Zero-Sequence Vs. Gap Protection in Power Transformers

Transformer neutral point protection is critical for system stability. Zero-sequence current protection acts as a backup for internal faults and adjacent ground faults. Its configuration depends on the grounding status. When the neutral is directly grounded, a two-stage overcurrent scheme is used: short time delays limit fault scope, while long time delays trip all transformer breakers.

In systems with multiple parallel transformers, only some neutrals are grounded. If a grounded transformer trips due to a fault, ungrounded units face dangerous overvoltages. Protection varies by insulation type: fully insulated transformers use zero-sequence overvoltage protection, while graded insulation transformers require discharge gap protection to prevent insulation breakdown.

A key operational rule is that zero-sequence and gap protection must not operate simultaneously. The CT locations differ: neutral zero-sequence CTs are at the bushing outlet, while gap CTs are behind them. If the gap breaks down with the neutral disconnected, the zero-sequence CT still detects current, causing misoperation. Furthermore, zero-sequence settings are high for direct grounding, whereas gap settings are low for intermittent discharges. Therefore, gap protection must be disengaged when the neutral grounding switch is closed to ensure selectivity and safety.