipment to retain itsmeasurement accuracy and to avoid angleerrors whatever the fault. The problem of sizingthe CT disappears.These measurement reducers, comprising alarge number of turns wound around an non-magnetic core, are described in CahierTechnique n170.

Resistive voltage dividers, with their low cost andlow space requirements, are installed in cubicles,near to the directional protection equipment : the

voltage measurement cabling is much morereliable than when VT's are used: VTs are nolonger a common failure mode of the protectionsystem.The development of sensors goes further tostrengthen the interest of directional protectionequipment, by improving performance levels andmaking them easier to incorporate.

5.3 In conclusion

Technological advances (digital protectionequipment, new sensors, etc.), as well as logicalselectivity make directional protection equipmenteasier to use.

Today, this high performance and easy toincorporate protection equipment is provinginvaluable in improving the dependability of the

electrical power supply. It is increasingly beingused to protect networks and machines, whetherfor phase to phase fault protection or for earthingfault protection.

Readers interested in more general informationon the various types of protection equipment usedin MV can refer to Cahier Technique n 174.

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