Neutral point grounding operation mode for 110kV～220kV power grid transformers
Field: Manufacturing
China
The arrangement of neutral point grounding operation modes for 110kV～220kV power grid transformers shall meet the insulation withstand requirements of transformer neutral points, and shall also strive to keep the zero-sequence impedance of substations basically unchanged, while ensuring that the zero-sequence comprehensive impedance at any short-circuit point in the system does not exceed three times the positive-sequence comprehensive impedance.
For 220kV and 110kV transformers in new construction and technical renovation projects, their neutral point grounding modes shall strictly comply with the following requirements:
1. Autotransformers
The neutral point of an autotransformer must be directly grounded or grounded via a small reactance.
2. Thin-insulation Transformers (Unmodified)
The neutral point of an unmodified thin-insulation transformer should preferably be operated in a directly grounded manner.
3. 220kV Transformers
- If the insulation class of the 110kV side neutral point of a 220kV transformer is 35kV, the neutral points of both the 220kV side and 110kV side shall be operated in a directly grounded manner.
- The grounding modes of the neutral points on the 220kV and 110kV sides of the transformer should preferably be the same, and the neutral point grounding disconnecting switch should preferably be equipped with remote operation functionality.
- In a 220kV substation/power plant, one transformer should preferably be operated with its neutral point directly grounded. For 220kV substations/power plants with two or more transformers, when the 220kV or 110kV side busbars are split on either or both sides, one transformer with a directly grounded neutral point shall be maintained on each busbar section.
4. 110kV Transformers
- If the insulation class of the 110kV neutral point of a 110kV transformer is 66kV, the neutral point may not be operated in a directly grounded manner.
- For transformers where the insulation class of the 110kV neutral point is 44kV or lower, the neutral point should preferably be operated in a directly grounded manner.
- In a power plant or a substation with a power source on the medium/low-voltage side, one transformer should preferably be maintained with its neutral point directly grounded within the plant/substation.
- The neutral point of a terminal transformer without a local power supply should not be operated in a directly grounded manner.
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