Risk Factors regarding the Application of Degraded Voltage Relaying at Nuclear Generating Stations

Nuclear generating stations utilize degraded voltage relaying schemes to protect electrical equipment against sustained low-voltage conditions. Based on regulatory guidance, typical design involves the use of undervoltage relays which, when actuated, automatically disconnect the safety buses from the degraded offsite power supply and initiate reconnection to an altemate supply such as the diesel generator. This automatic disconnection creates the risk of an unwanted separation from the offsite power supply. Although the industry recognizes the importance of ensuring that adequate voltage is supplied to critical plant equipment, the method of achieving this goal is a matter of ongoing discussion. The potential benefits of degraded voltage relaying schemes do not necessarily outweigh the additional risks that they create, and other measures not explicitly described in the regulations, including detailed analyses of the transmission grid and onsite distribution system, and procedural controls, are essential elements of a reliable offsite power supply.