Subsynchronous Resonance Mitigation Options For

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Assessment of FACTSRequirements on the PSE&G

SystemSubsynchronous Resonance Mitigation Options for the Pennsylanvia-New Jersey-MarylandInterconnection

TR-106463

Final Report, March 1996

Prepared byGENERAL ELECTRIC COMPANY1 River RoadSchenectady, NY 12345

Principal Contributors: Ann T. HillEinar V. LarsenDaniel H. Baker

PUBLIC SERVICE ELECTRIC & GAS80 Park PlazaNewark, NJ 07101

Principal Contributor:Emile Hyman

Prepared for Electric Power Research Institute3412 Hillview AvenuePalo Alto, California 94304

EPRI Project Manager Rambabu Adapa


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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSOREDOR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OFEPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THEUSE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT,INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOTINFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUALPROPERTY, OR (III) THAT THIS REPORT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANYCONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THEPOSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS REPORT OR ANYINFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT.

ORGANIZATION(S) THAT PREPARED THIS REPORT

General Electric CompanyPublic Service Electric & Gas

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 CogginsDrive, P.O. Box 23205, Pleasant Hill, CA 94523, (510) 934-4212.

Electric Power Research Institute and EPRI are registered service marks of Electric Power Research Institute, Inc.

Copyright 1995 Electric Power Research Institute, Inc. All rights reserved.


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REPORT SUMMARY

Subsynchronous resonance (SSR) due to nearby generating units and capacitors haslimited the application of series capacitors to increase bulk power transfer capability.This report investigated use of thyristor control on a portion of the series capacitorsystem to mitigate SSR.

BACKGROUND

The 500-kV Pennsylvania-New Jersey-Maryland (PJM) transmission grid deliverspower from a number of jointly-owned PJM generation projects. The operatingeconomies of the PJM pool drive the movement of power from the most economicgeneration, regardless of location or actual ownership, to the load served. As a result,typically 5000 MW flows from west to east on the PJM system. From 1982-1993, 4200MVARs of shunt capacitors were added to the PJM system to overcome post-contingency voltage drop constraints that limit transfers. Studies in 1988 and 1989,which ultimately led to the second round of shunt capacitor additions, suggested 50%series compensation for three of the 500-kV lines: Keystone-Juniata, Conemaugh- Juniata, and Juniata-Alburtis. SSR screening studies identified the potential for SSR atseveral generators in multiple contingency scenarios of varying severity. Hence,several studies considered thyristor control of series capacitors to prevent this SSR.However, such control for the total series capacitor system significantly raises costs.

OBJECTIVES

To investigate the benefit of using thyristor control in a portion of a series capacitorsystem to mitigate SSR.

APPROACH

The analysis focused on a portion of the 500-kV transmission grid of the PJMInterconnection. The project team performed eigenvalue analysis and time domainsimulations for the PJM 50% series compensation application. Simulations modeled thethyristor control algorithm developed for the EPRI-sponsored Thyristor ControlledSeries Capacitor (TCSC) demonstration at Bonneville Power Administrations (BPA)Slatt Substation, as well as special modulation controls developed by General Electric(GE) after the Slatt installation was completed. The team also explored use of specialSSR damping controls.


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RESULTS

Emulating the technology and control algorithms employed in the GE/EPRI TCSCdemonstration at BPAs Slatt Substation, this study shows that the SSR threat can bemitigated by using TCSC for 34-44 percent of the total series compensation on the

Conemaugh-Juniata line, depending on the degree of conservatism of the TCSC design.Using a novel modulation scheme, the range of TCSC compensation may be reduced to24-30 percent. A passive filtering technique for SSR mitigation also merits furtherconsideration.

EPRI PERSPECTIVE

This study demonstrated the use of thyristor control in only a portion of the seriescapacitor system to mitigate SSR and thus reduce the total cost of the series capacitorinstallation. FACTS technologies are offering competitive solutions to utilities byenabling increased power transfers, improved system damping, and better systemcontrol (see EPRI reports EL-6943 volumes 1-2, TR-100504, TR-101784, TR-101932, TR-101933 volumes 1-2, TR-103164, TR-103166, TR-103167, TR-103168, TR-103641, TR-103701, and TR-103906). EPRI has planned several FACTS assessment studies andprojects to enhance device hardware through Tailored Collaboration with memberutilities.

INTEREST CATEGORIES

FACTS and substations, communication, protection and controlTransmission access evaluationPower system operations and control

KEYWORDS

Power system stabilityPower system controlPower system dynamicsPower system planningFlexible AC transmission systems


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ABSTRACT

The application of series capacitors has become more common as utilities attempt toincrease transfer capability over existing bulk transmission paths. Many seriescapacitor applications have been limited though by the potential for subsynchronousresonance (SSR) of nearby steam generating units with the series capacitors. This studyinvestigates the benefit of using thyristor control in only a portion of a series capacitorsystem to mitigate SSR. This analysis utilizes a portion of the 500kV transmission gridof the Pennsylvania-Jersey-Maryland (PJM) Interconnection for this case study.Previous PSE&G studies suggested 50% series capacitor compensation of the Keystone- Juniata, Conemaugh-Junita, and Juniata-Alburtis 500kV lines to improve the prevailingwest-to-east PJM power transfer capability. SSR screening studies performed at thattime showed potential for SSR at several generators in multiple contingency scenarios.Emulating the technology and control algorithms employed in the GE/EPRI TCSCdemonstration at BPAs Slatt Substation, this study shows that the SSR threat could bemitigated by utilizing TCSC for between 34% and 44% of the total series compensationon the Conemaugh-Juniata 500kV line, depending on the degree of conservatism of theTCSC design. Using a novel modulation scheme, the range of TCSC compensation maybe reduced to between 24% and 30% of the total series compensation of theConemaugh-Juniata 500kV line. A passive filtering technique for SSR mitigation isshown to also merit further consideration. Other proposed schemes are enumeratedand briefly described.


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ACKNOWLEDGEMENTS

We are grateful to PJM for permission to use the engineering data from which this caseis drawn, and to Dennis Sobieski and Jim Hebson of PSE&G for making thearrangements. The technical guidance and support provided by Daniel H. Baker of GEfor this project is respectfully acknowledged. GE and PSE&G also acknowledge thesupport for this project provided by EPRI and Ram Adapa.


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EXECUTIVE SUMMARY

This simulation study demonstrates the potential for SSR suppression by using thyristorcontrol of only a portion of a series capacitor system. This approach offers the potentialof significantly lower equipment cost relative to the cost of using thyristor control of theentire installation. Additionally, adding special modulation to the TCSC control showspromise for further reducing the required size of the thyristor-controlled portion, andtherefore total cost. It should be noted that the study case was only one of severalcontingencies with potential to cause SSR within the study system.

Eigenvalue analysis and time simulations, whose results were in essential agreement,were performed for the PJM 50% series compensation application using specialsimulation tools developed by GE. Simulations modeled the thyristor control algorithmdeveloped for the EPRI-sponsored TCSC demonstration at BPA's Slatt Substation, plusspecial modulation controls developed by GE after the Slatt installation was completed.

