real time digital power system simulator design considerations and relay performance evaluation

IEEE Transactions on Power Delivery, Vol. 14, No. 3, July 1999 773

REAL TIME DIGITAL POWER SSTEM SIMULATOR DESIGN CONSIDERATIONS AND RELA* PERFORMANCE EVALUATON

D. Jakominich

Ral SA S € . & A

R. Krebs, D. Retzmann Siemens AG

Erlaryyen, Germany

A. Kumar, Member, IEEE Siemens AG

Erlangen, Germany

Abstract- Environmental pressures, right of way problems for new transmisson Ilnes, large scale system interconnection and increasing power demand especially in developing countries call for new transmission technologies. Use of FACTS and HVDC is gaining momentum. Relaying problems in such systems require sophisticated approach and numerical technology using high grade processors offers a promising solution. To establish the performance of such relays which are designed more as powerful schemes rather than as a conglomeration of individual relays, detailed testing is required using an elaborate real-time simulator model. These tests are not only necessary during design and development but also are required by the utilities as a part of their acceptance. Highlighting the requirements of a hybrid real-time simulator including the complete spectrum of power system dynamics, this report describes an analog cum digital model used for real-time assessment of system performance. SVCs, HVDC, ASC and CSC can be modelled fully digital by computer simulation and transient real-time data Injection as well as analog by using physical simulation with original converter controllers for performance evaluation of numerical relays under real system conditions. As an example of the use of this simulator, tests on a numerical EHV-line relay are described and the relay performance is analyzed.

I. INTRODUCTION

Changes in the political !andscape especially in Europe are having a strong impact on the power industry. The different segments of this industry are undergoing rapid changes. On the generation side, nu- clear power is still very much disputed and many countries are forced to buy environmental friendly power for example hydel from countries which have this in plenty to meet their demands. As a consequence of the removal of political obstacles, there is a strong trend tdwards the interconnection of different grids to improve the quality of power transmission. For instance, the grids of UCPTE of West Europe, NORDEL of the Scandinavian countries, EES and VES of Eastern Europe are going to be interlinked. ?he entry of many more countries into the EEC (now called the EU) is an added impetus to this development. Such changes in the political scenario and proposed links are not without problems. Practically, this requires the use of new transmission,technologies. In Europe, there is a marked increase in HVDC links partly as sea cables and partly as overhead lines [5,12] Func- tionally they serve either as a back to back connection or are used for bulk power transfer. Another development which has a promising future is FACTS [3, 7, 9, 10,221. These technologies offer a wide range of stability improvment features leading to better system performance. On the protection and control side, numerical technology (using powerful microprocessors and signal processors) is being in- creasingly used for such systems. Testing of such protection and control systems under realistic conditions demands a sophisticated real-time simulation of all system

96 SM 411-9 PWRD A paper recommended and approved by the IEEE Power System Relaying Committee of the IEEE Power Engineering Society for presentation at the 1996 IEEUPES Summer Meeting, July 28 - August 1, 1996, in Denver, Colorado. Manuscript submitted April 15, 1996; made available for printing May 20, 1996.

components. The classical way of using only analog devices has reached a physical limit because of the complexity of the systems. There is therefore a strong movement internationally towards the use of hybrid simulators (a combination of analog and digital) The first part of this paper describes a flexible analog cum digital real- time simulator which combines the advantages of both ways of simulation. ?he second part of this paper is devoted to the use of the simulator for testing different protection functions and includes details of the simulated systems, fault types and associated relaying problems.

11. WHY REAL-TIME SIMULATOR TESTS FOR NUM ER IC AL RELAYS?

Gernerally, one can observe a strong trend towards the use of numerical relaying for all power apparatus and at all voltage levels of generation, transmission and distribution. Due to a basic difference in the way numerical relays and their analog counterparts process the measuring signals, there is still an understandable skepsis from the utilities in the use of numerical relays because the user is confronted with terms like antialiasing, digital filters, sample and hold, multiplexers, A/D converters and measuring algorithms. The decisions of numericai relays are based on algorithms which manipulate the digitalized measuring signals mathematically. Well known methods used include solution of differential and integral equations,. DFTs, FFTs, correlation, convolution etc. The performance of the relays therefore depends very much on the ingenuity of the measuring algorithm. Added to this, one is confronted with the quality of the software and the use in the measuring algorithm of discrete measuring values which are digitalized at regular time instants. Many utilities therefore insist on acceptance tests whith their systems modelled as completely and as accurately as possible before they go into a large scale refurbishment of their relaying schemes. During these acceptance tests, the operators and engineers could also be trained. Also during the design stage, simulator tests are required to opti- mize the algorithm. Real--time simulator tests can also be used to validate r,ew equipment settings. Shott tripping times of numerical relays claimed by different man- ufacturers imply that the relay has to process signals with transients still on. Conventional testing using a standard stationary test kit is insufficient to establish the performance. The increasing complexity of power systems demands a critical look and a thorough understanding of the behaviour of the network, the control and the protection schemes adopted.

