Behavior of Protective Relays During Subsynchronous Resonance in Transmission Line and Adaptation of Generator Out-of-Step Protection

Abstract

Subsynchronous resonance (SSR) is a phenomenon in which electrical energy is exchanged between the generator and the transmission system below power frequency. SSR occurs due to the interaction of a series-compensated transmission line with a generator. It results in shaft oscillation and out-of-step (OOS) condition. During SSR, the magnitude of voltage and current is increased. It also increases probable occurrence of ferroresonance. It is obviously clear that protective relays are affected in such conditions. In this article, Manitoba Hydro electrical network is examined with series capacitors by PSCAD/EMTDC to investigate the impact of SSR on the operation of different types of protective relays. SSR is mostly concerned with transmission lines. In addition, swing characteristic of impedance during SSR causes activation of the OOS blocking (OSB) element of distance relay and OOS protection of generator. Hence, an adaptive algorithm based on subharmonic measurement and ferroresonance analysis in the time domain is proposed for OOS characteristic of generator protective relay to recognize SSR condition and make a decision on behavior of the function. Finally, the algorithm is examined in SSR condition to certify the correct operation.