Transformer operations can pose several risks, especially when dealing with no-load conditions. One major concern is the occurrence of operating overvoltage during the process of adapting to a no-load transformer, which can severely damage the insulation system. Another issue is the potential rise in no-load voltage, which may lead to insulation failure and even equipment damage. Understanding these risks is essential for ensuring safe and reliable operation of transformers.
To mitigate the risk of operating overvoltage caused by no-load transformers, special attention must be given to the grounding of the transformer's neutral point. In high-voltage systems (110 kV and above), the neutral point is often grounded to prevent overvoltage from occurring during switching operations. However, in some cases, especially where fault currents need to be limited, certain transformers may not have their neutral points directly grounded. This decision depends on factors such as system configuration, insulation coordination, and relay protection requirements.
When the neutral point is grounded, it helps prevent overvoltages caused by asymmetric switching or capacitor coupling. For example, if a three-phase switch trips or operates out of sync, the grounded neutral point can help limit the resulting overvoltage. Therefore, proper handling of the neutral-point grounding switch is crucial to avoid damage due to unexpected overvoltage events.
The grounding of the transformer’s neutral point should follow specific guidelines:
- If multiple transformers are connected to different busbars, at least one transformer on each busbar must have its neutral point directly grounded. This ensures that no busbar remains ungrounded after a fault or switching event.
- If there is power on the low-voltage side of the transformer, the neutral point must be grounded to prevent the transformer from becoming an insulated system if the high-voltage switch trips.
- In parallel operation, only one transformer should have its neutral point directly grounded at any given time. Before disconnecting a grounded transformer, another one must be switched on first to maintain system stability.
- Before energizing or de-energizing a transformer, ensure that the neutral point is grounded to prevent inductive overvoltage due to non-synchronous switching. After charging, the grounding method should be adjusted according to the normal operating mode.
In addition to managing overvoltage, it is also important to control the no-load voltage rise. Operators should avoid situations where the transformer experiences excessive voltage when no load is connected. This can be done by using reactors, adjusting inductive loads, or changing the tap settings of on-load tap changers to lower the receiving voltage. If the source is a power plant feeding a substation, the plant's output voltage can be reduced as needed. If the plant has other loads, the busbar voltage can be adjusted separately to meet equipment requirements.
By following these precautions, operators can significantly reduce the risk of insulation damage and ensure the safe and stable operation of the power system.
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