Simulation results show that, without thyristor control and approximately 50%compensation of three lines, loss of the Keystone-Conemaugh line with one Conemaughgenerator and the Conemaugh-Hunterstown line out of service produced an unstableSSR interaction with the first torsional mode of the Conemaugh HP unit. Thyristorcontrol of only a portion of the Conemaugh-Juniata series compensation using theconstant-impedance control scheme of the Slatt installation sufficiently mitigated thepotential for SSR within the context of the contingency studied. Simulating asuperimposed firing modulation scheme for special SSR damping further increased theeffectiveness of the thyristor control.

While this study shows a potential for use of special SSR damping controls, adaptationto any of a set of possible contingencies would require further study. It is possible thatthe usefulness of special controls will depend on the richness of the potential SSRinteractions within a particular case or within particular contingencies of that case.

The PJM series compensation case, studied here, has illustrated a number of features forSSR suppression by thyristor control. This case provides a useful benchmark forstudying candidate control schemes for SSR suppression in highly interconnectedsystems.


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CONTENTS

1 INTRODUCTION.......................................................................................................1-1

2 SUBSYNCHRONOUS RESONANCE ISSUES.........................................................2-12.1 Analysis Method for SSR Damping.....................................................................2-12.2 Transient Torque Analysis Methods ...................................................................2-3

3 CONEMAUGH SSR POTENTIAL WITH CONVENTIONAL SERIES CAPACITORS3-1

4 TCSC DESIGN CONSIDERATIONS.........................................................................4-14.1 TCSC Operating Characteristics........................................................................4-14.2 SSR Damping Characteristics of TCSC..............................................................4-24.3 Selection of Nominal X ord ....................................................................................4-34.4 Special Considerations at Low Line Current.......................................................4-4

5 TCSC/CONVENTIONAL HYBRID SERIES COMPENSATION ................................5-1

5.1 TCSC Rating Selection .......................................................................................5-15.2 Operating Conditions ..........................................................................................5-25.3 SSR-Mitigation Performance ..............................................................................5-35.4 TCSC/Fixed Series Capacitor Combination ........................................................5-95.5 Enhanced SSR Mitigation with Modulation Control ..........................................5-10

6 OTHER MITIGATION METHODS.............................................................................6-16.1 Series Capacitor Bypass Filters..........................................................................6-16.2 Discussion of Other SSR Countermeasures.......................................................6-5

7 CONCLUSIONS........................................................................................................7-1

8 REFERENCES..........................................................................................................8-1

A CONEMAUGH GENERATOR #1 TORSIONAL CHARACTERISTICS.................... A-1


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1-1

1 INTRODUCTION

PJM, the Pennsylvania-Jersey-Maryland Interconnection, is the world's oldest powerpool. Eight member utilities coordinate the planning, operation and maintenance oftheir own generation and transmission resources, import economy power from externalsuppliers, and share the benefits of minute-to-minute uniform incremental cost dispatchas though they were a single utility with 46,000 MW peak load. The pool conceptallowed PJM utilities to participate in large base load generating stations outside theirown service territories where there was advantage to doing so, sharing ownership andrisk with one or more partner utilities. Examples include the Keystone and Conemaugh

mine-mouth plants in western Pennsylvania, and the Peach Bottom, Salem, and HopeCreek nuclear units in eastern Pennsylvania and New Jersey.

A 500 kV PJM transmission grid (Figure 1-1) evolved in the past three decades todeliver power from a number of jointly owned PJM generation projects. This grid ismultiply connected to adjacent control areas to its south and west. These utilities joinwith PJM in a regional plan for coordinated operation, and, in turn, interconnect withthe ECAR 765 kV system in the Midwest.

The operating economies of the PJM pool derive in large measure from the movementof power from the most economic generation, regardless of location or actualownership, to the load served. As a result, throughout the year, there is a heavyprevailing west-to-east flow on the PJM 500 kV system, amounting to (typically) 5000MW. From 1982 to 1993, 4200 MVArs of shunt capacitors have been added to the PJMsystem to overcome post-contingency voltage drop constraints that limit transfers.

Studies in 1988-89 [1], which ultimately led to the second round of shunt capacitoradditions, suggested 50% series capacitor compensation of three of the 500 kV circuitsshown in Figure 1-1: Keystone-Juniata; Conemaugh-Juniata; and Juniata-Alburtis.

Subsynchronous resonance (SSR) screening studies, performed as part of the 1988-89

series compensation study, showed potential for SSR at several generators in multiplecontingency scenarios of varying severity. A number of application studies haveconsidered thyristor control of series capacitors (TCSC) for preventing SSR. However,thyristor control of the total series capacitor system raises the cost of an installationsignificantly. Could the SSR prevention objective be achieved with thyristor control ofonly a portion of the series capacitor installation?


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Introduction

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Paralleling a PSE&G study considering the practical limitations and enhancementpotential of the PJM 500 kV system, EPRI has funded GE to study thyristor control of aportion of the series capacitor compensation of PJM, described above, to determine thefraction of series capacitor installation(s) that would require thyristor control for SSRsuppression. The simulation has been designed to emulate technology and control

algorithms employed in the GE/EPRI TCSC demonstration project at BPA's SlattSubstation [2]. The simulation tools used for the evaluation have been validated for thiscontrol system by comparison to both real-time simulator results [2] and to field tests.

This report descibes the results of this simulation study, specifically, with the Keystone- Juniata, Conemaugh-Juniata, and Juniata-Alburtis circuits 50% compensated, and theportion of series capacitor installations on the Conemaugh-Juniata circuit with thyristorcontrol to prevent SSR at the Conemaugh generating station. The Conemaugh unitswere identified among the PJM units with a potential for SSR effects in the 1988-89studies.

Figure 1-1. The PJM 500 kV system and interconnections.


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2 SUBSYNCHRONOUS RESONANCE ISSUES

Two key aspects of series capacitor application must be addressed to ensure integrity ofturbine-generator units [3]:

1. Torsional Interaction. This aspect relates to the tendency of the series compensated actransmission network to act like a negative damping influence on torsional vibrations.When this negative damping influence overcomes the inherent "mechanical" dampingof the shaft torsional system, the torsional vibrations will grow exponentially untildamage occurs. Generally such growth in shaft vibrations occurs with a rather long

time-constant, on the order of many seconds.2. Transient Torque. This aspect relates to the tendency of series capacitors to amplifythe shaft stresses during major network transient events, over and above that whichwould exist without the series capacitors. Generally, the critical aspect of thisphenomenon is the magnitudes of shaft vibrations after the electrical transients havedecayed substantially, which is on the order of one second.

SSR studies are typically done in various stages [4]. The first stage involves a screeninganalysis that determines the risk for SSR from the impedance-vs-frequencycharacteristics of the network. This "frequency-scanning" technique is an approximateassessment of potential torsional interaction, and is useful to define the criticalcontingencies that contribute to the overall risk of a torsional instability.

Evaluation of actual interaction potential and shaft transient-stresses requires more in-depth analysis. This is particularly true when the network includes large power-electronic systems, such as HVDC, SVC, or TCSC. These systems are not amenable toimpedance-scanning techniques, and the details of control action are crucial toobtaining valid results.

General concepts and methods typically used for these detailed evaluations are

described in the following subsections.

2.1 Analysis Method for SSR Damping

The interaction between a subsynchronous mode of torsional vibration on a turbine-generator shaft and a series compensated ac transmission line is known assubsynchronous resonance (SSR). Small-signal torsional damping is one aspect of SSR


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that can be described by the transfer function from generator speed to electrical torque.The real portion of this transfer function represents the so-called "electrical damping"contribution to torsional vibration decay rate.