111. DEMANDS OF CONTROL AND PROTECTION TESTING ON A REAL-TIME SIMULATOR

A. Coinponents of the Used Real-Time Siniulator

For a realistic evaluation’ of the control strategies and protection schemes, realbtime simulators have to satisfy many stringent requirements. These requirements are dictated by conditions which exist during normal system operation and during faults. The different components of the real--time simulator are:

Machines with turbine and excitation control Transformers with saturation characteristic - L.oads - static as well as dynamic HVDC systems with detailed controls for thyristors and reactive power cornponents

0885-8977/99/$10.00 0 1996 IEEE

774

TABLE 1: SlMULAflON REQUIREMENTS

I T 1 Transient System Requirements Network Components

PROS

0 An exact representation of the system even with a large X/R ratio Is possible, i.e. component tolerances are no limitation

@ System size for the simulation is practically unlimited. AC and DClinks. FACTS with controls, exact machine modelling with turbine and excitation controls, and power systems stabilizers can all be included in the sirnolalion

@ Non-linearities can be exactly simulated @ it is easy to calculate the measuring signals with complex fault sequen-

ces, for example for testing the transient blocking feature in distance relays

@ Fault locations can be changed in very small steps

~~ ~

@ Machines, transformers. FACTS, single- and double-circuit lines. as well as multi-terminal lines

0 Circuit breakers with facility for single pole breaker opening and auto reclosing with settable time delays

0 Instrument transformers (CTs and PTs) 0 Signal transmission delay for transfer trip schemes 0 Power amplifiers for voltages and currents 0 Bidirectional load transfer (active and reactive power) of

variable magnitudes

CONS

@ Modiflcation of test sequence is time consuming 0 Changes in test condition for example system changes implies a new

set of calculations 0 Since the measuring signals are calculated off line, real-time interacti-

on between protection and the modelled systenl is not possible @ Calculated measuring signals being digital values, they have to be con-

vetted to analog quantities. using D/A converters, before amplification. @ Testing of an interactive scheme for example pilot relaying is not possi-

ble

0 Different types of faults with and without fault resistances at

@I Faults at selectable Instants on the voltage chicle to simulate

@ Power system oscillations, subsynchronous risonances 0 Current reversal on double-circuit lines due to sequential tripping 0 Faults with weak-infeed condition 0 Sequential- and inter-system faults @ Closing onto baked and unbolted faults

desired locations

different transient conditions

I I 1 Transmission lines with their frequency characteristics

* FACTS components and series capacitors with their protection circuits, ASC (Advanced Series Compensation) and CSC (Con- trolled Series Compensation) Static var and synchronous corn ensators with controls Circuit breakers with pole controf(l-pole trip and reclosure) Instrument tranformers (CTs and PTs) Communication links between line ends for exchange of protectl- on signals Data acquisition and retrieving units Power amplifiers for generation of current and voltage signal levels suitable for the relay

B. Simulator Requirements for Testing Relays

From a relaying viewpoint the simulation requirements are summarized in table 1 The number of n-elements to be used for the lines depends upon the desired tests. If the effects of travelling waves are to be considered, the number of n-elements should be at least 10. For double-circuit lines, the mutual coupling between the circuits should be a part of simulation. A detailed machine representation is of special importance for generator-close relay testing because of the variation of machine reac- tances from sybtransient to transient to steady state values. Study of power swings also requires a fairly accurate representation of the machines With series compensated lines, the location of the series capacitors and the type of protection used for them - gaps or MOVs - has a

significant effect on relay performance and has to be properly simulated. Relays reacting to subsynchronuos resonances (SSR) demand their exact simulation. If relays are being tested in FACTS environoment, the different components of the FACTS like high-voltage DC transmission (HVDCT), advanced series compensation (ASC), thyristor controlled series capacitor (TCSC) and SVCs must be exactly simulated. Since the performance of the relay is considerably influenced by instrument transformer transients (CT saturation, CVT tansients) their simulation must be as realistic as possible, he short-ciruit currents and voltages being strongly dependent on Lult location and source impedance ration(SIR), the amplifiers used must faithfully repro- duce the frequencies and magnitudes over the desired range.

IV. DETAILS OF AN ADVANCED HYBRID TRANSIENT-NEMORK ANALYZER

Theoretically, it is possible to use either an analog or a digital simulator to test the relays. Each of these methods has advantages and disadvantages which are summarized in tables 2a) and 2b) . As can be seen from these tables. the advantages of the one are the disadvantages of the other. Hence for testing relays, a hybrid model is used which combines the advantages of both.

