STATIC VAR COMPENSATOR (SVC) CAPACITOR PROTECTION

SVCs are used in strategic installations to rapidly compensate reactive power for maintaining an acceptable system voltage profile and for improving the overall stability of the power system. Voltage flickers are also reduced in industrial applications when SVCs and harmonic filters are provided.

An example of an SVC installation is illustrated in Figure 1.

Figure 1—Typical SVC

Protection of SVCs is provided by a combination of conventional protective relays and protective functions contained in the SVC control system (IEEE Power System Relay Committee [B7]). These functions are made up of a number of zones to include the SVC step-down transformer, low-voltage buses, reactor branches, capacitor branches, filters, and thyristors.

This guide applies equally to the protection of the fixed or thyristor-switched capacitor (TSC) banks provided as an integral part of an SVC installation. In this regard, Clause 7 and Clause 8 describe various protection methods that can be applied by conventional relays to shunt capacitor banks along with additional methods illustrated in Figure 2 and Table 1.

Figure 2—Capacitor unbalance protection in the TSC mode

Table 1—Suggested SVC protection methods

SVC capacitor banks in the TSC mode are provided with air-core reactors connected in series to limit the inrush generated from thyristor switching. The selection and setting of the overcurrent protection should consider a misfire in the TSC valve (accounting for the magnitude and time duration of inrush and outrush currents) and should coordinate with other protective functions provided in the controls.

Harmonics are an important factor to consider in the protection of capacitors in the TSC mode. Adequate overcurrent and overvoltage protection types and settings shall be provided to counter the effect of harmonics generated by the thyristor-controlled reactors (TCRs) and by other unusual harmonic distortion such as those resulting from geomagnetic disturbances (Benmouyal et al. [B3]).

In this regard, true rms-based overcurrent relays are required to adequately protect the series limiting reactors against overloads while peak-measuring voltage relays should be provided for the protection of capacitors against overvoltages.