Combining Tranmission Line Protection with Voltage Stability Monitoring

This paper introduces a Relay-based Local Index (RLI) of voltage emergency for voltage instability detection and considers ways to avoid undesirable line disconnections by distance protection during long-term voltage instability. The RLI stems from the previously presented New LIVES Index (NLI), which is calculated by processing voltage/current phasor measurements from Phasor Measurement Units (PMUs) at transmission substations on the boundaries of a weak system area. Distance relays of transmission lines can similarly calculate the RLI in a similar time scale. The RLI should not provide false alarm during the occurrence of a remote short-circuit fault, whereas it should provide an early alarm before any of the critical transmission lines forming a corridor is tripped by any distance element. Comparison between the alarms provided by NLI and RLI is performed using simulation studies of the Nordic Test System where a case of unwanted remote Zone-3 tripping is identified.