TABLE 2A): PROS AND CONS OF DIGITAL SIMULATION

TABLE 28): PROS AND CONS OF ANALOG SIMULATION

PROS CONS

@ A modification of a certain test sequence is possible within a short time @ Since the elements are physically modelled, acheck on the faithfulness

of the model is always possible within a short time @ Any changes in the model can be done very fast @ Interaction between protection and model is easy lo realize; for instance

current interruption or current flow can be controlled by prolection trip and reclose commands

@ The analog current and voltage signals from the model could be directly amplified to feed protective relays without complex hardware

@ Pilot relaying can be tested including transmissiondelays, as in original systems

I

0 Component tolerances prevent exact representation of the original system. A large WR ratio as required by tranforrners and EHV lines is difficult to realize

@ Complex systems for instance, large interconnected grids are difficult to simulate

0 Non-linearities like differept saturation characteristics and fault arcs are difficult lo simulate

@ Avery large number of Jt- elements are required. if the fault locations have lo be changed in small steps.

0 Complex fault sequences are not easy to realize 0 Simulation of machine dynamics including turbine and excitation con-

trols needs complex models @ Non transposed transmission lines are difficult to simiilate without loss

of accuracy

775

Computer and Data Acquisit ion System

Dlgltal Sequence Controllers

Posltlve and Zero Sequence Components

Measuring Protection Control

Signal Generatlon and Recordlng

Computer Simulation

Fig. 1 : Advanced AC/DC Real-Time Simulator Facilities

A. AC/DC Real-Time Simulator

The simulator described in this paper has originally been developed as a transient network analyzer UNA) with special regard to the higher-frequency requirements for the development and testing of FACTS devices such as HVDC, static var compensators and advanced series compensators (ASC) with controlled thyristors [E, 14, 16, 261. The band of frequency that can be examined ranges from:

a few Hz for power swings to above 5 kHz for analyzing events related closely to the valves.

The simulator mainly comprises analog elements which function on a continuous time basis in order to cope with the fast dynamic response of modern converter control systems. The time resolution of an HVDC trigger set for example is around a few microseconds and the requirements for the simulation process in the analyzer are simi- larily demanding. Therefore, the real-time simulation must be continuous in order to avoid any step changes in time and amplitude [13, 231. From the power source to the load, the simulation covers every item of AC power equipment relevant to a thorough study of the system either as a detailed model or grouped together as a benchmark for simulation. With these features, the simulation is fast, flexible, precise and re- producible. The test conditions can be changed quickly, especially during final acceptance tests by the customers. This is the strength of reai-time simulation with physical models. New data recording and logging systems of the steady-state and transient network conditions with high-performance analog-digital and digital-analog in- terfaces were developed for the AC/DC simulator. These are supplemented by new digital sequence controllers. This permits specifying precisely of any fault sequence in cycle and degree steps for short-circuits and for switching adjacent lines, also with automat- ic reclosing. The sequence controller also simulates communication links between the two line ends (including the propagation delay time), such as are used in distance and differential protection, and the setting of the circuit-breaker opening delay. An overview of the actual simulator equipment is given in Fig. 1 : Var- ious power-system infeeds such as complete generator models and controlled electronic sources are available. AC/DC transmission lines are simulated including the zero sequence impedances. Five test stations are installed for independant system studies. Most of the simulator equipment is based on a modular structure, so the size and kind of a test station can be easily adapted to the actual requirements of a new investigation. Each test station can be operated independant or in combination with any other test station. The data acquisition system has a bidirectional access to these test stations for both recording and data injection purposes. At present, the test stations of the simulator are in use as follows: -one SVC test station (for 2 compensators) is connected with the

HVDC test station. Here is simulated the parallel operation of a large HVDC long distance transmission in a series compensated

system with two compensators, which are tested and commis- sioned in the simulator for the project Mead Adelanto, USA, with the original equipment for closed-loop and open-loop control,

-the third test station is used for the commissioning of 3 static compensators for a project in South Africa, also with closed-loop and open-loop control,

-the fourth test station is mounted with a prototype of the Advanced Series Compensation for current system studies to test new control functions for power oscillation damping and suppression of subsynchronous resonances,

-at the protection test station, the behavior of system protection schemes in any of these FACTS devices environoment can be in- vestigated.

An example of the simulator models is given in Fig. 2. This test station has been in use for various HVDC investigation with back-to- back and long distance transmission schemes and is now combined with one of the SVC test stations. The digital HVDC control is based on the control system SIMADYN D which allows high flexibility for the implementation of any closed- and open-loop application.

