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Study Committee B5 Colloquium2005 September 14-16

Calgary, CANADA

SERIES CAPACITORS AND DISTANCE PROTECTION SUBSYNCHRONOUS FREQUENCIES

AND REAL-TIME TEST OF DISTANCE PROTECTION RELAYS

Rainer Krebs, Volker Henn Jürgen Holbach Patricia Morais , Gustavo Arruda Siemens AG, PTD SE PTI Siemens, SPT&D CHESF-DOPR Po-Box: 3220 Po-Box: 29503 D- 91050 Erlangen, Germany Wendell, NC 27626-0503, USA 50761-901-Recife-PE, Brazil

Abstract: Beyond the lots of benefits of series capacitor installation in HV and EHV power systems regarding the increase of transmission capability, they also produce some disturbing quantities which affect the protection systems.

The paper presents as one of the disturbing quantities, the calculation of the subsynchronous frequencies and their influence on measuring algorithms. Hints for settings calculation were given.

In the second part the setup of the digital real-time test simulator (RTDS) of Siemens PTI Germany will be presented. As an example for one of the lots of extensive tests, results of customer approval test of 7SA522 relay on applications in the 500 kV system of CHESF, Brazil. will be shown.

Index terms: Series capacitor, subsynchrounous frequency, distance protection, settings calculation, real-time simulation

I. INTRODUCTION

Transmission lines are of ohmic-inductive-capacitive character. For load transfer below the natural power the transmission line is ohmic-capacitive, for load transfer higher than natural power the transmission line reacts as ohmic-inductive. Especially for long overhead lines, the reactance limits the transfer capability to values far below the thermal limit of the line with respect to the static stability. It was obvious, already in the past, to compensate the reactance or part of the reactance by series connection of a series capacitor to increase the transmission capability of fundamental frequency. In the system without series capacitors, faults are of inductive character, which results in the corresponding directional characteristic of distance relays: forward = ohmic inductive impedance. The capacitive or apparent capacitive nature of the fault current, that will exist in power systems with series capacitors for different fault scenarios, may cause the line protection to fail to operate, or to operate incorrectly, unless careful measures are taken to acknowledge this problem.

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Additionally, the series capacitors, in series with the reactances of the power system react as series resonant circuits producing subsynchronous, synchronous and interharmonic components of the voltages and currents. These frequency components are influencing the measuring algorithms of protection relays. Series compensated power systems require distance protection relays with especially designed features like the Siemens relays 7SA522 and 7SA6. The development of these relays must be accompanied and supported by real-time simulation and tests in especially equipped labs like the Siemens PTI Lab in Germany.

II. FUNDAMENTAL BEHAVIOUR OF FSC AND TCSC Series capacitor banks are equipped with platform protection to protect the capacitors against the high possible voltages during line faults. For this purpose, MOVs, spark gaps and bypass breakers are installed to bridge the capacitor during faults. When the limiting voltage of the spark-gap (if installed) is exceeded, the network is restored to an ohmic-inductive nature for short-circuit current and impedance, so that no special measures have to be applied to the relays. If MOV and GAP are installed, depending on the magnitude of the short circuit current, the capacitor and varistor in parallel provide variable, current sensitive values of impedance as shown in fig. 1. If the MOV is working, the shown circular, current-depending impedance is produced by the parallel connection. If any thermal limit of the MOV will be exceeded, the energy monitor will trigger the spark gap via a fibre-optical path to protect now the MOV. If it’s only necessary to increase the power transfer capability with respect to the fundamental frequency, the installation of fixed series capacitors is necessary. However if additional transient performance like power-oscillation damping or load-flow control is required from the system, thyristor controlled series capacitors (TCSC) are required. With the installation of these equipments, also during load transfer, there is no longer a constant impedance seen by the distance protection.