A positive electrical damping component indicates that if the generator shaft speed

increases, then the electrical torque will also increase by an amount proportional to thevalue of the damping component. This increase in electrical torque will oppose theoriginal perturbing force, and cause the succeeding oscillations to decay more quickly.A negative electrical-damping component indicates that if the generator shaft speedincreases, then the electrical torque will decrease. This decrease in electrical torque willaid the original perturbing force, and cause subsequent oscillations to have lessdamping, or to become unstable.

The net damping of a torsional mode of vibration consists of the electrical component,described in the previous two paragraphs, and the so-called mechanical component.The mechanical damping is due to all factors other than changes in electrical torque onthe generator shaft (primarily steam flow, friction, and windage). Mechanical dampingis quite small, but it is always positive, and increases with generator load. A torsionalmode of vibration will be stable when the net damping is positive. A torsional modewill be unstable if electrical damping at the torsional frequency is negative and larger inmagnitude than the inherent mechanical damping. Mechanical damping is minimumwhen a machine is at no-load, and monotonically increases with the machine load.

Figure 2.1-1. Typical electrical damping curve.

The torsional stability concept is illustrated in Figure 2.1-1. A typical electrical dampingcurve and mechanical damping values for one torsional mode are shown. Themechanical damping values are plotted as negative values for ease of comparison with


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2-3

the electrical damping curve. This is shown as a bar in the plot, where the extremes arefor no-load and full-load on the unit (-D NL and -D FL, respectively). The inherentmechanical damping increases with load, so it is minimum at no-load and maximum atfull-load. If the electrical damping curve is below the mechanical damping point, themode is unstable. In Figure 2.1-1, the mode shown is unstable at no load (-D NL) and

stable at full load (-D FL).

This type of analysis is somewhat approximate for situations off resonance, since theactual frequency of torsional vibration will vary a little due to the synchronizing effectof the electrical network. By using such analysis to determine a worst-case electricaldamping within a certain frequency window, e.g., 1Hz, the procedure can be used fordesign purposes.

2.2 Transient Torque Analysis Methods

The consequences of transient torque amplification are related to fatigue which meansan accelerated loss-of-life of the turbine-generator shaft. A complete analysis of fatiguedamage involves performing a large number of transient simulations with the completenetwork, where all conceivable switching sequences and fault timings are considered ina statistical manner. For each case, the torque responses of each shaft section areevaluated in a nonlinear manner to estimate the fatigue loss-of-life for the event.

Such a detailed study is quite involved, and is usually performed as a final step prior toimplementing a solution. Generally, some approximations can be made based oncomparing the torsional vibration level of a few critical cases with the worst case that

would exist without series compensation. This worst case is generally a full-loadsituation, where a fault is imposed near the generator and cleared in the worst timingwith respect to the particular torsional vibration mode of concern.


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3-1

3 CONEMAUGH SSR POTENTIAL WITH CONVENTIONAL

SERIES CAPACITORS

The locations of the PJM series compensation used as the basis for the 1988-89 Study areshown in Figure 3-1 and will be referred to as follows:

Xc1: Compensation as a percent of the Keystone-Juniata 500 kV line inductivereactance

Xc2: Compensation as a percent of the Conemaugh-Juniata 500 kV line inductivereactance. Conemaugh-Juniata 500 kV line inductive reactance will be referredto as X line.

Xc3: Compensation as a percent of the Juniata-Alburtis 500 kV line inductivereactance

Figure 3-1. SSR study equivalent system showing the location of PJM series compensation.


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Conemaugh SSR Potential with Conventional Series Capacitors

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The Conemaugh station consists of two cross-compound pairs of units. Each unit hasseveral torsional modes of shaft vibration. The only modes that have a potential forinteraction with the transmission network are the first mode of the HP unit, which has afrequency of approximately 27 Hz, and the first and third modes of the LP unit (9.5 Hzand 19Hz, respectively). Other modes have very low interaction with the ac network.

For this analysis, conservative estimates are used for no-load and full-load damping ofthe torsional modes

The 1989 Screening Study evaluated the PJM system with 50% compensation values forXc1, Xc2, and Xc 3. The limiting case for Conemaugh is when the 500 kV line to Keystoneis out of service. This contingency results in a small risk of torsional instability underlight unit-load conditions. A greater SSR interaction exists when one Conemaugh cross-compound unit is radial to Juniata (i.e., Conemaugh-Keystone & Conemaugh-Hunterstown lines and one of the Conemaugh units are out of service). This radial caseis selected for purposes of the TCSC design study.

Figures 3-2 and 3-3 illustrate the calculated electrical damping, De, versus frequency forboth the HP and LP units of Conemaugh, for a number of contingencies. Figure 3-2 isfor the pre-contingency network, with and without 50% compensation in all threelocations.

Figure 3-2. De -vs- frequency for HP and LP #1 unit of Conemaugh. Pre-contingency network with and without 50% compensation in all three locations.


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Figure 3-3 illustrates electrical damping for the critical triple contingency case, whereone Conemaugh unit is left radial from Juniata. Only the 27 Hz mode of the HP unit isaffected. The 19 Hz and 9.5 Hz modes of the LP unit are low enough in frequency toavoid significant SSR interaction with the transmission system resonances.

Figure 3-3. De -vs- frequency for HP and LP #1 unit of Conemaugh. Crit ical conti ngency case where Conemaugh is radial from Juniata.

Due to the topology of this contingency network, the frequency of the critical electricalresonance is affected mostly by the external capacitors (Xc 1 and Xc 3). The SSR impact ofthis particular resonance on Conemaugh is amplified by Xc 2. Figure 3-4 illustrates thesensitivity of the SSR impact to the value of Xc 1 and Xc 3, with Xc 2 fixed at 50%. Note thefrequency of the resonance shifts significantly as the value of Xc 1 and Xc 3 are changed.


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3-4

Figure 3-4. De -vs- frequency for Conemaugh HP Unit #1. Critical contingency case whereConemaugh Unit #1 is radial from Juniata. Variations of Xc 1 & Xc3 with Xc2=50%.

The damping impact on the Conemaugh HP 27Hz torsional mode is illustrated as afunction of Xc 2 in Figure 3-5, for selected values of the external capacitors. Note that theworst case is always with Xc 1 and Xc 3 at 48%. For the purposes of this study Xc 1 and Xc 3 were set at 48% instead of 50% to exhibit maximum interaction with the 27 Hz torsional


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3-5

mode of Conemaugh HP. These results can be used to determine the maximum level ofXc2.

Figure 3-5. De -vs- series compensation XC2. Critical contingency case where ConemaughUnit #1 is radial from Juniata. Each curve represents a constant value for Xc 1%=Xc3% series

compensation.

Two criteria can be considered, each representing different degrees of conservatism.The most conservative approach would be to choose the point of zero electricaldamping (De = 0), which means that the inherent mechanical damping will bemaintained to guarantee stability. In this case, the maximum possible Xc 2 withXc1=Xc3=48% would be near 15% (point A on Figure 3-5). Another approach is to selectthe point where the dashed-dotted curve crosses the no-load (-D m,light load ) dampingline, which means that any perturbation to the torsional vibrations will causeoscillations without growth or decay. This criterion would permit Xc 2 to be as high as

25% (point B on Figure 3-5).For the contingency that isolates Conemaugh on the line to Juniata, the HP 27 Hztorsional mode is unstable at no load, and will be very close to a zero-margin situationat full load on the unit. Figures 3-6 and 3-7 illustrate both situations in the time domain.


