Coordinating generator protection with transmission protection and generator control — NERC standards and pending requirements

Abstract

Recent misoperations of generation protection during major system disturbances have highlighted the need for better coordination of generator protection with generator capability, generator Automatic Voltage Regulator (AVR) control and transmission system protection. Generator protection misoperations contributed to the 1996 outages in the western U.S. and played a key role in the 2003 U.S. East Coast blackout. Since most recent major power system disturbances are the result of voltage collapse, generator protection must be secure during low voltage system conditions while still providing generator protection. The generator AVR needs to properly control VAr support to rapidly stabilize system voltage during major disturbances. In addition, turbine control should not trip generators for recoverable undervoltage conditions. As a result of the 2003 blackouts, NERC (North Electric Reliability Council) has developed protection standards that generator operators and owner must follow. NERC is also conducting audits to ensure that generator owners and operators are meeting those standards. These standards also address maintenance of the generator protection system. The record of generator trips (290 units totaling 52,743.9 MW) during the North American disturbance on August 14, 2003, included thirteen types of generator protection relay functions that operated to initiate generator tripping. A list of the protection elements that tripped included: generator system backup protection, over-excitation (volts/hertz), undervoltage, reverse power, loss-of-field, under/overfrequency and inadvertent generator energizing protection. Of the 290 trippings, 96 are unknown trippings by relaying or controls which could not be identified from the monitoring available at these plants. There is no information available that directly addresses which of the 290 trippings were appropriate for the Bulk Electric System (BES) conditions, and which were nuisance trips. In addition to traditional generator protective relay tripping, there were trippings of generator controls for BES voltage dips. Examples are “lean blowout trips” of combustion turbines, Power Load Unbalance (PLU) actuations during system disturbances as well as response of nuclear and other types of generation to system low voltage. The above factors have motivated NERC to become pro-active in addressing the coordination of generator and BPS protection.