B. Digital Model

With powerful modern workstations and PCs the NETOMAC or EMTP program system can be used, not only for special individual cases, but also for complex system studies in order to determine the currents and voltages for protection equipment. The simulation capability extends up to complete power stations and FACTS equipment with detailed control. By running the computer variables in the AC/DC simulator in real time on the original equipment it is possible to overcome the limitations of the earlier, purely analog models. Especially the program NETOMAC (20, 41 provides a large number of simulation options. These include selectable variable time steps, almost unlimited network size and large interconnected grids with several HVDC systems including original control systems, whereby the protection and FACTS simulation is made substantially more complete. In [I 91 a comparison of the two powerful simulation pack- ages EMTP and NETOMAC is made, regarding the simulation of a detailed HVDC system. The resulting transient responses from ATP version of the EMTP and from NETOMAC show that identical results can be obtained from the two programms, in spite of the many program differences. The authors show that to overcome time delays and lack of interpolation in'the EMTP, however, a much smaller time-step has to be chosen than in the NETOMAC program. One has to select a 5 times smaller time step in the EMTP program than in NETOMAC to get identical results. The corresponding calculation time is then about 4 times longer than with the NETOMAC program. This advantage in calculation time led to the use of NETOMAC for the digital system simulation for testing protection relays, where on one hand hundreds of dynamic system situations have to be calculated and on the other hand interconnected grids including HVDC and power plants have to be considered.

776

Current trasformers

Voltage transformers

Machines

Power transformers

Lines

Circuit breakers

System network faults

Line Load

Faults

System network

Fig. 2: View of the HVDC Test Station In the Advanced ACIDC Real-Time Simulator

~~

All protection schemes using current as a measuring input e g overcurrent, distance, transformer-differential, buszone etc All protection schemes using voltage inputs e g. distance and directional

Line distance relays and machine protection schemes

Basically transformer differential

Llne distance relays

Line distance relay and earth fault protection schemes

Line distance schemes

Earth fault protection

Llne distance schemes

Overvoltage and overcurrent relays

V STUDIES FOR RELAYING SYSTEMS

A. Evaluation and Choice of Measuring Algorithms

Numerical relaying techniques include a measuring algorithm as a software module, which decides whether to trip or not. Advantage can be taken of this fact and different algorithms can be pro- grammed on a PC and evaluated with fault data from a hybrid simulator. For this purpose, the fault data stored on a digital data acquisition system is retrieved and input to the measuring algorithm. In this manner, the algorithm can be optimized before it is built into the relay. The effects influencing the algorithm are partlyfrom the primary network and partly from the instrument transformers. The reaction to these parasitic effects depends on thetype of relay involved. For instance, magnetizing current inrush may primarily effect transformer protection schemes. Series capacitors with their protective circuits are a concern to line protection. CT-saturation generally influences the performance of all relays, whereas CVT transients are of importance only to relays using voltages as input, for instance distance relays In Table 3, these effects are divided into two groups:-one caused by the network and the other by the instruments transformers. The modelling requirements, therefore, are dictated by the type of protection under test. The real-time simulator described here can be used for testing all types of protection schemes for lines, transformers, machines and busbars. In most of the cases, simulator tests are required to evaluate relay performance for fast tripping under transient condltions; the transient being a result of system faults. There are, however, some cases when tests on protection systems have to be done without

faults, for instance under system conditions1 (power swings) caused by load changes, switching in or out of series and shunt capacitors etc. Fig. 3 shows as an example, the currents during the start-up of an HVDCT which have been used for testing AC line protection relays. The most Common phenomena that must be anticipated in HVDC- environoment are inrush effects due to saturation of the converter transformers. This occurs most frequently on energizing and startup of HVDC systems, as shown in Fig. 3, and after AC system faults close to the bus. In this case, it is particularly important for the digital protection filters to provide good suppression of the exponentially decaying DC component and the 2nd harmonic in order to avoid an unwanted tripping (overreach in distance protection). Temporarily very slow decaying DC current components have to be considered which are particularly likely In case of commutation fail- ures of the converters. Typical transients from the HVDC system in short-circuit currents and voltages are shown in Fig. 4. The amount of reaction depends on the ratio. between the infeed power of the HVDC system and short-circuit capacity of the AC system. An extreme case is when the HVDC system feeds directly into the protection zone, e.g. when the HVDC station is linked to the AC system by a radial line. The dis- torted HVDC currents come into effect without short-circuit contri- bution from the AC system. The high DC component after incidence of the short circuit can have, depending on the control of the HVDC inverter, no current zero crossing at all even for several cycles. For satisfactory operation of protection close to an HVDC system it is especially important to use filters and algorithms for the distance protection which, in addition to supressing the DC components, provide very good damping of exponential function and second and higher harmonics. Extensive tests have been carried out on distance protection and it

TABLE 3: PARASITIC EFFECTS INFLUENCING PROTECTION SYSTEMS

Nature of Disturbance

1. CT saturation

2. CVT transients

3. Power swings

4. Magnetizing current inrush

5. Line transients on faults

6. Unequal pole opening and closing

7. Sequential and simultaneous faults

8. Single pole breaker opening

9. Arcing and high resistive faults

1O.Dynamic nelwork changes e.g. swi!ching in, switching on loads, series and shunt capacitors

Source I Protection Effected

777

1 .o] I i&

0 0

Energize Switch in HVDC

Fig. 3: Transformer primary currents at energizing and startup of a 600 MW HVDC system with inrush currents from the converter transformer

V

IIA 0

5

WQ

Fig. 4: Effect of an HVDC station in a very weak AC system (SCR = 1.5) on the reactance measurement of the 7SA513 arising from a line-to-earth fault

has become apparent that an algorithm based on the line differential equation still gives excellent results with extremely weak AC systems. Even with a short-circuit ratio of SCCACsyste&nHvDC= 1.5, at the stability limit of the HVDC control, there IS no malor effect on the measuring accuracy of the numerical distance protection 7SA513. Fig. 4 shows how rapidly the measuring reactance moves to the actual value within only one cycle. A test of phase-comparison protection and current differential protection under the same conditions has given similarly good results.