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3-6

Figure 3-6. Transient response following a trip of the Conemaugh-Keystone line leavingConemaugh Unit #1 operating at l ight load radial from Juniata.


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3-7

Figure 3-7. Transient response following a trip of the Conemaugh-Keystone line leavingConemaugh Unit #1 operating at fu l l load radial from Juniata.


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3-8

These cases represent the transient response following a trip of the Conemaugh-Keystone line. The Conemaugh breakers are assumed already in a position to isolateone of the Conemaugh units onto the Juniata line after clearing the line to Keystone.Note the growth of the 27 Hz oscillations in the light-load case, as predicted by thedamping curve of Figure 3-4. This is an example of an SSR torsional instability. At fullload on the unit, the inherent mechanical damping is sufficient to overcome thenegative influence caused by the series capacitor, and the 27 Hz mode continues withnearly constant amplitude rather than growing.


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4-1

4 TCSC DESIGN CONSIDERATIONS

This section describes basic characteristics of the TCSC, and specific aspects of designthat are important for the SSR-mitigation objective of this study.

4.1 TCSC Operating Characteri stics

Figure 4-1 illustrates the power circuit of a single TCSC module. A TCSC moduleconsists of a series capacitor with a parallel path that includes a thyristor switch withsurge inductor. A complete TCSC system may be composed of several such modules inseries and may accompany a conventional series capacitor bank as part of an overallproject to aid system performance.

TCSC modules have three basic modes ofoperation;

thyristors blocked (no gating and zerothyristor conduction),

thyristor bypassed (continuous gating

and full thyristor conduction), and operating in vernier mode with phase

control of gate signals and subsequentpartial thyristor conduction.

The various modes of operation aredescribed in depth in [5].

Since thyristor control can alter the apparent 60 Hz impedance of the module, it isimportant to clarify certain terms:

Xc = "Nominal" reactance of capacitors only (@60 Hz), ohms

ILine = Line current magnitude

Xord = Net reactance of TCSC in per unit of X c (@60 Hz)

Figure 4-1. TCSC module power circuit.


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For example,

Xord = +1.0 pu means operation with no thyristor current

Xord = +1.5 pu means operation with thyristor firing such that the 60 Hz

component of capacitor voltage is 1.5*X c*ILine and lags current by 90 (capacitive). Or I c = 1.5 * ILine

Xord = -0.5 pu means operation with thyristor firing such that the 60 Hzcomponent of capacitor voltage is 0.5*X c*ILine and leads current by 90 (inductive)

The convention used here defines positive reactance as capacitive and negativereactance as inductive. Since we are evaluating a series capacitor, the capacitive value istaken as positive ohms.

XTCSC is defined as,

XTCSC (ohms) = X ord (pu) * Xc (ohms)

The total series compensation will be defined as:

XC = Xfixed + XTCSC

where, X fixed is the inductive reactance of the fixed (conventional) portion of the seriescapacitor installation.

4.2 SSR Damping Characteristics of TCSC

The natural response of a series compensated line involves an electrical mode ofoscillation between the voltage on the series capacitor and the current in the lineinductance. These oscillations are below the operating frequency, i.e., subsynchronous,when the line is less than 100% compensated.

Proper control of a TCSC can yield a beneficial damping influence on the

subsynchronous electrical modes of oscillation. This is illustrated by the exampleassociated with Figure 5-1 in Reference [6]. The sample system consists of two infinitebuses connected by an inductor in series with a capacitor representing a seriescompensated transmission line. If this system is stimulated by a minor disturbance, itoscillates at its natural frequency of 15 Hz. The line current and capacitor voltage are 60Hz modulated by the 15 Hz subsynchronous oscillations. If the series capacitor in theline is replaced by an equivalent TCSC operating in the vernier mode, the brief


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conduction periods of the thyristor valve prevent the capacitor from participating in theresonant oscillation. The TCSC looks effectively as a resistor at the subsynchronousfrequency and reduces the resonance [7]. The net response shows only 60 Hzwaveforms for capacitor voltage and line current.

There are a number of unfavorable aspects associated with these subsynchronousoscillations. The most widely recognized, however, is the interaction with turbine-generator torsional modes of vibration, commonly referred to as subsynchronousresonance, or SSR. The basic constant reactance control of the TCSC inherentlyprovides critical damping of the subsynchronous electrical mode. The SSR performancebenefits discussed in this application are the result of a specific control algorithm. GE'sproprietary TCSC control design is used in this report to demonstrate the benefits ofTCSC for SSR damping. A compensated system that consists entirely of TCSC can beshown to be SSR-neutral; the system will exhibit no resonances at subsynchronousfrequencies.

Recent field tests have confirmed these predicted performance characteristics [8] at theBonneville Power Administration's Slatt substation to demonstrate SSR mitigationutilizing TCSC.

Further simulation of systems with only part of the compensation being TCSC suggeststhat the TCSC acts similar to a resistive element in the circuit in terms of damping tooscillations. A small portion of TCSC with a total compensation system can be effectivein preventing the otherwise lightly damped electrical oscillations from causingundesirable behavior on the overall network.

4.3 Selection of Nominal X ord

When designing a TCSC, one of the criteria is to meet a certain effective reactance innominal conditions. A degree of freedom available to the designer is how this isachieved, in terms of X c and X ord . For example, a 10 ohm effective reactance can beachieved with an 8 ohm X c designed to operate with X ord of 1.25, a 5 ohm X c with X ord of2.0, or any other such combination.

For the specific case of using a small TCSC to help mitigate SSR caused by other fixedcapacitors in the network, past studies have shown that a design X ord between 1.5 and2.0 is a good selection. The higher degree of vernier operation provides more dynamicrange around the nominal operating point, and is at a more stable point on the TCSCcharacteristic than with operation with X ord below 1.5 or above 2.0.


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4.4 Special Considerations at Low Line Current

The TCSC is a unique application of thyristors, in that the voltage across the valvevaries substantially over the operating range. In nearly all other applications of

thyristor valves, the voltage is nearly constant, at most varying over a 2:1 range innormal operation. With the series compensation application the voltage will beproportional to line current, which can vary over a range exceeding 20:1. The challengeis to design a thyristor valve that has enough insulation to withstand the high-currentextremes (while retaining normal control), and yet can ensure secure gating at a voltageof only a few percentage of maximum. Thyristors have an inherent limit on how low avoltage such gating can be considered secure, so there will exist a point at which thethyristor valve must be blocked, and normal thyristor control will be lost.

This consideration leads to the operating strategy of deliberately blocking the thyristorvalves when line current falls below some predetermined level. Experience to datesuggests that the threshold for this action will be from 5% to 10% of the ratedcontinuous current.

When low-current blocking is active, the benefit of firing the thyristors for SSR dampingwill be lost. Therefore, for applications where SSR is a concern, the bypass breaker mustalso be closed when this low-current blocking action is taken to remove the seriescapacitors from the power circuit. Because the SSR oscillations take several seconds tobecome a problem, this bypass function can be designed to have a substantial delay.Since speed is not required, the control can be designed for robustness. Bypassing forextended low-current conditions should not present any problem for power system

operations, since at low current, the series compensation is not needed for its primarypurpose of improving transfer capability.


