110kV Transformer Zero-Sequence Protection: Issues & Improvement Measures

Problems with Zero-Sequence Protection of 110 kV Transformers

In an effectively grounded system, the neutral-to-ground displacement voltage of a transformer is limited to a certain level, and the neutral-point gap protection does not operate. The purpose of installing gap protection is to prevent damage to transformer insulation caused by elevated zero-sequence voltage in non-effectively grounded systems. The discharge gap only operates when a single-phase ground fault occurs, all directly grounded neutral-point transformers are tripped out, and energized transformers with ungrounded neutral points remain connected to the faulted grid. In this case, the gap discharges to reduce the neutral-to-ground voltage and avoid insulation damage. 

However, gap breakdown generates chopped waves, which are detrimental to the transformer’s turn-to-turn insulation. Therefore, when zero-sequence voltage rises due to a single-phase ground fault, it is preferable for zero-sequence overvoltage protection—not gap current protection—to trip the transformer. In contrast, gap current protection involves a degree of randomness and may fail to operate for various reasons. From this perspective, for protecting transformer neutral-point insulation, zero-sequence overvoltage protection is more critical than gap current protection. 

Typically, zero-sequence overvoltage protection and gap current protection are used together to form a complete neutral-point insulation protection scheme. Hence, installing only gap current protection without zero-sequence overvoltage protection is inadequate—especially during intermittent gap breakdown, where the discharge current cannot be sustained, rendering gap current protection ineffective.

Most currently commissioned 110 kV substations are equipped only with neutral-point rod gaps but lack corresponding protective relaying. This configuration is disadvantageous. When the grid’s zero-sequence voltage rises close to the rated phase voltage, all ungrounded-neutral transformers simultaneously experience zero-sequence overvoltage. If a terminal transformer without gap overcurrent protection has its neutral-point gap discharge prematurely—and the discharge cannot be sustained—the energized ungrounded-neutral transformer will remain connected to the faulted grid. 

Therefore, for terminal transformers without low-voltage-side power sources, if complete gap current protection and zero-sequence overvoltage protection are not installed, the neutral-point rod gap should either be removed or its distance intentionally increased to prevent premature discharge.

For substations with internal bridge connections, the conventional practice of using the first time setting of the neutral-grounded transformer’s zero-sequence current protection to trip breakers 900 and the 100 bus tie is not optimal. When the low-voltage sides are operating in parallel, tripping breaker 900 results in the unnecessary loss of one bus section. Meanwhile, the low-voltage-side breaker of the ungrounded transformer remains closed. 

In the absence of zero-sequence overvoltage protection, if a temporary low-voltage power source exists (e.g., due to 10 kV power transfer), the ungrounded transformer is at risk of overvoltage. Therefore, given that three-phase voltage transformers (VTs) are already installed on the 110 kV side, adding zero-sequence overvoltage protection is a simple and effective safety measure.

Control of Transformer Neutral Grounding Methods and Improvement Measures for Zero-Sequence Protection

First, it is essential to ensure that the 110 kV system operates as an effectively grounded system. Preventing misoperation is the most fundamental approach—ensuring that the 110 kV neutral point of the source-end transformer is effectively grounded. If permitted by protection coordination settings, both paralleled source-side transformers can have their neutral points grounded simultaneously.

If a power-supplying transformer loses its grounded neutral point, the system may become non-effectively grounded. Therefore, during the design phase, source-end transformers—or those that may supply power in the future—should be equipped with complete neutral-point gap protection, including neutral-point zero-sequence overcurrent protection, neutral-point gap current protection, and open-delta zero-sequence voltage protection on the busbar.

On 110 kV outgoing feeders, regardless of how many transformers are connected in parallel, terminal transformers may operate with ungrounded neutral points as long as the source-side neutral point is grounded. In actual operation, to mitigate potential risks, one transformer neutral point may be grounded. When selecting which neutral point to ground, the following priority order should be applied:

• Prefer transformers whose low-voltage side temporarily supplies power;

• Next, consider transformers whose high-voltage side lacks a circuit breaker;

• Finally, select the transformer closest to the power source.

For the majority of already-commissioned 110 kV terminal substations that currently lack open-delta zero-sequence voltage protection (from bus VTs) and neutral-point gap current protection, the originally installed neutral-point rod gaps should either be removed or their spacing intentionally increased to avoid premature discharge.

For future 110 kV substation designs, three-phase voltage transformers should be considered on the high-voltage side, along with zero-sequence overvoltage protection and transformer neutral-point gap current protection. This configuration provides operational flexibility and adapts to future changes in grid structure.

For internal bridge-connected substations, the first time setting of the main transformer’s neutral-point zero-sequence current protection should trip the other ungrounded transformer to avoid expanding the outage area or causing power-frequency overvoltage.