B. Tests on Numerical Protection Systems

Real-time simulators can be very successfully used for a quantita- tive evaluation of relay performance [16, 17, 181. The simulator described above was used for developing and testing a new EHV numerical line protection.

61. Systems Modelled The following 400 kV systems were simulated for testing EH?/ line. protection

400km, 300km and 100km single- and double-circuit lines 12km, single and double circuit lines 400km series compensated lines with the series capacitor either at the end or in the middle of the line Lines with ASC (Advanced Series Compensation)

The lines were modelled as three-phase systems with distributed impedances (resistances, inductances and capacitances) in the three phases as well as in the neutral. The simulated double-circuit lines included the mutual coupling between the circuits. The line sections were modelled as n-elements. The series capacitors were modelled with protective gaps which short-circuit the series capacitor at the instant the capacitor voltage exceeds the flashover voltage. The gap modelling permits the setting of any desired flashover voltage. In the tests, it was set to 2.5 Irated. The system infeed permits independent settings for frequency, voltage magnitude and phase angle. Hence magnitudes and direction of active and reactive power can be controlled and power system oscillations could be simulated. The CTs were simulated linear and the PTs as CVTs with realistic transient response. The circuit breakers were set for single-pole tripping with an opening time of 2 cycles (40 ms on a 50 Hz system or 33 ms on a 60 Hz system). Furthermore a signal propogation delay of 20 ms was modelled for the transfer trip signals between the protective relays. For a study with the ASC-scheme parts of the 230 kV WAPA system was used as model (see fig. 5 and 6) .

62. Description of Tests Carried Out 1000 testswere carried out to assess the performance of EHV line protection, 800 of them were done using the analog model and 200 using the digital model with NETOMAC simulation. The tests included the following:

Short circuits with varying infeed ratios in the positive- and zero-- sequence systems, includin weak-infeed conditions variable load flow, includingaeavy load conditions Single and multi-phase faults with low and high fault resistances Faults at different locations and at different switching angles on voltage cycle Sequential faults during the auto-reclosing dead time Short circuits on parallel lines Short circuits between the two parallel lines Evolving and simultaneous fault sat various locations in the power system Reversal of current direction during short circuits on parallel lines due to sequential breaker opening Closing of a circuit breaker onto a fault.

The protection devices were also subjected to a number of tests with non-faulty conditions which included:

Energizing of an unfaulted line Shunt reactor switching Fuse failure in the voltage transformer circuit Load swings Interruption to the relay' s auxiliary power supply

Additional tests were simulated by connecting the protection test station to the FACTS test stations and by digital NETOMAC simulation to ensure correct relay operation even in FACTS enviroment, e.g. High voltage Direct Current Transmission and Advanced Series Compensation.

83. Relay Functions Adtive During'Tests The systems described above have been used for elaborate testing of an EHV numerical line protection on a simulated 400 kV series compensated and 230kV-ASC system. Both line ends were protected by the relays whereby the following functions (integrated software modules) were active:

Zone packaged multi zone distance 9 Directional earth fault comparison

Selectable transfer trip scheme

778

Trip, Trip2 Trip3

Line 1 -1

SCC Navajo Bus

--I_

I I

I I

r I

f3- :red:; breaker MOV R

Line 2

2,. t, .. " ' SCC Shlprock Bus

Kayenta Substation

Fig. 5: WAPA Kayenta: ASC-TNA Representatlon 230 kV, 60 Hz

. Weak infeed logic Power swing detection with the associated logic Overvoltage protection Distance to fault location Phase selection for single pole trips Directional and noc-directionai phase- and earth-fault overcurrent back up rotection Line check &se onto a fault) Single and 3pole autoreclosing Breaker failure Fuse failure monitor

on single phase faults and vice versa. Residual ground currents because of load under 1-pole-open conditions [2] can influence on-line distance measurement leading to a threepole trip during dead time following a single-pole trip on line to ground faults. Single-pole trips under weak-infeed conditions with load transfer has to be ensured usihg a phase segregated weak-infeed logic. The tested relay incorpbrates load compensating methods and appropriate software logic to ensure proper phase selection.

