Study On Single Phase Grounding Fault Protection For Hydro-generator Stator

The grounding faults in the generator stator are the common faults which generators are subjected. Internal short-circuit faults, which may immediately damage generator, are always preceded by the grounding faults. The possibility of internal faults and damage caused by them can be decreased largely if the stator grounding faults protection reliably and sensitively operates. Therefore, the stator grounding fault protection plays an important role in the protection system of generator.Currently, the main schemes of stator single phase grounding fault protection, based on zero-sequence voltage and third harmonic voltage, are non-selective for the stator grounding, internal and external grounding of enlarging unit connection, which makes trouble for finding the fault point and effects the safety and stability operation of the generator when grounding fault occurs.In order to analyze the characteristics of stator grounding fault and develop its protection schemes, a mathematical method for large-sized hydro-generators is established based on the analysis of transient grounding fault information which is using sine wave to simulate the transient signal changing. By this way, the instantaneous value and angle of the voltage and current can be calculated easily. In the meantime, it also can minimize the calculate interval which means that the transient value and angle of the voltage and current can be calculated in a short time. So the method is very important for the generator protect scheme, which based on theory of transient information.The analysis of simulation has been done for stator grounding of enlarging unit connection and one generator connected with one transformer, which clarify the characteristic of large-sized hydro-generator stator single phase grounding fault. Power system instantaneous signal Sine-approximation method has been employed to calculate the zero-sequence power in this paper.For verifying the validity and correct of the proposed method, the simulation has been done for the protection of generator stator grounding. The result of simulation indicated that the protection of generator stator grounding based on zero-sequence power frequency transient power direction is valid and correct.