Study on a New Loss-of-Field Protection Strategy

This paper presents a new loss-of-field (LOF) protection scheme. Through theory analyses to generator and the power system in LOF event, three characteristics were found: 1) generator terminal voltage (GTV) decreases faster than the high side voltage of step-up transformer(HVSUT) does; 2) reactive power output of the generator (QO), GTV, HVSUT all decrease; 3) generator angle exceeds the static stability limit (SSL) angle gradually unless QO reverses. All this three characteristics are different to that in power system oscillation (PSO). Thus the corresponding three criteria are proposed: "voltage change" criterion, "voltage, reactive power change" criterion, "angel, reactive power change" criterion. A new LOF protection scheme then is constructed combining with the existing SSL criterion. Simulation results show that the three criteria are valid and the new protection has advantages of high speed in operation and good selectivity comparing with existing protection schemes.