Avoiding loss of voltage sensing runaway for generator excitation systems

The excitation system, synchronous generator and connected loads of an electrical power system can incur serious damage if a generator's sensing voltage input to the automatic voltage regulator (AVR) is lost. There are various conditions that may cause loss of sensing voltage to the excitation system, such as blown Potential Transformer (PT) fuses, failed PTs, mechanical connection problems, and sometimes even human error. The design and features of a voltage regulator/excitation system will define how the excitation system will react to a Loss of Sensing Voltage condition. Normal voltage regulation operation requires that if the Automatic Voltage Regulator's sensed generator voltage is lower than the AVR's regulation set point, the AVR will respond by increasing field current until the sensed generator voltage satisfies the AVR. Without additional excitation limiters or protective functions to detect a problem, an AVR operating without generator sensing voltage will produce maximum available field current and potentially cause damage to some element of the electrical power system. This paper addresses the protective safeguards commonly built into excitation systems to manage Loss of Voltage Sensing events and prevent power system equipment damage.