B. Faults on Double-Circuit Lines

Earth faults on doublecircuit lines influence the distance relays' performance due to mutual coupling [27. The relay in the faulted line might underreach or overreach depending on system operating conditions [l]. If both lines are in service, the relay tends to underreach if no compensation methods are used for the mutual coupling. It is the common practice not to use any compensation but to solve the underreach problem by using a permissive overreaching transfer-trip scheme. When the parallel line is earthed, the relay in the line in service tends to overreach. A well designed numerical line relay permits the change over of settings using an aux. contact of the earthing switch of the parallel line. Loads and single pole open conditions result in the distribution of the false residual ground current between the two circuits. Relay software logic must be able to distinguish between the false and the true residual ground currents, to prevent the loss of both the lines.

C. Fault Resistance

System faults with resistances in combination with load influences the measured fault distance. Whether the relay overreaches or un- derreaches depends on the direction of load flow. Using suitable load compensating methods, the relay can measure correctly.

D. Current-Reversal Guard VI. IDENTIFICATION OF RELAYING PROBLEMS

AND SOLUTIONS

The following relaying problems are well known, and were simulated to examine the ielays' performance [16, 17, 181.

A. Effect of Load

The amount of load transfer influences the selection of faulty phase or phases. Single pole tripping on single-phase faults and three pole tripping on multi-phase faults are not always ensured if the phase selection is not properly designed. With series compensated lines, the magnitude and direction of the load current can influence the fault current in such a manner that an unequal gap flashover on two phase to ground faults might occur. Because EHV relays are generally designed as non switched, that means that all impedance loops are al- lowed to be measured, it is possible that the impedances measured by the ground loops are different, since the earth fault residual compensating factor (&&) ist not the same for both the loops on un- equai gap flashover. Proper phase selection is also of extreme importance in permissive underreach transfer trip scheme for the relay measuring the fault in the carrier zone. Wrong phase selection can mean three-pole trip

Sequential fault clearence on double circuit lines results in fault current reversal, therefore special measures are necessary in the form of transient blocking to prevent wrong trips from the relay in the unfaulted line. With a ski( loop distance measurement, the transient blocking logic must consider the fact that the load continues to flow in one direction which need not to be the same as the direction of fault current, i.e. the fault current in the unfaulted line may reverse its direction and not the load current, due to sequential trip on the other line with single line to ground faults.

E. Fast Trip for Close in Faults

Close in faults demand fast trip. "his is a fairly critical problem with short lines because the relay's first zone impedance setting is very low. Numerical techniques used in modern EHV line-protection relays offer a good solution to this problem by examination of the voltages, currents, impedances and angles to achieve fast trip. High set current elements and undervoltage controlled current fault detection are some of the possible methods.

I

779

E Line-Check Feature

Fast tripping with trip times less. than 1 cycle when closing onto a bolted fault is one of the desirable features of a good EHV line-protection relay. A classical way of dealing with this situation is to use the breaker closing command via a binary input. With numerical relays, one can realize a fast trip by using the "no current, no voltage" condition prior to the fault. The insecurity of auxiliary inputs is thereby overcome and one can use the measuring signals to detect close-onte-a-fault condition.

G. Series Compensation

Series compensation influences relay performance considerably [SI. The correctness of distance to fault calculation depends upon the behaviour of the protection circuit used across the capacitor as mentioned before. If the relay logic does not include any means of detecting the state of the capacitor with its protective circuit, it should include other methods like voltage memory and cross polarisation for proper directional measurement. Care must be taken to prevent overreach on 1 st-zone faults and underreach on carrier-zones through proper settings. For instance, if series capacitors are protected by gaps, the 1 st zone setting must consider the full capacitance. For setting the carrier zones the capacitors must be considered as short circuited.

H. Transient performance of CTs

EHV lines are generally equipped with CTs which do not saturate, since they are designed to transform the primary current accurately. Of course the measuring algorithm used in the relay should be in- sensitive to the dc component of the short-circuit current without their reach being effected. With HV and MV systems, however, CT saturation could occur and the evaluation of these relays demands a suitable simulation of the transient response of the CT including remanence, which is particularly of importance when single-pole breaker opening is practiced. With bus protection schemes, CT saturation is of special importance and the effectiveness of the measuring algorithm used can be criti- cally analyzed [21].

1. Effect of line transients in volfages

Voltage signals in EHV systems could be a cause of concern to distance relays, because of the high frequency content introduced by the line capacitance. The voltage form on the secondary side depends on the type of VTs used. Inductively coupled VTs exhibit a different frequency response than the CVTs. Whereas inductively coupled VTs transform the high frequency signals without much modification of the frequency spectrum on the primary side, the CVTs influence the waveform of the voltage considerably. The frequencies to be expected vary from the DC component to a few hundreds of Hz. The amount of damping used decides how long these parasitic frequencies continue. A detailed analysis of the transient response of the CVTs is included in references [15, 251. On the relay side, errors in measured reach and directional determination are to be expected, if no suitable measures are included in the relay, The relay under test includes band- pass filters, sound phase voltage and voltage memory to ensure correct relay performance with CVT transients.

