A static var compensator (SVC) combines thyristor-controlled reactors, capacitor banks and filters. IEEE C37.99-2000 (clauses 9.3 and 9.4) says its protection is divided into zones and that the guide applies to the fixed or thyristor-switched (TSC) banks that are part of the SVC. TCR harmonics call for true rms relays for the reactors and peak-measuring relays for the capacitors.
Short answer
A static var compensator, or SVC, compensates reactive power quickly to hold the voltage profile and improve system stability. In industrial installations, the SVC with filters also reduces voltage flicker. Its capacitors are part of the equipment: fixed banks or thyristor-switched capacitor (TSC) banks, together with thyristor-controlled reactors (TCR) and harmonic filters.
IEEE C37.99-2000 (clauses 9.3 and 9.4) says that SVC protection combines conventional relays with functions of the SVC control system itself, and that the guide applies equally to the fixed or TSC banks integrated into the SVC. Clauses 7 and 8 of the standard apply to them, plus the additional methods of Figure 44 and Table 14.
What an SVC consists of
The typical SVC of the standard (Figure 43) has:
- Main breaker and main reactor with a thyristor switch.
- Capacitor banks with their breaker.
- Filter reactors and harmonic filters of the 3rd, 5th and 7th order.
- Current and voltage transformers.
- Electronic controller.
Protection is organized by zones: the step-down transformer, the low-voltage buses, the reactor branches, the capacitor branches, the filters and the thyristors. Conventional relays cover part of them and the control protection functions cover the rest.
Protections suggested by the standard
Table 14 of IEEE C37.99-2000 summarizes the suggested methods:
| Zone | Device | Function | Standard’s note |
|---|---|---|---|
| TSC | 60 C | Unbalance | Unbalance measurement with H-bridge |
| TSC and TCR | 50/51 | Overcurrent | Branch faults and overloads of the limiting reactors |
| TCR | 46 | Negative sequence | Unbalance |
| TSC and TCR | 60 | Zero-sequence unbalance between phases | Unbalance, instead of the 46 |
| TSC | 59 | Overload | Capacitor overvoltage measured with the current |
| TSC and TCR | 50N | Ground overcurrent | Branch faults |
Figure 44 shows the capacitor unbalance in TSC mode: one unbalance relay in each leg of the delta (60-1, 60-2 and 60-3) and a negative-sequence protection (46) connected to the current transformers.
What is specific to the TSC: switching and harmonics
Series reactor and valve failure
TSC banks have air-core series reactors, which limit the inrush current produced by thyristor switching. The standard asks that the selection and setting of overcurrent consider a firing failure of the TSC valve, with the magnitude and duration of the inrush and outrush currents. They must also be coordinated with the other control functions.
Harmonics
Harmonics are an important factor in protecting capacitors in TSC mode. They are generated by thyristor-controlled reactors (TCR) and by other unusual distortions, such as those produced by geomagnetic disturbances. The standard asks that the types and settings of overcurrent and overvoltage protection consider those effects. Specifically:
- True rms overcurrent relays to protect the limiting reactors against overloads.
- Voltage relays that measure the peak value to protect the capacitors against overvoltages.
To measure the distortion produced by thyristors you can use our harmonic distortion (THD) tool.
SVC filters
The switching elements of the SVC produce harmonics that may require filters. A six-pulse unit has three TCRs connected in delta. In balanced operation, it injects odd harmonics into the system: the 5th, 7th, 11th and 13th.
Filtering effectiveness depends mainly on the system impedance, which is represented by the R-X locus. That impedance can determine the need for additional filters, single or double tuned. At system frequency, the filters behave as a capacitor bank and generate reactive power.
A filter bank includes several protection zones. The standard lists the representative functions of one of them:
- Differential.
- Phase and ground overcurrent.
- Overvoltage.
- Capacitor unbalance.
- Overload.
Some of the methods in clauses 7 and 8 apply to these filters. But unbalance protection can be more demanding depending on the filter configuration (tuned to one frequency or to several) and the voltage of the system it is connected to. The details of those protections, including single- and multi-frequency filters, are in clauses 9.1 and 9.2 of the standard.
The protection of a tuned filter is detailed in harmonic filter protection.
What is taken from the rest of the standard
Because the guide applies equally to the SVC banks, what it says about a shunt bank remains valid for their capacitors:
- The unbalance protection of clause 8, according to the bank connection and the fuse type, with the natural unbalance of the bank and of the system that the signal must distinguish.
- Protection against system overvoltage, which removes the bank when voltage exceeds what the capacitors withstand. The supporting function is explained in overvoltage protection 59.
- Bank breaker failure and surge arresters, with the same criterion as in a substation bank.
The difference is that the SVC measures and controls the bank with its own control system, so the conventional protections and those of the control must be coordinated with each other.
From clauses 7 and 8, the bank voltage protection (59, 59B and 27B), the overcurrent and breaker failure in a bank and, for capacitor switching, switching a bank apply.
Common mistakes
- Overlooking harmonics in a TSC. TCRs and switching change the current and voltage the capacitors withstand.
- Measuring the wrong quantity on each element. The standard asks for true rms for the limiting reactors and peak measurement for the capacitors.
- Setting overcurrent without considering a valve failure. The resulting inrush and outrush current must be coordinated with the control.
- Designing the filter without the system impedance. Filter effectiveness depends on the R-X locus.
What we do at Tiza Group
In our engineering design and studies service we analyze the harmonics and impedance of your network to size filters and define the capacitor protections. We work remotely, worldwide. Contact us and we will quote the study.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Harmonic filter protection per IEEE C37.99
- AccuSine and PQF active filters, SVC and STATCOM
- Harmonic filters: IEEE 1531 criteria
- Resonance in capacitor banks: why they fail
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 9.3 and 9.4, Figures 43 and 44, Table 14.
- IEEE Std 1531, IEEE Guide for Application and Specification of Harmonic Filters.
Frequently asked questions
What is a TSC and why does it have a series reactor?
It is a capacitor bank that is connected and disconnected with thyristors. According to IEEE C37.99-2000, TSC-mode banks include air-core series reactors to limit the inrush current produced by thyristor switching.
Which relay measures the overvoltage of a capacitor in an SVC with harmonics?
The standard calls for voltage relays that measure the peak value to protect the capacitors, and true rms overcurrent relays to protect the series limiting reactors from overload.
Why does an SVC need harmonic filters?
Because the switching elements of the SVC produce harmonics. With three delta-connected controlled reactors (a six-pulse unit), odd harmonics such as the 5th, 7th, 11th and 13th are injected. The need for a filter and its type depend on the system impedance.