III. SUBSYNCHRONOUS FREQUENCIES PRODUCED BY FSC

In series-compensated systems, a short-circuit e.g. on line-side of the series capacitor leads to a resonant circuit consisting of the series capacitor, the line- and source reactance and the damping line

ILine

UCap

ICap

IMOV

IGap

GAP - Firing

Load Flow Fault Inception

20 40 60 80 100 120ms

GAP

MOV

Un=400kV

-XC

Gap fired

with MOV

without Gap,without MOV

IL

XC

ZLZC

ZL

ZC

ZS

ZS

R

X

ILUL

EMOV

3-Phase Short-Circuit

Fig 1: Series compensation: Principle, diagram of instantaneous values and impedance seen by distance protection

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and source resistance. At a short-circuit behind the series capacitor without gap-flashover, the short-circuit current contains a frequency component of resonant-frequency of the circuit. This subsynchronous frequency may lead on one hand to subsynchronous resonances with the shafts of installed turbine-generator systems and on the other hand to a disturbance of measuring algorithms of protection relays. In the following, the influence of these subsynchronous current components on the impedance measurement of distance protection relays will be calculated. Setting recommendations for independent first zone of distance relays will be given.

A. Max. Subsynchronous Current Component For the calculation of the main equations for the subsynchronous current components the differential equation of the short-circuit loop with series capacitor has to be solved. To reach a series resonant circuit with the highest amplitude of subsynchronous current component, a line-to-line short-circuit loop will be regarded, which has the lowest damping. Line-to-earth short-circuit loops include the zero-sequence impedance with a lower impedance angle than the positive-sequence impedance with corresponding higher damping.

{ {

Fig. 2: Single-line diagram of a line-to-line short-circuit with series capacitor and no gap-flashover

Fig. 3: Single-line diagram of reduced system for the calculation of the subsynchronous current components

The simple system diagram acc. to fig. 3 results after combining the system and line impedances. It can be described by the following differential equations with a source voltage e(t) only with fundamental frequency of e.g. 50Hz or 60Hz.

)(2

2

teudt

duRC

dtud

LC ccc =++ (1)

With t0ωτ = and the resonant frequency 0ω and the quality factor Q of the resonant circuit:

⎟⎟⎠

⎞⎜⎜⎝

⎛=++

02

2 1ωτ

ττeu

ddu

Qdud

ccc (2)

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And the short-circuit current:

ττ

ddu

LCi c=)( (3)

The source voltage e(t) has only one frequency component namely 0f nominal frequency. For the subsynchronous component during short-circuit, the source voltage is 0 and the source can be regarded as short-circuited. With this condition, the following solution of the above differential equations can be given: Resonant capacitor voltage:

)cos()( 00 ucc eUu ψβττ ατ −= − (4) Resonant current:

)cos()( 220 ieU

LCi ψβτβατ ατ −+= − (5)

Where βα j+ are the eigenvalues of the system acc. to fig. 3. The starting condition for maximum subsynchronous current is reached for a maximum energy in the capacitor without gap-flashover. The maximum energy is reached for a capacitor voltage just below the gap-flashover voltage. With these constraints, the constants of subsynchronous current component can be calculated. a) Resonant frequency of the undamped series-resonant circuit:

LC1

0 =ω (6)

b) Frequency of the damped series-resonant circuit:

0

2

0 214ωω

QQ

a

−= (7)

c) Quality factor of the damped series resonant circuit

0ωRLQ = (8)

d) Time constant of subsynchronous frequency:

0

2ωQT = (9)

e) RMS-value of subsynchronous frequency current component:

1412

20−

=

∧

QRUI gap

a (10)

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B. Influence of subsynchronous current component on impedance measurement Numerical distance relays are measuring the fundamental component of currents and voltages and the corresponding fundamental impedance. A subsynchronous current component influences this impedance measurement by the transfer function of the relaying algorithm. Acc. to fig. 4, the transfer function can be roughly estimated as linear between DC and fundamental frequency. Taking such a standard filter characteristic into account, the influence on the impedance measurement behaves like the ratio of filtered subsynchronous current to fundamental short-circuit current acc. to equation 11.

Fig. 4: Transfer functions of sine and cosine filter of full cycle Fourierfilter.

