A second look at generator 51-V relays

ANSI/DEEE device type 51-V relays have long been used as back-up overcurrent protection for industrial generators. Many systems have experienced a steady increase in size since the introduction of the ANSI/IEEE device type 51-V voltage-restrained overcurrent relay. In some cases, this growth has resulted in the need for several levels of coordination timing delays between phase overcurrent relays, and has seen the introduction of tie circuits (synchronizing reactor buses, tie circuit breakers, etc.) rated for ampacities greater than one or more of the generators present. The settings required for certain ANSI/IEEE device type 51-V relays, in order to coordinate on a standard basis with other phase overcurrent relays in the power system, may result in large time delays and/or high trip current levels. This could make for a severe compromise between protection and selectivity. In some cases, an auxiliary or alternative device may be an inverse characteristic undervoltage relay. The need for such a device will be influenced by the exciter type present on a specific generator, the number of levels of phase overcurrent relay coordination in the system, and the philosophy of the individual engineer with regard to the issue of compromising between protection and coordination.