J. Tapped lines

Use of distance relays on tapped lines has always been associated with problems. The reach measured by distance elements depends on the system conditions. Classical way of overcomming this is to use underreaching or overreaching tranfer trip schemes. There is, however, a good chance of improving relay performence using communication channels to transfer the state of the breakers at the line ends to switch the pararneter sets of the numerical distance relay. Such and other ideas are a part of the adaptive relaying [11 , 241

K. Influence of SVCS, HVDC and ASC

Under transient system conditions. a wide range of harmonics and non-periodic disturbances might be injected by these thyristor-controlled devices into the system. As examples see [7,8,9,23] and section V

L Power Swings

Power swings influence relay performance considerably Real-time simulators can be used to test novel methods for the detection of power swings One such method incoporated in the tested relay de- tects power swings by monitoring the rate of change of resistance measured by the relay. Due to the detailed representation of the system, it is possible to de- cide which of the relays have to be set for out of step tripping and which of them for out of step blocking. If out of step relaying is being tested as a part of machine protection scheme, one can determine the power swing counter to be set to ensure system stability.

VII. CONCLUSION

Real time system simulation is an essential tool in estabilishing the performance of modern protection and control equipment. With the NETOMAC (registered trade mark of Siemens) digital simulaton program, the simulator can also be used as a tool for development of protection in FACTS environment. Additionally, the fault data of any earlier real-time simulation or from the fault records stored in the numerical relsy subsequent to a system fault, can be used for performance analysis. For this purpose the IEEE standard common format for transient data exchange (COMTRADE) offers an excellent solution Another possible application of this fault data is a learn- ing model for a system like Expert and Neural nets and development of new measuring algorithms for protection and control. From the system operators view point, such simulators can be very helpful in indeep understanding and analysing the system behaviour under transient conditions. This is becoming more and more necessary due to the increasing complexity of the system. Very often, one can reduce the commisioning time of complex control systems used in ASC, HVDC and SVC since the commissioning engineer can acquire a thorough knowledge and train himself at the simulator before the actual commisioning work at site.

VIII. REFERENCES

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Geeves, S.; Bergmann, K.; Retzmann, D.; Witzmann, R.: Im- provement of System Stability by the Harker Static Var Com- pensatorslUK - Verification of System Performance by Digital and Real--Time Simulation. ICPST, Beijing, 1994 Hedin, R.A.; Henn, V.; Montoya, AH.; Torgerson, D.R.; Weiss, S.: Advanced Series Compensation (ASC): Transient Network Analyzer Studies Compared with Digital Simulation Studies. EPRI, Proc.. FACTS Conference, Boston, 1992 Hedin, R.A.; Weiss, S.; Torgerson, D.; Eilts, L.E.: SSR Charac- teristics of Alternative Types of Series Compensation

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Schemes. IEEUPES, San Francisco. 1994, paper 534-8 PWRS Hingorani, N.G.: A new Scheme for Subsynchronous Reso- nance Dampin of Torsional Oscillations and Transient Torque, Part I. IEEE Trans. PAS-100, No. 4, April 1981, pp.

Horowitz S.H. Phadke, A.G.; Thorp, J.S.: Adaptive Transmis- sion Systems Relaying. IEEE Trans. Power Delivery 1988, pp 1436-1 445 Hugelschafer, L.; Knittler, D.: Digitale Leittechnik in HGU- und Blindleistungskompensationsanlagen. Elektrie 45 (1 991) H. 3, s. 108-110 Karlecik-Maier, F.; Retzmann, D.; Rittiger, J.: Simulation von Hochspannungsgleichstromubertragung und Statischen Kompensatoren. Elektrie 45 (1991), H. 3, S. 100-103 Kaufhold, W.; Retzmann, D.; Schultz, W.: Echtzeitsiniulator zur Untersuchung des Systems Stromrichter und Netz. etz Bd. 113, H. 3, S. 116-123 Kezunovic, M.; Fromen, C.W.; Nilsson, S.L.: Digital Models of Coupling Capacitor Voltage Transformers for Protective Relay Transient Studies. IEEE Trans. Power Delivery, Vol. , 110.4,

Krebs, R : Kumar, A.; Pretorious, C.; Retzmann, D.: Real- Time System Studies for FACTS and Protection. ICPST, Beij- ing, 1994 Krebs, R.; Kumar, A.; Retzmann, D.: Real-Time Simulation for Evaluation of Transmission-Line Protection Under Transient Conditions. 81h NPSC, New Delhi, India 1994 Krebs, R . ; Retzmann, D.; Ziegler, G.: Auslegung und Priifung von Schutzsystemen. etz. Bd. 115, H. 18, S. 103S1043 Krilicke, B.; Lehn, P.; Rittiger, J.: Comparison of the ATP Ver- sion of the EMTP and the NETOMAC Program for Simulation of HVDC Systems. IEEE Trans. PES, New York, 1995 Kulicke, 6.: NETOMAC Digital Pro ram for Simulating Electro- mechanical and Electromagnetic fransient Phenomena in AC