⎟⎟⎠

⎞⎜⎜⎝

⎛

−

−+±=

⎟⎟⎠

⎞⎜⎜⎝

⎛

±==

∧

n

acgap

k

n

aa

RX ff

QR

XXREU

Iff

IKK 0

2

22

50

00

influenceinfluence14

)(2 (11)

With equation 11, the influence of a suharmonic frequency component on the impedance measurement ca be calculated, which will be shown in the following example. However it is necessary to know the values of all impedances of the short-circuit loop. The impedances are varying with the source data and the short-circuit location, so that a worst case estimation is necessary. Taking into account, that for HV and EHV systems, the positive-sequence resistance is less than 10% of the positive-sequence reactance, equation 11 can be simplified to:

XXX

EU

Iff

IKK cgap

k

n

aa

RX)(

250

00

influenceinfluence−

±=⎟⎟⎠

⎞⎜⎜⎝

⎛

±==

∧

(12)

Taking as reactance X a calculated value, for which the fundamental short-circuit current gives a capacitor voltage of gap firing voltage, the following simple equation can be derived:

EU

KKgap

RX

21

1influenceinfluence ∧

+

== (13)

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To avoid overreaches of the first, undelayed zone, it must be reduced to following setting, where K is the impedance grading factor ( e.g.: K=0.8 …… 0.9):

)(

21

11 XcX

EU

KXgap

zone −

+

= ∧ (14)

For typical applications, independent and undelayed zone 1 has to be reduced to values in a region of less than 50 %. Fig. 5 shows as an example R and X values of impedance calculation, based on fourier-filtered voltages and currents, as well as typical and proposed setting values.

Fig. 5: Measured impedance based on Fourier-filtered voltages and currents during

sub- synchronous oscillation

IV. PERFORMANCE VERIFICATION OF PROTECTION FOR SERIES COMPENSATED LINES

A. Setup of used transient network analyzer For the network simulation a fully digital real time network analyzer RTDS with two racks was used. The analog outputs were amplified acc. to fig. 6 to match the needed input for analog signals for testing the numerical distance protection relays 7SA6 and 7SA522. Three sets of voltage amplifiers with maximum 130V rms line to ground and three sets of current amplifiers with 40A rms were available. A data acquisition system with 16 analog and 64 digital channels recorded the analog inputs at the relays’ terminals as well as the reaction of the relays e.g. trip or teleprotection signals. A semi automatic documentation system generated the final report for the customer CHESF, Brazil.

RTDSTM

Power System Simulation

Data Recording

DIGSI

Relay 1 Relay 2

DIGSI

Voltages

Currents

CommunicationLink Simulation

Fig. 6: Hardwaresetup of TNA for relay testing

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B. Power system under Test CHESF in Brazil introduced new series capacitors in their 500kV system at substation S.J. Piaui. The setup of the network included acc. to fig. 7: - 3 single lines, - two double-circuit lines with mutual coupling, - 4 sets of series capacitors with MOV and bypass, - 14 shunt reactors and - 6 sources. The protected line was equipped with capacitive voltage transformers (CVTs) and current transformers (simulated as saturable CTs). The lines Boa Esperanca to Piaui and Piaui to Sobradinho were investigated, each with two typical loadflows. Tests were done with faults on the protected lines, on busbars and adjacent lines, both low and high resistive.

Fig. 7: Structure of the 500kV series compensated system for test of relays

C. Relais under test For closed loop relay testing in complex networks, always both ends of the line must be equipped with the full scheme protection relays including the communication. So we used 7SA612 with distance protection, high-impedance earth-fault protection and autoreclosure. 7SA612 was equipped with special settings for series compensation, including the use of memorized voltages for correct direction determination on series compensated lines.

D. Test results With adequate and optimized settings, the relay behaviour was fully satisfying. Relays located in Boa eEsperanca and Sobradinho, looking to S.J. Piaui will see a fault on the busbar at S.J. Piaui with 70% compensation, this means at 30% of line length. A fault on the adjacent line will be seen with more than 140% compensation. Therefore these relays cannot use independent zone 1, they must use teleprotection with POTT scheme. A relay in J.Piaui looking to Boa Esperanca will see a fault on the line to Sobradinho with more than 140% of line impedance. So the relay in J.Piaui needs a huge reverse zone to pickup and prevent echoes in the telecommunication and to serve as backup protection. For the detection of ground faults, it is necessary to disable the U0 criterion, since phase to phase faults with MOV operating produce due to the nonlinearity false U0 voltages, which may be interpreted as ground faults. In all the cases of ground faults, the ground current was large enough for a secure earth fault detection. For high resistive ground faults, were the distane protection could not operate the high impedance earth fault protection was used. This function operates with U0 and I0. In same cases it could happen that the direction measurement was disturbed by the false U0 voltages, introduces by the MOVs. The

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direction measurent was good, as soon as the bypass operated. So, if we expect frequent high resistive ground faults, a delay should be introduced. Doing relay tests on closed loop simulators is not only relay performance verification. More over it is

d between

training for developers, network planners, commissioning engineers and operation people.

