Casing selection test plan

1) 1 to 3 years

2) After overhaul (including major equipment overhaul)

3) If necessary

1) The main insulation resistance should not be less than 10,000 MΩ

2) The end screen to ground insulation resistance should not be less than 1,000 MΩ

Use a 2500V megohmmeter

1) 1 to 3 years

2) After overhaul (including major equipment overhaul)

3) When necessary

1) The value of tgδ(%) at 20°C should not exceed the values in the table below;

1) Oil-paper capacitor type bushings generally do not perform temperature conversion for tgδ. When tgδ and factory value or previous test results significantly increase or approach the values in the table, a comprehensive analysis of the relationship between tgδ and temperature/voltage should be conducted. If tgδ increases significantly with temperature or when the voltage rises from 10kV to Um/√3, and the tgδ increment exceeds ±0.3%, the device should not continue to operate.

2) Pure porcelain sleeves below 20kV and hydraulic sleeves connected to transformer oil do not measure tgδ.

3) When measuring the transformer bushing tgδ, all winding terminals connected to the tested bushing are shorted and pressurized, while other winding terminals are grounded. The last screen is connected to the bridge, and the positive wiring method is used for measurement.

Voltage level kV

20~35

66~110

220～500

Big
Repair
Rear

Oil-filled

3.0

1.5

——

Oil paper capacitor

1.0

1.0

8.0

Filled type

3.0

2.5

——

Tape capacitor type

2.0

1.5

1.0

Tape type

2.5

2.0

——

2) If the end screen to ground insulation resistance of the capacitor-type bushing is less than 1000MΩ, the end screen to ground tgδ should be measured, and its value should not exceed 2%.

3) If the capacitance value of the capacitor-type bushing differs by more than ±5% from the factory value or the previous test result, the cause should be investigated.

1) After overhaul

2) When necessary

The test voltage is 85% of the factory value.

Pure porcelain wall bushings of 35kV and below can be tested together with busbar insulators.