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5-1

5 TCSC/CONVENTIONAL HYBRID SERIES

COMPENSATION

To reach the desired 50% total compensation on the three selected PJM lines, TCSC canbe used in some proportion to mitigate the SSR problem defined in Section 3. Thissection addresses the design of a TCSC whose primary purpose is SSR mitigation.

Previous studies have shown that equipping all of the series compensation withthyristor control will avoid the problematic resonance situation, and result in an SSR-neutral system. For economic reasons, however, it is desirable to minimize the rating ofTCSC needed and design a hybrid system including both conventional seriescompensation and TCSC. The challenge is to determine the minimum amount of TCSCneeded to meet the application needs.

The following analysis focuses on quantifying the stability aspect of the application.The transient torque aspect is dealt with qualitatively.

5.1 TCSC Rating Selection

For each effective X TCSC in ohms, the nominal X ord [5] is a variable that can be selectedby the designer. Table 5.1-1 shows a selection of possible combinations to meet thedesired total compensation of 50% with varying fractions of TCSC and various choicesfor nominal X ord . The XTCSC value in the X TCSC/ Fixed ratio is given in percent of theConemaugh-Juniata line inductive reactance. This percentage convention for X TCSC willbe used throughout the remainder of this paper.


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TCSC/Conventional Hybrid Series Compensation

5-2

Table 5.1-1

Possible Conventional Series Capacitor and TCSC Choices for Xc2 = 50% as a Percentage of Conemaugh-Juniata Line (76.25 ohms)

XTCSC/Fixed Fixed X TCSC XC (ohms) @ Nominal X ord

(percent) (ohms) (ohms) 1.1 1.5 2.0

0/50 38.1 0 0 0 0

10/40 30.5 7.6 6.9 5.1 3.8

15/35 26.7 11.4 10.4 7.6 5.725/25 19.1 19.1 17.3 12.7 9.5

35/15 11.4 26.7 24.3 17.8 13.4

50/0 0 38.1 34.6 25.4 19.1

For this study, a nominal X ord of 1.5 is selected. The continuous rating of the seriesequipment is assumed to be 2500 amps.

5.2 Operating Conditions

A large number of factors must be considered in designing the control loops for theTCSC. Robustness must be guaranteed over a large range of operating conditions, andattention to this aspect is required prior to any evaluation of TCSC performance for SSRdamping. A number of power system network operating configurations and powerflows through the TCSC were evaluated in this project to ensure an adequate stabilitymargin on the inner control loops for the TCSC options studied.

For the transient simulations subsequently presented, the post-contingency scenario isthe case where Conemaugh #1 is isolated on the line to Juniata and is near minimumload (215 MW). In this case the line current is approximately 250 amps, whichcorresponds to approximately 10% of the TCSC rating. This will be considered a levelwhere TCSC firing control is able to operate.


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5-3

5.3 SSR-Mitigation Performance

Figure 5.3-1 illustrates the benefit of TCSC in terms of small-signal damping versusfrequency curves similar to those presented in Section 2. The different curves representdifferent ratios of X TCSC to conventional series capacitors. These are the same ratios as

defined in Table 4.1-1 with X ord constant at 1.5 pu.

These results for X ord at 1.5 pu are summarized in Figure 5.3-2. This shows the worstexpected damping on the critical first mode of the HP unit as a function of X TCSC (inpercent of the total line impedance).

Figure 5.3-3 illustrates a time-domain view of this improvement, which comparesresponses with different TCSC ratios for the same fault and line clearing event as shownin Figure 3-6. In each group of three, the top trace is for the 50% conventional seriescapacitor as a reference, the middle is for the 10/40 case, and the bottom is for the 25/25

case.


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5-4

Figure 5.3-3 illustrates the important points regarding the phenomena of interest andthe performance of the TCSC. Among the key points are the following:

1. The ringdown of electrical oscillations after the fault show two important impacts ofthe TCSC. These oscillations are an exchange of energy between the capacitors andthe inductance of the line, which is stimulated by the electrical disturbance. Theseare most readily observed in the time period from about 0.2 to 0.7 seconds in theplots, and are most evident in the voltage across the fixed capacitor ( Fixed Capacitor,kV ) portion. Note that the frequency of these oscillations significantly reduces as thepercentage of fixed capacitance decreases -- this indicates that the line resonance isbeing moved away from the torsional mode. Second, note that the decay rateimproves as the percentage of TCSC increases. This is a consequence of the TCSCappearing as a resistive component to the line resonances.

2. As shown by the HP Generator Speed curves, the torsional vibrations experience less

stimulation as the portion of TCSC increases. This is an indication that transienttorque amplification will be attenuated by the TCSC, so a trend similar to the impacton torsional damping can be expected.

3. As shown by the HP Generator Speedcurves, the torsional vibrations are betterdamped as the portion of X TCSC increases. This is consistent with the precedingdiscussion of damping based on frequency-domain analysis. These three curves areenlarged and shown for a longer time period in Figure 5.3-4 for closer inspection. Inthe case without TCSC, note the continued subsynchronous component in thecapacitor voltage. This is evidence of significant interaction with the turbine-

generator shaft vibrations. Even a relatively small amount of X TCSC providessignificant attenuation to this oscillation.


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5-5

Figure 5.3-1. Comparison of Conemaugh HP torsional damping with various X TCSC /Fixed capacitor ratios to maintain a constant Xc 2=50%. Post-contingency systemwith Conemaugh Unit #1 operating at low load and radial from Juniata. X TCSC at X ord =1.5.


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5-6

Figure 5.3-2. Expected damping of the critical first mode of the HP unit as a function of the X TCSC component of the total 50% compensation. Post-contingency system withConemaugh Unit #1 operating at low load and radial from Juniata.


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5-7

Figure 5.3-3. Initial transient response of selected variables. Responses follow a fault &trip of the Conemaugh-Keystone leaving Conemaugh Unit #1 operating at light load and radial from Juniata. Various X TCSC /Fixed ratios to maintain Xc 2=50%.


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5-8

Figure 5.3-4. Long term transient response of HP generator speed. Response follows a fault & trip of the Conemaugh-Keystone leaving Conemaugh Unit #1 operating at light load and radial from Juniata. Various X TCSC /Fixed ratios to maintain Xc 2=50%.


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5-9

5.4 TCSC/Fixed Series Capacito r Combination

As described in Section 3, various criteria can be applied when making a selection forthe ratio of X TCSC and fixed compensation. Two possible criteria noted in Section 3 are touse zero margin on no-load damping, or zero electrical damping. From Figure 4.3-1,

this would require an X TCSC that was approximately 17% or 21%, respectively, of the lineimpedance.

Consideration must be given to the possible need to bypass the TCSC under certainconditions (e.g., very low line currents for extended duration). In this condition, theremaining fixed compensation must be below the level at which SSR concerns wouldexist. These have been identified in Section 2, and are summarized in Table 5.4-1. Thetable also indicates the minimum portion of fixed compensation that must be bypassedwith the TCSC. Figure 5.4-1 illustrates the circuit concept.

