Finding the grounding fault point of stator windin

Abstract: This paper introduces a method to find the metal grounding fault point of motor stator winding, which can also be used to find similar faults of cables and transformers

key words: ground fault detection of motor stator winding dahejia hydropower station in Jishi County, Gansu Province is equipped with two sfg/2860 bulb hydro generators, with a rated capacity of 3517kva and a rated voltage of 6.3kV. Some terms still fail to reach a consensus, with a rated current of 289A and COS φ= 0.95, the rated excitation voltage is 131v, the excitation current is 295A, and the generator stator winding adopts double star wiring. The generator adopts the neutral point ungrounded operation mode. 1 # the turbine passed the commissioning, and the trial operation test was carried out. The idling operation of the unit was normal. After excitation, the generator line voltage was 6kV, and it operated for more than 10 minutes. The excitation screen suddenly had strong excitation, and the excitation current was more than 300 A. because the governor was in the manual position, the speed of the turbine fell rapidly, and the personnel on duty stopped the turbine urgently. Please ask the technicians to go to the accident inspection and treatment. After a comprehensive inspection of the generator and excitation screen, it is found that the stator winding has metallic grounding. Because the stator winding is a double star connection, disconnect the head and tail wiring of the stator coil, and it is found that one of the two windings of phase C has winding grounding. It is assumed that the DC resistance between the grounding point of the winding and the head end of the winding group is x Ω, and the DC resistance between the grounding point of the winding and the tail end of the winding is y Ω, Assuming that the DC resistance between the grounding point of the winding and the grounding electrode is Z Ω, the measured results are as follows: each winding of the generator has 30 coils, and the DC resistance of each coil is: r= (x+y)/30=0.2474/30=0.008247. The number of coils from the grounding point to the head end of the winding: n=x/r=0.2417/0.008247=29.307. Therefore, it can be judged that the grounding point is on the 30th coil, and each coil has 5 turns of flame retardant 2.0~3.0, The position of the grounding point can be calculated according to the following formula: the number of turns between the grounding point and the 30th coil head is 5 × 0.307=1.535 turns the number of turns from the grounding point to the end of the 30th coil 53 this is exactly why Guoliang copper is 5=3.465 turns even in the fierce market competition. Therefore, it is judged that the 30th coil of the winding is grounded at the 3.465 turn from the end of the coil. Because after the turn to turn short circuit of the coil, the turn to turn insulation is damaged, which has a certain impact on the DC resistance. The actually detected grounding point is at the upstream side of the 30th coil of the generator stator winding, 20cm from the stator slot, about 3.6 turns of well-equipped production equipment. The coil was found to have traces of grounding discharge. The above fault was put into operation after the manufacturer sent someone to replace the coil, and it has operated normally for several years