Identification of ferroresonance as the cause of SVC instability in a degraded series compensated network

This paper identifies ferroresonance as the cause of an instability case of a static VAr compensator (SVC) used in a degraded configuration of the Hydro-Quebec series compensated network. An electric circuit, which has a sub-synchronous resonance and a hyper-synchronous one, is used as a model for the network. The SVC is composed of a power transformer which supplies a thyristor-controlled reactor (TCR) and a bank of thyristor-switched capacitors (TSC). The analogical simulations, which were realized at the IREQ Transients Network Analyzer (TNA) in order to study the stability of the SVC, are presented. An instability is illustrated and analyzed. In this paper, it is shown that during the instability, the SVC transformer is magnetically saturated. A sub-synchronous voltage oscillation at the SVC terminal is identified as the cause of the magnetic saturation of the SVC transformer. It is proved that the cause of the instability is the ferroresonance of the SVC transformer. Finally it is shown that this ferroresonance is a quasi-periodic one