Table 5.4-1TCSC & Fixed Capacitor Segments

SSR CriterionTCSC

(% of XLine)XF1

(% of XLine)XF2

(% of XLine)Total

(% of XLine)

De > No-Load 17 8 25 50De > 0 21 14 15 50

Figure 5.4-1. TCSC & Fixed Capacitor Switching Segments


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5-10

5.5 Enhanced SSR Mitigation with Modulation Control

The benefits previously described were realized with the basic "constant-reactance"mode of TCSC control, developed for the Slatt project [2]. A limited investigation has

been undertaken here to estimate the potential benefits of augmenting the control witha special modulation signal to enhance the damping of torsional vibrations.

Several factors are important when designing such a modulation control. One key isthat an input signal local to the TCSC be utilized, since secure transmission of a wide-band signal from a remote location would be a significant reliability concern. For thisevaluation, the input signal is derived solely from the line current flowing through theTCSC.

Another factor to consider is the robustness of such a control function. It must act in away that is at least neutral to other performance aspects of all power system operatingconditions. Important performance aspects include the impact of control on all turbine-generator unit torsional modes of vibration in the electrical vicinity, and the impact ofambient noise in the system on the increased sensitivity of the control system. For thisevaluation, robustness was checked for conditions with all-lines in service and for thecritical contingency. It is expected that the performance indicated in the followingdiscussion can be achieved while satisfying the robustness criteria, although significantstudy effort is needed to finalize the control design.

Figures 5.5-1 and 5.5-2 illustrates the beneficial impact of this modulation control. The10%/40% X TCSC/Fixed choice for TCSC rating was selected for this illustration, since

with constant-reactance control it is too small to remedy the SSR concern withConemaugh.

The first trace illustrates the base case, without modulation, which is the same aspresented in Section 4.3 of this report. HP generator shaft speed is shown in Figure 5.5-1, while TCSC firing advance angle ( ) is shown in Figure 5.5-2. Note that the firingangle is not constant in this base case, due to the inherent action of the control to hold aconstant-reactance effect.

The second and third plots in Figures 5.5-1 and 5.5-2 illustrate cases with modulation, attwo different gain settings. The gain in the second plot in each figure represents what islikely to be an acceptable level for a robust control. The gain in the third plot in eachfigure represents twice the expected acceptable level. Note the increased motion infiring angle in Figure 5.5-2 as modulation gain is increased. This indicates that thedynamic range of the TCSC control is being utilized to aid damping of the torsionalvibrations. The damping is seen in the improved decay rate of the 27Hz vibrations ingenerator speed shown Figure 5.5-1.


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5-11

Figure 5.5-1. Beneficial impact of modulation control. Transient response of HP

generator speed following a fault and trip of the Conemaugh-Keystone leavingConemaugh Unit #1 operating at light load and radial from Juniata. Various modulationgains.


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5-12

Figure 5.5-2. Beneficial impact of modulation control. Transient response of firing angle advance in degrees following a fault and trip of the Conemaugh-Keystone leavingConemaugh Unit #1 operating at light load and radial from Juniata. Various modulationgains.


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5-13

The expected benefit of such modulation control is summarized in Figure 5.5-3, wherethe modulation gain is selected as the value which is likely to satisfy the robustnesscriteria. With modulation, the size of the TCSC needed to alleviate the SSR concern issmaller than without modulation. For the example of De>No-Load criterion, includingmodulation would mean that the TCSC could be 12%, compared to the 17% needed

without modulation.

Figure 5.5-3. Expected benefit of modulation control. Electrical damping with modulation(bold line) and without modulation (thin line).


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6-1

6 OTHER MITIGATION METHODS

This section discusses other existing SSR countermeasures that are generator-based orseries capacitor bank-based. The TCSC case study system is used as a benchmark todemonstrate the potential of a series capacitor bypass filter for mitigating SSR. OtherSSR counter measures are discussed briefly and existing applications are noted.

6.1 Series Capacitor Bypass Filters

Capacitor bypass filtering (CBF), Figure 6-1, is a passive technique to short circuitsubsynchronous capacitor voltage while blocking the bypass for synchronous capacitorvoltage through parallel resonance.

Figure 6-1. Series capacitor bypass filter.

The SSR mitigation performance of a bypass filter was demonstrated with theConemaugh-Juniata 500kV series compensated system. Variations were made to thebypass filter capacitance and the filter resistance to see how they each affected the 27Hzsystem resonance that occurs when the lightly loaded Conemaugh units is left radial onthe series compensated line. The relationship between capacitance, capacitanceimpedance, and filter rating relative to the series capacitor are given in Table 6-1.


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Other Mitigation Methods

6-2

Table 6-1Series Capacitor Bypass Filter Elements

Cf Capacitance Xc f Impedance Xc f Rating(pu on C 2) (pu on Xc 2) (pu on Xc 2)

0 0.20 5 .20.50 2 .501.0 1 1.0

Figure 6-2 shows how the damping characteristics change as the filter capacitance, C f, isincreased from 0, to 20%, to 50%, and 100%, of the line series capacitance, C 2. The 60Hzbypass filter reactance impedance, X lf, is set equal to the 60Hz filter capacitanceimpedance, Xc

f, in each case.


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6-3

Figure 6-2. Series capacitor bypass filter on Conemaugh-Juniata 500kV line.Conemaugh Unit #1, at low load, is radial on the Juniata line. Series capacitor Xc

2is

fixed at 50% and all other external series capacitors are set to 48%. Filter resistance, R f = 2*Xc 2. Vary filter capacitor impedance, Xc f .

The weakness in the bypass filter is in protection of the lowest subsynchronousfrequency mechanical modes, typically below 20Hz mechanical (40Hz electrical). InFigure 6-2, the initial resonance at 27Hz (solid line) moves down to 25Hz as the filtercapacitance is increased to 20% of C 2. The initial resonance moves even further withincreasing C f.

The bar labeled -DM in each right-hand plot [b] identifies the 27Hz mode dampingfrom no-load (-.20) to full load (-2.0) for the Conemaugh unit. Figure 6-2[b] shows thatthe filter capacitance (C f) must be greater than half of the line series capacitance (C 2) toachieve marginal damping of the 27Hz mode. This can be seen by comparing thedashed line with the dash-dotted line. The filter size (rating, costs) increase withdecreasing torsional frequency.


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6-4

The initial resistance was chosen as R f = 2*Xc2. Figure 6-3 shows how the dampingcharacteristics change as the filter resistance, R f, is varied from 5x, 2x, 1x, and .5x theimpedance value of the series capacitance, Xc 2. The bypass filter capacitanceimpedance, Xc f, was set equal to 2*Xc 2. As show in Figure 6-3[a], fine tuning the bypassfilter moves the resonance point to less than 20Hz. Setting filter resistance equal to or

less than the impedance value of the series capacitance avoids the instability at 27Hz.However, if there were a torsional mode near 20Hz, then the 20Hz mode would beadversely affected by the lower values of R f.

Figure 6-3. Series capacitor bypass filter on Conemaugh-Juniata 500kV line.Conemaugh Unit #1, at low load, is radial on the Juniata line. Series capacitor Xc 2 is

fixed at 50% and all other external series capacitors are set to 48%. Filter capacitance, Xc f = 2*Xc 2. Vary filter resistance, R f .