Kumar A.; Hansen, P.: Digital Buszone Protection. IEEE Com- puter Application in Power, 1993, pp 29-34 Povh, D.; T II H : Static Var Compensation for High-Voltage Systems, T k II SEPOPE, Sao Paulo/Brasilien 1989 Retzmann, D.; Schraudolph, M.: Any Stud Can Benefit from Simulation - System Studies for HVDC fransmission. Sie- mens EV.Re ort (1991), H. 4, S.4-7 Rockefeller, &I.; Wagner, C.L.; Linders, J.R.; Hicks, K.L.; Rizy, D.T.: Adaptive Transmission Relaying Concepts for Im- proved Performance. iEEE Trans. Power Delivery, Vol3, no. 4, 1988, pp 1446-1 458 Sweetana, A.: Transient Res onse Characterisitics of Capaci- tance Potential Devices. IEEE Trans on PAS, Vol. PAS-89, 1970, pp 1989-1997 Welsh, G.; Bergmann, K.; Retzmann, D.; Schmidt, M.: Tran- sient AC/DC Simulator and Field Tests of the Closed Loop Control of the Pelham SVCs. EPE Power Electronics and

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IX. BIOGRAPHY

Dan Jakominlch received his Bachelor of Science in Electrical Engi- neering from Villanova University in 1992. He worked as a relay engineer in the protection department of Siemens Germany for two years, While in Germany his primary focus was to assist in the development of microprocessor based relays. This work Included testing of a 1 cycle EHVdistancerelay. Heisnowworking asanapplicationengineerinthe relay department of Siemens Energy & Automation Raleigh. NC.

Ralner Krebs was born in Pfledelbach. Germany on January 19,1958. He received the Dipl.-lng degree in Electrical Englneerlng and Power Systems in 1982 and the Dr.-lng. degree in 4 990 from Friedrich-Alexander Univer- sity Erlangen. He joined the 'Institut fur Eiektrische Energieversorgung' at the University of Erlangen in 1983 working on calculation and measurement of power system disturbances and unbalances using spoce-phasor theory. In 1990 he joined Siemens AG in Erlangen as a member of the System Planning De- partment. He started his work in the fields of power-system planning, power-system relaying and protection simulation and fault analysis. He is member of VDE, its associated section ETG and CIGRE Working Group 34.08 'Protection Against Voltage Collapse'.

Ani1 Kumar (M64) received his B.Tech, ME and Ph. D from lhe indian Insti- tute of Science Bangalore in 1962,1964 and 1969 respectively. From 1970 - 1972, he was a post doctoraifellow of the Alexander von Humboldt Foun- daton al the technical university of Braunschweig, Germany. Since 1973 he has been with the Protection Department of Siemens, Germany where he Is now a senior engineer. He has been involved in the design and development of digital protection systems from the beginning. In his 22 years service with Siemens, he has also carried out design and sales work for EHV line protection and customer oriented design tebts of protection schemes on the AC/DC Simulator. From 1980 to 1990. Dr. Kumar was a membev of the CIGRE Working Groups34.01 and 34.02dealing with digital protection and control of power systems. At present his main field of work involves design and development of digital busbar protection. He is a member of the IEEE and PES.

Dietmar Retzmann was born in Pfalzfeld, Germany, on November 4, 1947. He received Dip!.-hg. degree from Technische Hochschule Darmstadt in 1974 and Dr.-lng. degree (Real Time AC Signal Process- ing) from Friedrich-Alexander University of Erlangen in 1983. Dietmar Retzmann joined Brown Boveri AG, Mannheim 1974, sales department for Switchgear and Installations. He worked in the technical group for system planning and as commissioning engineer for system protection equipment. In 1976, he was called for research and teaching at the new founded "Institut fur Elektrische Energieversorgung" at the University of Erlangen. Dr. Retzmannjoined SIEMENSAG, Erlangen, System Planning department in 1982. He is Technical Manager of the AC/DC Real-Time Simu- lator group (TNA). He is member of VDE and its associated sections ETG. GMA. GME.

78 1

Discussion

Norton Savage, P.E., Silver Spring, MD:

This paper provides a neatly

protective relay testing by means of analog and digital simulation; it is enlightening to the non-specialist and would appear to be useful also to the relay specialist. Being one of the former, I have only a few questions, the answers to which may be well-known to protection engineers. The methods and equipment described in the paper appear to be intended for use in overhead transmission system studies; are they

developed discussion of

readily adaptable for use with systems that consist primarily of underground cables? For &systems comprising a mixture of overhead and underground transmission, dotraveling wave reflections complicate the simulations? Are any records available that compare the results of the simulated testing with the results of actual fault conditions that may have occurred? Can the simulation apparatus be used to replicate faults that have occurred, in order to determine if the protection scheme operated as intended?

Closure was not provided by the Author.

real time digital power system simulator design considerations and relay performance evaluation

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