Development Of Stator Ground Fault Protection With Outside Sources

With the generator capacity increasing,the requirement of the stator earth-fault protection becoming more and more high for the generator.Therefore,the generator protection devices put forward higher requirements,more accurate and faster identification of various types of failures,reliable action.In the relay protection system of the generator,one of the non-negligible parts is stator ground protection.The stator single-phase grounding protection is divided into two categories according to the principle: non-injection and injection.Non-injection protection in the phase of the stator ground fault occurs when the protection can not play a role,and the sensitivity is low,affected by the operation of large,poor reliability.The injection protection can compensate for these shortcomings,with higher reliability and effectiveness,so the proposed 20 Hz plus AC power supply type injection protection scheme.The first,this paper analyzed the stator ground protection status,pointed out their shortcomings.The stator grounding protection logic of AC power supply is proposed by combining the grounding resistance criterion with the power frequency zero sequence voltage criterion: resistance criterion and power frequency zero sequence Current criterion.Resistance criterion By detecting the grounding device secondary side of the 20 Hz voltage and current to calculate the grounding resistance,always monitor the stator insulation level,when the grounding resistance is lower than the value of action in the signal or trip.At the same time,the protection setting method is analyzed.Secondly,the design requirements of the parameters of the external 20 Hz power supply stator grounding protection system are analyzed in detail.Including a low-frequency voltage injected into the system,the ground variable load resistance,grounding transformer to determine the ratio.And the setting of a conventional system to protect the measurement stage intermediate converter ratio,the device small CT ratio,the secondary load resistance,the voltage divider ratio,the device PT PT ratio determination.And thirdly,a new protection filtering algorithm for 20 Hz power supply stator grounding protection is proposed.The correctness of the theoretical analysis is verified by the actual increase of the protection device.Last,the simulation of the stator grounding protection proposed in this paper is carried out by RTDS model.The simulation results show that the stator grounding resistance of the stator,the equivalent capacitance of the stator windings and the grounding position of the stator are different.Protection scheme can accurately measure the stator grounding resistance.The stator earth resistance measurement can satisfy the absolute error <0.5 k? when the grounding resistance is less than 5 k? and the relative error is less than 10% when the grounding resistance is more than 5 k?.Grounding resistance measurement accuracy,measurement results are stable,measurement error within the allowable range.