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6-5

This cursory analysis shows that this SSR mitigation scheme has potential for movingthe resonance point away from the critical frequency. An initial bypass filter designwould include the following components relative to the series capacitor Xc 2:

Xcf Impedance Xc f Rating X lf Impedance R f Impedance(pu on Xc 2) (pu on Xc 2) (pu on Xc 2) (pu on Xc 2)

2 .50 2 .5

These results reflect a single system condition with the Conemaugh unit at low loading.Stronger conclusions can be drawn only after other system configurations anddispatches have been examined.

6.2 Discussion of Other SSR Countermeasures

Since the early 1970s there have been many technical papers published on the subject ofSSR. The IEEE Torsional Issues Working Group has recently published the fourthsupplement to A Bibliography for the Study of Subsynchronous Resonance BetweenRotating Machines and Power Systems [10]. This section provides a short descriptionof other SSR countermeasures. These countermeasures have been segregated by thosetechniques that are unit-based and those techniques that are series capacitor-based.

The following countermeasures are installed at the unit that is at risk for SSR.

Series Blocking Filter - This is the only complete solution to the transient torque problem athigh levels of series compensation. The filter is installed in series with the generator byplacing it in the neutral end of the generator step-up (GSU) transformer high-side. This is atuned filter with inductance and capacitance in parallel with one filter stage for each torsionalmode. This type of filter has been in service at the Navajo Plant in Arizona for 20 years [11].

Supplementary Excitation Damping Control (SEDC) - This controller modulates generatorfield voltage to add torsional damping. The SEDC control [12,13] is useful for moderatedestabilization problems or as a supplement to a Series Blocking Filter. The SEDC requires a

high response excitation system and requires measured shaft oscillations as an input signal.This controller is in operation at the Navajo, Jim Bridger, and Coronado generating units in theWestern USA.

Synchronous Machine Frequency (SMF) Relay and Digital Pulsation Relay (DPR) - Theserelays provide protection by tripping the unit for excessive torsional oscillations. Shaftoscillations of the unit are monitored directly. These relays are used as backup protection only,


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6-6

not primary protection. The SMF and DPR are also used in cases where an SSR problem occursonly after multiple contingencies. These relays are installed on many units [14,15].

Subsynchronous Oscillation (SSO) Relays - The SSO relays provide protection by tripping theunit for excessive subsynchronous frequencies in armature current. However, it is difficult tocoordinate armature current frequency content to torsional oscillations. Therefore, these relaysare prone to false trips. Like the SMF and DPR, these relays are intended for backup protectiononly, not primary protection. The SSO relays are also used in cases where an SSR problemoccurs only after multiple contingencies.

Shunt Damping Controls are known as Torsional Dynamic Stabilizers [16], Shunt ReactorStabilizers, and Static VAr Stabilizers. These controls are essentially a reactor installed in shuntwith the generator either on the high-side or low-side of the GSU. One dynamic stabilizer hasbeen in service on one unit at San Juan for about 15 years [17].

Series Damping Controls - A separate thyristor control device is placed in the high-side

neutral end of the GSU. The thyristor bridge is in series with the generator and is gated inresponse to measurement of shaft oscillations. There is additional inductance in parallel withthe thyristor device. There are no known prototype or in-service applications.

Torsional Vibration Monitoring Systems (TVMS) and Torsional Stress Analayzer (TSA) -While not strictly protection, these devices provide information on the torsional performance ofthe unit and other protective devices. They can be used to provide post disturbanceinformation on shaft torques, shaft fatigue, and torsional damping.

The following countermeasures are installed at the series capacitor bank:

Dual or Low Gap Spark Over - These gaps remove the series capacitors during transients atlower levels than the normal Spark Over Gap voltage thereby reducing the transient torqueproblem in the generators. Low Gaps are not in service during capacitor bank insertion. Adisadvantage of this countermeasure is that it removes the series capacitors when they are mostdesired. These gaps have been installed on many capacitor banks in WSCC since he mid 1970s.

Metal Oxide Varistors (MOV) Protection - MOVs in parallel with the series capacitors limitthe voltage on the capacitors during transients while the series capacitors remain in service.Limiting the voltage reduces the transient torque problem on the generators. MOV protectionhas been installed on some new series capacitor banks since the late 1980s.

Transfer Tripping / Capacitor Insertion Schemes - Several series capacitor installations in theUSA only allow the series capacitors to be inserted when line flows and/or generation levelsrequire them. At Jim Bridger in Wyoming, all units on-line must be above 50% load before thehigh levels of series capacitors are used. Other schemes will bypass series capacitors uponcertain line contingencies that expose the generation to SSR problems.

Capacitor Bank Filters - Different filters, with resistors and/or inductors, for series capacitorshave been analyzed over the years for SSR stability control. These filters have also been studied


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6-7

with special MOV devices to provide transient torque control. None of these filters has beeninstalled.

Phase Unbalanced Series Capacitors - A phase unbalance in the series capacitors can beachieved by having additional capacitors offset at 60 Hz with additional series inductance.This scheme has been shown to reduce the peak SSR undamping. The trade-off is that thedestabilization is then spread over a broader frequency range. There have been no actualinstallations.


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7-1

7 CONCLUSIONS

This simulation study demonstrates the potential for SSR suppression by using thyristorcontrol of only a portion of a series capacitor system. This approach offers the potentialof significantly lower equipment cost relative to the cost of using thyristor control of theentire installation. Additionally, adding special modulation to the TCSC control showspromise for further reducing the required size of the thyristor-controlled portion, andtherefore total cost. It should be noted that the study case was only one of severalcontingencies with potential to cause SSR within the study system.

Eigenvalue analysis and time simulations, whose results were in essential agreement,were performed for the PJM 50% series compensation application using specialsimulation tools developed by GE. Simulations modeled the thyristor control algorithmdeveloped for the EPRI-sponsored TCSC demonstration at BPA's Slatt Substation, plusspecial modulation controls developed by GE after the Slatt installation was completed.

Simulation results show that, without thyristor control and approximately 50%compensation of three lines, loss of the Keystone-Conemaugh line with one Conemaughgenerator and the Conemaugh-Hunterstown line out of service produced an unstableSSR interaction with the first torsional of the Conemaugh HP unit. Thyristor control ofonly a portion of the Conemaugh-Juniata series compensation using the constant-impedance control scheme of the Slatt installation sufficiently mitigated the potentialfor SSR within the context of the contingency studied. Simulating a superimposedfiring modulation scheme for special SSR damping further increased the effectiveness ofthe thyristor control.

While this study shows a potential for use of special SSR damping controls, adaptationto any of a set of possible contingencies would require further study. It is possible thatthe usefulness of special controls will depend on the richness of the potential SSRinteractions within a particular case or within particular contingencies of that case.

An important consideration in the application of such technology to mitigate SSR is theconfidence in study results. One aspect of such a study is the ability to foresee and testall contingencies which affect the interaction with generator shaft torsional modes. Anunderstanding of the key issues permits this to be accomplished with confidence.Another key aspect is the confidence in the modeling of the power system, thegenerators, and the TCSC. The former has been well established over the past twodecades, in both time-simulation and eigenvalue domains. The TCSC model can be


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Conclusions

7-2

constructed from basic principles for both time-simulation and eigenvalue studies aswell, and the simulation tools used for this analysis have been proven versus resultsfrom a real-time simulator and from the field. Thus, the basic aspects required forconfident application exist and such a concept should be considered a realistic optionfor the future.