Figures 8 and 9 show the oscillograms of a double-phase-to-ground fault in Piaui, locatethe capacitor and the circuit breaker. One can see the subharmonics in the current of the left relay and in the voltage of the right one. The relays tripped in zone 1 on right side and in zone 1B on the left side. After opening the breakers on both sides, one can see a damped resonant frequency on the healthy phase caused by line capacity and shunt reactor as well as a low frequent voltage, caused by a resonance between series capacitor and shunt reactor. The automatic reclosure energizes the line from the right side, automatic reclosure from left side enables loadflow again.

Fig. 8: Relay in Boa Esperanca on line to Piaui

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Fig. 9: Relay in Piaui on line to Boa Esperanca

V. BIBLIOGRAPHY [1] Krebs, R.; Kumar, A.; Pretorius, C.; Retzmann, D.: Real-Time System Studies for FACTS and

Protection. International Conference on Power System Technology (ICPST), Beijing-China, 1994. [2] Jakominich, D.; Krebs, R.; Kumar, A.; Retzmann, D.: Real-time digital power system simulator - design

considerations and relay performance evaluation. First International Conference on Digital Power System Simulators (ICDS). College Station, Texas USA, 1995.

[3] Jakominich, D.; Krebs, R.; Kumar, A.; Retzmann, D.: Real-time digital power system simulator - design considerations and relay performance evaluation. IEEE / PES, Denver Colorado, paper 96 SM 411 - 9 PWRD.

[4] Krebs, R.; Retzmann, D.; Ziegler, G.: Performance Verification of Protection in FACTS Environment. PSP-96, Slovenia, 1996, pp. 211 - 217. (ISBN 961-6210-07-6)

[5] Claus, M.; Henn, V.; Krebs, R.: Simulation knows no bounds: Digital real-time simulation for closed-loop testing of numerical communicating relays. 7th international conference and exhibition on developments in power-system protection, RAI, Amsterdam, 9th-12th april 2001.

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VI. BIOGRAPHY

Dr. Rainer Krebs, born 1958 in Germany (member of VDE and CIGRE), received his Dipl.-Ing. degree from the University of Erlangen in 1982. From 1983 to 1990 he worked as an assistant professor at the Institute for Electrical Power Systems at the same University. In 1990 he received his Dr.-Ing. degree in Electrical Engineering. 1990 he joined Siemens AG, Power Transmission and Distribution, System Planning Department. Since 1998 he is director of the System-Protection and System-Analysis Tools Department. Since 2003, he is in parallel lecturer for ‘system protection and control’ at the Otto-von-Guericke University of Magdeburg, Germany.

Volker Henn, born 1959 in Germany, received his Dipl.-Ing. degree from the University of Karlsruhe in 1985. From 1985 to 1990 he worked at Siemens AG, Power Transmission and Distribution, Protection Department. In 1991 he joined the System Planning Department, where he is working in the field of real-time simulation and protection testing.

Dr. Juergen Holbach was born in Saarlouis Germany, on June 17, 1961. He graduated from the University of Berlin with a PhD in Electrical Engineering. He jointed the Siemens AG in 1992 as a development engineer in Berlin Germany. In 1994 he moved to the product management group for protection relays in Nuremberg, Germany. Since 2000 he is the product manager for transmission relays in Raleigh, NC, USA.

Gustavo Arruda received his BSc in Electrical Engineering from Federal University of Pernambuco-UFPE in 1980, and has been with CHESF since 1982. He is presently the manager of the Protection and Control Division. His main interests are digital simulations for relay scheme tests and the application of software based tools for investigating relay performance.

Patricia Morais received her BSc in Electrical Engineering from Federal University of Pernambuco-UFPE in 1983, and has been with CHESF since 1984. She is presently a senior protection engineer at the Protection and Control Division. Her main interests are system simulations for protection studies, and the application tools for the management of the protective devices database.

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