Improvement and Analysis on Ping-Pong Type of Ground Protection for Field Winding

Current ping-pong type of field winding ground protections cannot detect grounding fault of field winding while generator is in free-running operation or shut down. To remedy this defect, the authors propose an improved scheme for ping-pong type field winding ground protection, that is, a DC voltage is injected into field winding and switching-over circuit is connected in parallel with the injected voltage. The principle of this improving scheme and the criterion for protection action are given; several problems such as the calculation method of grounding resistance, the selection of parameters in switching-over circuit and that of switching-over period are analyzed. Simulation results show that the accuracy and sensitivity of the improved scheme can meet the requirement of practical operation. By means of calculating grounding resistance by sampling redundant algorithm, the affects of bad sampled data and non-steady DC operating condition on protection accuracy can be effectively reduced.