The Influence of the Combination-type Grounding Method on the Voltage-injected Grounding Protection

A combination-type grounding method, which combines with some advantages of resonant grounding method and is able to reduce the ground fault current, is adopted or will be adopted for large-sized generators, such as hydro-generators or nuclear power generators. In this method, an inductor is installed in parallel with the load resistor at LV-side of the neutral grounding transformer (NGT). In this paper, a digital simulation system with quasi-distributed capacitance model is established by using MATLAB SIMULINK for studying stator ground fault and voltage-injected grounding fault protection. The same algorithm as protection device is used. Four ground fault protection schemes, which one is using conventional high-resistance grounding and other three are using combination-type grounding, are designed and the simulations are carried out on an example for a large-sized generator in China. By comparing the analysis, the influence of the combination-type grounding method on the voltage-injected grounding protection is studied. The results show that the parallel inductor can worsen the measurement results of ground fault resistance. And if the parallel inductor is installed on HV-side of the NGT, it is best to the voltage-injected grounding protection, however, it will increase equipment cost. With the range of 10A-25A of ground fault current limitation, the greater the detuning degree, the better the measurement of ground fault resistance.