Relay Element Performance During Power System Frequency Excursions

Many voltage and current protection elements in microprocessor relays use the fundamental frequency component of current and voltage. Distance relays also calculate apparent fault impedance at the power system frequency. Most microprocessor relays track system frequency to calculate the current, voltage, and impedance quantities. When the integrity of the power system is in jeopardy, system frequency can experience a large and rapid excursion because of generation and load imbalances from system separations into regional islands. Such separations occur in major system events such as the U.S.-Canadian blackout of August 14, 2003, that affected 50 million people in eight U.S. states and two Canadian provinces. Relay elements in such a disturbed system state must perform reliably to prevent any protective element misoperations from aggravating the disturbance and causing widespread outages. Relay element performance during a system frequency excursion depends on factors including the system frequency rate of change, the tracking rate and tracking limit of the relay, and the element type. This paper reviews overcurrent, distance, and current differential element designs. It then examines the performance of these elements during frequency excursions.