What is transformer gap protection?

Transformer gap protection is a relay protection method specifically designed to protect the neutral point insulation of transformers.

Neutral Point Grounding Method: In my country's 110kV and above power grids, a directly grounded neutral point system is commonly used. However, to limit single-phase short-circuit current and ensure system stability, not all transformer neutral points are directly grounded; some are ungrounded (i.e., neutral point insulation operation).

Weaknesses of Graded Insulation: The neutral point insulation level of this type of transformer is much lower than that at the winding ends. When a single-phase ground fault or line operation occurs, the transformer with an ungrounded neutral point will experience very high overvoltages (up to 2-3 times the phase voltage), which is sufficient to break down its neutral point insulation.

Protection Task: The task of gap protection is to quickly break down the discharge gap when a dangerous overvoltage occurs at the neutral point, forming an artificial grounding path, thereby protecting the neutral point insulation from damage.

The core components of gap protection are a discharge gap and a set of zero-sequence current transformers.

Discharge Gap: Consists of two opposing metal electrodes (rods or balls) with a certain distance of air gap between them. This distance is precisely calculated, and its breakdown voltage is lower than the transformer neutral point withstand voltage but higher than the maximum voltage during normal operation.

Working Process:

Normal Operation: The neutral point voltage is very low, the gap does not discharge, and the protection does not operate.

Overvoltage Occurrence: When a ground fault or resonance occurs in the system, the neutral point voltage rises above the gap's set value, the air gap breaks down, forming an arc that short-circuits the neutral point to ground.

Zero-Sequence Current Generation: After the gap breaks down, a large zero-sequence current (or gap current) is generated.

Protection Action: The zero-sequence current transformer detects this current and transmits it to the protection device.

Main Protected Object: Transformer neutral point insulation.

Backup Protection Role: Gap protection typically serves as a backup or supplement to zero-sequence overcurrent protection. For transformers with an ungrounded neutral point, zero-sequence overcurrent protection cannot operate (because there is no path), in which case gap protection takes over.

Coordination with Surge Arresters: Gap protection is usually used in parallel with surge arresters. Surge arresters limit the amplitude of overvoltages, while gaps provide a path for large currents. The discharge voltage of the gap is slightly higher than the operating voltage of the surge arrester.

Therefore, gap protection can be understood as a defense mechanism that "sacrifices itself (the gap) to protect the core (neutral point insulation)," and is an indispensable safety barrier for graded insulated transformers.

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      Shaanxi West Power Tongzhong Electrical Co., Ltd.
Contact: Ms.Grace Liu (Director of Sales Department)

Email:xdtz04@westpowerelectric.com

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