Operational Use of Synchrophasor Technology for Wide-Area Power System Phase Angle Monitoring at California ISO

Abstract

Phase angle monitoring is carried out at CAISO considering two different purposes, one for validation of conflicting SE results between neighboring Utilities and another for making operating decision to reconnect a disconnected transmission equipment. Validation of State Estimator results in certain portions of the network providing conflicting results is done by model-independent phase angle measurements. Validation of SE results involves comparing calculated angle as the output of State Estimator and actual observed angle as measured by PMUs. State Estimator uses traditional SCADA measurements (such as bus kV, MW, MVAR) along with transmission topology model, whereas PMU-based measurement is “model-less”. Comparing the two values for a certain period of time provided a good network topology model validation as well. When a transmission line trips out of service or is taken out on a planned outage, the open phase angle difference between the two bus terminals increases. When the line needs to be reclosed, a large phase angle difference across the terminals of the line can lead to significant power swings and power system instability or damage to electrical equipment. Both State Estimator solution and PMU calculated angle pairs provide immediate feedback on whether it is safe to restore the transmission line or if further system re-dispatch is needed prior to reclosing the line. Knowing how large the open angle is, an operator will not waste time in attempting to reclose the transmission line if the phase angle difference is larger than what the synch-check relay in the station would allow. CAISO uses State Estimator calculated values and PMU calculated values to measure the open angle difference. The State Estimator calculated values are updated (every 30 s). PMU calculated values are updated much more frequently.