A Tale of Two Out-of-Phase Synchronizing Events at BC Hydro

This paper tells the tale of two out-of-phase synchronizing (OOPS) events that occurred at BC Hydro. A generator was synchronized 180 degrees out-of-phase without staff awareness of the faulty synchronization. During testing, the staff suspected a poor synchronization, after which the transient event records were investigated. The investigation concluded that the reference voltage signal was inverted because of a wiring error in the auxiliary voltage transformer (VT) circuit, which provided a common input to the autosynchronizer, synchroscope, and synchronism-check relay. The 16 kV generator breaker that is normally used to sync-close the unit to the system was equipped with OOPS protection, but the protection was not enabled because the breaker was closed before the event. A 500 kV breaker was used for synchronization; however, it was not equipped with OOPS protection, because the protection was not expected to perform well with high bus current ratings. During the OOPS event, many elements picked up, but none tripped. For instance, the loss-of-field and current unbalance elements asserted. However, because of their long time delays, these elements did not trip. The out-of-step protection qualified this disturbance as an event that did not cause an unstable power swing. The protection performed as designed, but it did not indicate poor synchronization. This paper provides an analysis of the different generator protection elements and discusses considerations for dedicated OOPS alarming and protection. This paper also discusses the follow-up diagnostics that were performed and provides recommendations to prevent the occurrence of a future OOPS event.