How to Implement Transformer Gap Protection & Standard Shutdown Steps

How to Implement Transformer Neutral Grounding Gap Protection Measures?

In a certain power grid, when a single-phase ground fault occurs on a power supply line, both the transformer neutral grounding gap protection and the power supply line protection operate simultaneously, causing an outage of an otherwise healthy transformer. The main reason is that during a system single-phase ground fault, the zero-sequence overvoltage causes the transformer neutral grounding gap to break down. The resulting zero-sequence current flowing through the transformer neutral exceeds the operating threshold of the gap zero-sequence current protection, thereby tripping all circuit breakers on the transformer sides. Therefore, reasonably selecting the operating mode of the transformer neutral point and reducing the zero-sequence overvoltage applied to it are key to resolving the miscoordination between transformer gap protection and system zero-sequence protection.

Fault Phenomenon

When a ground fault occurs on the upstream power supply line of the transformer, the line’s zero-sequence stage II protection operates after 0.5 seconds to trip the line circuit breaker. Simultaneously, the transformer neutral grounding gap breaks down, and the gap current protection also operates after 0.5 seconds to trip all circuit breakers on the transformer sides. Due to the loss of coordination between the transformer gap protection and the system zero-sequence protection, both protections operate at the same time, causing simultaneous de-energization of the line and the main transformer. Even if the line fault is transient and the auto-reclosing successfully restores line power momentarily, the transformer remains out of service because its breakers were tripped by gap protection and will not automatically restore power just because the line is re-energized.

Cause Analysis

A single-phase ground fault causes unbalanced three-phase operation. In transformers operating with an ungrounded neutral, the neutral point voltage shifts, inevitably leading to overvoltage. If the single-phase ground fault occurs at the end of the power supply line or on the 110 kV busbar of a terminal substation, the zero-sequence voltage at the 110 kV transformer neutral point reaches its maximum, and the equivalent zero-sequence reactance is also the highest. Under this condition, the transformer neutral grounding gap breaks down, triggering both the line ground fault trip and the transformer gap zero-sequence current protection.

Solution Measures

To resolve the miscoordination between 110 kV main transformer gap protection and system zero-sequence protection, additional grounding points should be added for transformers in specific local areas of the 110 kV system.

What Steps Are Required to Shut Down a Transformer?

Transformer Shutdown Procedure

When shutting down a transformer, the load side should be disconnected first, followed by the power supply side. Operationally, the circuit breaker should be opened first, then the disconnect switches on both sides of the circuit breaker should be opened. If there is no circuit breaker installed on either the power supply side or the load side of the transformer, all outgoing feeders on both sides should be disconnected first. Then, with the transformer in no-load condition, the same load switch or fuse switch used during energization should be used to cut off the power supply and shut down the transformer.

For water-cooled transformers shut down during winter, all water in the coolers must be completely drained.