The PJM series compensation case, studied here, has illustrated a number of features forSSR suppression by thyristor control. This case provides a useful benchmark forstudying candidate control schemes for SSR suppression in highly interconnectedsystems. This report also illustrates the use of the benchmark case with a passivefiltering scheme; other known SSR suppression techniques are described briefly.


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8-1

8 REFERENCES

[1] C.E.J. Bowler, M.D. Kankam, R.E. Stickle, G.E. Boukarim, H.Othman, N.Rostamkolai, "SSR Screening Study and Torsional Test Analysis, PJM 500 kV SeriesCompensation Proposal for Public Service Electric and Gas of New Jersey," FinalReport, General Electric Company, June 7, 1989.

[2] S. Nyati, C.A. Wegner, R.W. Delmerico, D.H. Baker, R.J. Piwko, A. Edris,"Effectiveness of Thyristor Controlled Series Capacitor in Enhancing Power SystemDynamics: An Analog Simulator Study," IEEE Trans. PwrDelivery, vol.9, No.2,April

1994, pp1018-1027.[3] C.E.J. Bowler, "Series Capacitor Based SSR Mitigation Prospects," EPRI FACTS

Workshop, Cincinnati, Ohio, Nov. 14-16, 1990.

[4] C.E.J. Bowler, M.D. Kankam, "Subsynchronous Resonance Analysis - Case Study -A Systematic Approach to SSR Planning Study," Presented at Pennsylvania ElectricAssociation, System Operating Committee Meeting, Philadelphia, February 2-3,1989.

[5] E.V. Larsen, K. Clark, S.A. Miske, Jr., J. Urbanek, "Characteristics and RatingConsiderations of Thyristor Controlled Series Compensation," IEEE Trans.PwrDelivery, vol.9, No.2, April 1994, pp. 992-1000.

[6] E.V. Larsen, C.E.J. Bowler, B.Damsky, S.Nilsson, "Benefits of Thyristor ControlledSeries Compensation," CIGRE paper 14/37/38-04, 1992.

[7] W. Zhu, R. Spee, R.R. Mohler, G.C. Alexander, W.A. Mittelstadt, D. Maratukulam,"An EMTP Study of SSR Mitigation Using the Thyristor Controlled SeriesCapacitor," I EEE PES Paper 94-SM-477-0-PWRD . San Francisco, CA, July 1994.

[8] R.J. Piwko, C.A. Wegner, B.C. Furumasu, B.L. Damskey, J.D. Eden, The SlattThyristor-Controlled Series Capacitor Project - Design, Installation,Commissioning, and System Testing, CIGRE paper presentation at 1994 session inParis.


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References

8-2

[9] A.T. Hill, E.V. Larsen, E. Hyman, Thyristor Control for SSR Suppression - A CaseStudy, Presented at Flexible AC Transmission System (FACTS 3): The Future inHigh-Voltage Transmission, Sponsored by Electric Power Research Institute,October 5-7, 1994 Baltimore, Maryland.

[10] A Bibliography for the Study of Subsynchronous Resonance Between RotatingMachines and Power Systems, IEEE Transactions on Power Apparatus and Systems,Vol. PAS-95, No. 1, pp. 216-218 , Jan.-Feb. 1976.

[11] J.F. Tang, J.A. Young, Operating Experience of Navajo Static Blocking Filter,IEEE PES Special Publication 81TH0086-9-PWR, pp. 23-26.

[12] C.E.J. Bowler, D.H. Baker, Concepts of Supplementary Torsional Damping byExcitation Modulation, IEEE PES Special Publication 81TH0086-9-PWR, pp. 64-69.

[13] C.E.J. Bowler, R.A. Lawson, Operating Experience With Supplemental ExcitationDamping Controls, IEEE PES Special Publication 81TH0086-9-PWR, pp. 27-33.

[14] C.E.J. Bowler, J.A. Demcko, L.Mankoff, W.C.Kotheimer, D.Cordray, The NavajoSMF Type Subsynchronous Resonance Relay, Paper F78-253-7, presented at theIEEE PES 1978 Winter Power Meeting.

[15] C.E.J. Bowler, L.Mankoff, Experience With SMF Type Subsynchronous ResonanceRelays IEEE PES Special Publication 81TH0086-9-PWR, pp. 43-46.

[16] D.G.Ramey, I.A. White, J.H.Dorney, and F.H. Kroening, Application of theDynamic Stabilizer to Solve an SSR Problem, Proceedings of the American Power Conference, Vol. 43, 1981, pp. 605-609.

[17] D.G. Ramey, D.S. Kimmel, J.H.Dorney, F.H.Kroening, Dynamic StabilizerVerification Tests at the San Juan Station, IEEE Trans. on Power Apparatus andSystems, Vol. PAS-100, No. 12, Dec. 1981, p. 5011-5019.

[18] First Supplement to a Bibliography for the Study of Subsynchronous ResonanceBetween Rotating Machines and Power Systems, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-98, No. 6, pp. 1862-1975 , Nov.-Dec. 1979.

[19] The Second Supplement to a Bibliography for the Study of SubsynchronousResonance Between Rotating Machines and Power Systems, IEEE Transactions onPower Apparatus and Systems, Vol. PAS-104, No. 2, pp. 321-327 , Feb. 1985.

[20] Third Supplement to a Bibliography for the Study of SubsynchronousResonance Between Rotating Machines and Power Systems, IEEE Transactions onPower Systems, Vol. PWRS-6, No. 2, pp. 830-834, May 1991.


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A-1

A CONEMAUGH GENERATOR #1 TORSIONAL

CHARACTERISTICS

The following tables document torsional vibration modes considered in this study forthe two units of the cross-compound station. As noted in the tables, only mode 1(27.3Hz) of the HP unit and modes 1 (9.5Hz) and 3 (19.1Hz) of the LP unit havesignificant interaction with the electric transmission network.

The tables include conservative estimates of equivalent dashpots based on an amplitudedecay rates of:

NL = -.02 sec-1 @ no load

FL = -.2 sec-1 @ full load

These represent the lower bound of those seen from field experience over a wide range

of turbine-generator units.

Table A-1HP Unit Torsional Modes

Base = 545MVA

Mode Frequency(fi - Hz)

ModalInertia

(Mi - pu)

RelativeGenerator

Motion(QGi - pu)

QGi2

_____ 2 Mi

No-LoadDashpot

(DNLi - pu)

Full-LoadDashpot

(DFLi - pu)

1 27.3 0.344 .263 .101 .20 2.02 48.2 .0182 .015 .0062 3.2 32

3 55.6 .0490 .012 .0015 13 1304 56.4 .0156 .014 .0063 3.2 32


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Conemaugh Generator #1 Torsional Characteristics

Table A-2LP Unit Torsional Modes

Base = 495MVA

Mode Frequency(fi - Hz)

ModalInertia

(Mi - pu)

RelativeGenerator

Motion(QGi - pu)

QGi2

_____ 2 Mi

No-LoadDashpot

(DNLi - pu)

Full-LoadDashpot

(DFLi - pu)

1 9.5 1.04 .39 .0731 0.27 2.72 12.4 .014 .0051 .0009 22 220

3 19.1 5.82 .92 .0727 0.27 2.74 53.2 .0058 .00013 small large large5 58.1 5.4 .083 .0006 3.3 33

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