Understanding stability problems in actual system — Case studies

North American Electric Reliability Corporation (NERC) and each regional reliability coordinator such as Western Electricity Coordinating Council (WECC) have created planning standards that define the scope of transmission planning study and the criteria of assessing stability. Transmission planning study identifies transmission upgrades to meet these criteria. In a transmission planning study, however, criteria violations may be caused by different types of instability. Identifying the root causes of the instability is the key to developing the effective mitigation measures. This paper uses actual power system studies as examples to demonstrate the application of power system stability theories and methods in identifying the real instability problems and the transmission expansion needs.