Lessons Learned in Static VAR

Compensator Protection

Aaron Findley, Mychal HoffmanPOWER Engineers, Inc.

Dan Sullivan, Jan Paramalingam Mitsubishi Electric Power Product Inc.

Presented by: Aaron Findley

i. Introduction to SVC’s

ii. SVC components

iii. SVC protection and lessons learned

iv. Summary

v. Questions

Presentation Contents

Introduction to SVCs

• Static var compensators are shunt-connected var generators or absorbers• Outputs are varied to control specific reactive power flow• Comprised of capacitors an reactors that are placed in and out of service using power electronic switching devices• Term “static” no moving or rotating main components

Introduction to SVCs

The common types of reactive power devices that make up all or part of static var system include:• Thyristor-switched reactor (TSR)• Thyristor-controlled reactor (TCR)• Thyristor-switched capacitor (TSC)• TSCs and TCRs are commonly used

Introduction to SVCs

• Capable of controlling individual phase voltages of the buses to which they are connected• They can be used for control of negative-sequence as well as positive-sequence voltage deviations• Primarily used for three phase control of the power system

Introduction to SVCs

• Ideally suited for rapid control of voltage• They provide faster response time than mechanical switched devices• They Provide continuous and smooth control of VARS

Components of an SVC

Components of an SVC system

TSC

TSC: Thyristor Switched Capacitors,• Capacitor banks switched on and off by using Thyristors• In three phase applications, the units are connected in delta• Integral cycle control is used where a change can be made every half cycle• Avoid switching when bus an capacitorvoltages are unequal

TSC

TCR

• A reactor in series with a bidirectional switch• Anti‐parallel thyristors conduct on alternate half‐cycles depending on firing angle• Reactors are switched on for a controllable fraction of every half cycle• Will require AC filtering because the non‐sinusoidal current draw results in harmonics.

TCR

Relay event report showing TCR firing delay. The reactive current magnitude decreases as the firing delay increases

Harmonics

4 sin cos cos in

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Where:V is the system voltageXl is the inductive reactancea is the delay anglen is the harmonic.

• Harmonics are a function of the firingDelay angle.

• Note: tripling harmonics are also Created but canceled in the delta Connection.

Filtering Considerations2 2

∗

Where:R is the sampling rate in samples/cycle    

• Most relay elements operate on filtered 60Hz current.

• Consider RMS based overcurrent elements for filter branches if  sensitive protection is needed.

• Often components are oversized

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TSC/TCR Protection

• Unbalance (TSC Capacitors)• Differential• Overcurrent• Over/Under Voltage

Classic Differential Protection

• Traditional percentage differential protection 

• Three differential zones, one for each phase of the protected equipment

1 2

| 1| | 2|

1 , | 2|

TSC/TCR Differential Protection

Each phase of the differential element wraps one valve and associated reactive element.

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Valves are housed indoors, reactive components are typically in the yard.

The delta connection often installed in a phase‐over‐phase configuration at the control building interface

Increases the probability of phase‐to‐phase faults in the delta connected TCR or TSC. 

TSC/TCR Physical Considerations

TSC/TCR Current ProtectionOvercurrent settings and time delays must be provided by the manufacturer.

Each SVC is a custom design, made to order. 

Manufacturer input is critical.

Event: TSC Fault

Fault occurred when ice formed on the animal guards resulting in a short circuit between phases.

Connected B and C phases through the capacitor bank. 

The differential element was blocked by harmonic blocking logic

TSC Fault, Filtered Report

TSC Fault, Unfiltered Report

Purpose of Harmonic Blocking

. Harmonic blocking and restraint are based on detecting the signature harmonics generated by the saturation of a ferromagnetic core.

Inrush is not an issue in air core reactors

TSC Fault, Harmonic Blocking

. Inrush currents or other transient currents will travel through the zone of protection and be accounted for.

No additional restraint or blocking logic is required

TCR Controlled Shutdown

For less critical failures the SVC is taken offline in a more gradual “controlled” stop

The TCR valves are placed into full conduction for several cycles to allow the TCRs to quickly discharge the energy stored in the filter banks

• Discharge step is required due to the capacitive nature of filter banks, and the desire to be able to quickly bring the SVC back online

• Useful to quickly re‐configure in a degraded mode following a fault

TCR Controlled Shutdown

. 

TCR Trip During Shutdown

. • The trip/event was triggered during the discharge phase of the shutdown when the TCR valves were fully conducting.

• The relay measured a negative‐sequence current magnitude of approximately 30% of the rated current.

• Blocked conduction of the TCRs prematurely, latched a trip preventing any degraded mode operation.

TCR Trip During Shutdown

TCR Trip During Shutdown

• Depending on the SVC control design, significant unbalance currents may flow during a shutdown even after the breaker has opened. 

• Nuisance trips that occur during a controlled shutdown can interfere with SVC operations, and halt auto‐reconfiguration schemes.

• Negative‐sequence elements protecting SVC branches should consider any intentionally unbalanced operations, either due to un‐symmetrical operation or filter bank discharge.

TCR Turn to Turn FaultNegative‐sequence alarms indicated that the fault may have developed minutes before finally evolving to include an additional phase. 

TCR Turn to Turn Fault

TCR Turn to Turn Fault

• Turn to turn faults in air core reactors are notoriously difficult to detect. The change in current measured external to the reactor can be relatively small depending on the location of the fault.

• The reason turn‐to‐turn faults are so damaging can best be understood by considering the ideal transformer model.

TCR Turn to Turn Fault

• Negative‐sequence current elements can detect some of turn‐to‐turn faults but are less sensitive to lower level faults.

• For some SVCs, sensitive unbalance protection could be possible by summing the delta currents directly to measure the reactor unbalance current This approach cannot be applied on SVC’s that are intended to operate unsymmetrically

• The lesson learned from this event is to be aware of the limitations of the applied protection scheme, depending on the SVC design a high degree of sensitivity for all fault types may not be practical.

Harmonic Filter BanksOften have more than one filter protected by a single relay. 

This could require some creative logic to differentiate between filters, which will likely have unique alarm an pickup values.

Standard Unbalance Protection is used

Harmonic Filter Banks• Series/parallel combinations 

of resistors, capacitor banks, and tuning reactors can be applied on a single branch to improve the transient performance of an SVC. 

• In this case blown fuses in the LC circuit capacitor banks are detected by measuring the 60Hz current in the resistive branch.

• It may take several minutes for resistor to reach rated impedance at temperature.

• SVC protection involves atypical applications of traditional protective relay schemes.

• The protection engineer understand both the intended purpose of each algorithm as well as the context in which they are being applied.

• Protective relays must coordinate with SVC controls and consider intentionally unbalanced SVC operational modes.

Summary

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