The arrangement of transformer-neutral grounding methods should aim to maintain the zero-sequence impedance of the substation as constant as possible. In special operating conditions where the zero-sequence impedance of the substation significantly changes due to transformer maintenance or other reasons, temporary measures should be taken according to regulations or actual situations.

1. Single Transformer Substation:

The neutral point should be directly grounded. When calculating normal protection settings, only the normal operating condition of the transformer with its neutral point grounded needs to be considered.

During transformer maintenance, special operating conditions can be handled by adjusting settings or following specified procedures to disable or enable relevant protection sections.

2. Substations with Two or More Transformers:

Only one transformer's neutral point should be directly grounded during operation. When this transformer is out of service, the other transformer, which initially has its neutral point ungrounded, should be switched to direct grounding.

Suppose it is necessary to have two transformers with their neutral points directly grounded during normal operations, and one of these transformers goes out of service. In that case, the third transformer (if available) should be switched to direct grounding of its neutral point. If no third transformer is available, handle it as a special operating condition.

3. Substations with Three or More Transformers on Double Busbars:

Operate with the neutral points of two transformers directly grounded, and connect them to different busbars. If one of these directly grounded transformers goes out of service, switch another transformer with an ungrounded neutral point to direct grounding.

If it is not possible to maintain one grounding point on each busbar, handle it as a special operating condition.

4. Improving Protection Coordination:

When a short line is out of service for maintenance, the number of transformers with directly grounded neutral points can be increased to offset the impact on the zero-sequence current distribution caused by the line being out of service.

5. Autotransformers and Transformers with Insulation Requirements:

The neutral points must be directly grounded during the operation.

By adhering to these guidelines, the stability and safety of the substation can be maintained during normal and special operating conditions.