Indian operational experience of under excitation limiters coordination with under excitation protection

Excitation system provides necessary energy that keeps generator in-synchronism with power system. It also helps generator in exchanging reactive power with grid. Increasing excitation current increases injected reactive-power while decreasing excitation current reduces the same. Various control modes are available in excitation system; Automatic Voltage control mode (AVR) is the most common control mode which helps in maintaining the grid voltage. During abnormal power system condition, various limiters e.g. over excitation limiter (OEL), under excitation limiter (UEL), ensure that machine operates within its capability.

Outage of generators due to lack of coordination of UEL and reach of under excitation protection has been-experienced.

Although under excitation and loss of protection are set as per standard guidelines, some coordination issues have been encountered due to change in limiter and relay characteristics during dynamic conditions. Some of the case studies, test results and methodology to ensure proper coordination between excitation system, machine capability-and protection relays is discussed.