Precautions for Generator Insulation Measurement

Professional generators involve a great deal of professional knowledge, and one of them is the insulation testing of generators. Today, I will introduce to you the insulation testing of generators and the matters needing attention.

The insulation of a generator is divided into two items: stator insulation and rotor insulation.

Stator Insulation

The measurement location of the stator insulation is in the generator neutral grounding cabinet. The stator insulation can only be measured when the rotor is in a stationary or barring state, the generator is charged with hydrogen to the rated pressure, the stator cooling water has been put into operation, and the conductivity of the cooling water is around 0.2 μS/cm.

#1, the grounding point of the generator neutral should be disconnected, and the generator PT trolley should be pulled out. Then, a 1000V insulation resistance tester (megohmmeter) is used for measurement. It is considered qualified when the insulation between the phase and the ground is not less than 1MΩ.

#2, the grounding points of the voltage transformers at the generator neutral and the excitation transformer neutral should be disconnected, and the generator PT trolley should be pulled out. Then, a 2500V insulation resistance tester is used for measurement. The voltage application time for the measurement is 1 minute. When the temperature is below 20℃, the insulation between the phase and the ground should not be less than 0.5MΩ, and each phase should be measured once in the three-phase parallel state. If the measured value is lower than 80% of the above value or the difference in insulation values between phases is greater than 20%, the insulation is unqualified.

#3,#4,#5,the grounding knife switch of the generator neutral should be pulled out, and the generator PT trolley should be pulled out. Then, a 1000V insulation resistance tester is used for measurement. It is considered qualified when the insulation between the phase and the ground is not less than 0.8MΩ.

Rotor Insulation Measurement

For Unit #1, use a 500V insulation resistance tester (megohmmeter). The insulation resistance to the ground should be greater than 20MΩ.

The rotor insulation measurement of Unit #2 is carried out by maintenance personnel. It is required that the insulation resistance to the ground is greater than 100MΩ at 20℃ and greater than 50MΩ at 30℃.

For Unit #3, Unit #4 and Unit #5, use a 500V insulation resistance tester. The insulation resistance to the ground should be greater than 1MΩ.

Precautions for Generator Insulation Measurement

1) During the gas replacement process and the switching on and off test of the generator-transformer unit switches, it is strictly prohibited to measure the insulation of all parts of the generator.

2) Do not use an insulation resistance tester to measure the insulation resistance of the control part and electronic devices of the rectifier cabinet. If measurement is necessary, it should be carried out by professional personnel.

3) Connect the wiring of the water megohmmeter correctly to prevent damage to the water megohmmeter.

4) The collector pipe of the stator cooling water (terminals 94, 95, 96) should be reliably grounded after measurement.

5) When measuring the insulation of the main exciter rotor of Unit #1 and Unit #2, the insulation detection switch of the main exciter rotor should be switched to the position of (simulation for Unit #2) (test for Unit #1).

6) When measuring the rotor insulation of Generator #3 and Generator #4, the rotor grounding protection fuse and the rotor voltage isolation fuse should be pulled open. Otherwise, the fuse is likely to blow.

7) Before measuring the insulation of the low-voltage side winding of the excitation transformer, the fuses of the DC capacitor, auxiliary transformer and synchronous transformer in the rectifier cabinet should be pulled open.

8) When measuring the insulation of the generator-transformer unit, the outlet disconnecting switch of the main transformer and the branch switches of the high-voltage station service transformer should be in the open position, and the generator outlet PT should be in the disconnected position.

9) It is only allowed to measure the generator insulation when the generator rotor is in a stationary or barring state to prevent electric shock to personnel or damage to the water megohmmeter.

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