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Things to Know When Operating Voltage Transformers: De-energizing and Energizing Procedures

Echo
Echo
Field: Transformer Analysis
China

Q:What are the Operating Sequence Rules for the Secondary Miniature Circuit Breaker and High-Voltage Power Supply During Voltage Transformer De-energizing and Energizing?

A:For busbar voltage transformers, the principle for operating the secondary miniature circuit breaker during de-energizing and energizing is as follows:

  • De-energizing: First, open the secondary miniature circuit breaker, then disconnect the high-voltage power supply of the voltage transformer (VT).

  • Energizing: First, energize the high-voltage side of the VT, then close the secondary miniature circuit breaker.

This sequence primarily prevents back-charging of the de-energized VT from the low-voltage side through the secondary circuit. It applies to wiring configurations such as double-busbar or single-busbar with sectionalizer, where secondary paralleling of VTs may occur. To prevent back-charging due to rare miswiring and to standardize operational procedures, this sequence should be followed for all VT configurations.

Critical Risk in Double-Busbar or Sectionalized Single-Busbar Systems

When de-energizing a busbar VT while the secondary circuits of both busbar VTs are paralleled, if the high-voltage source is disconnected first (by opening the bus-tie or sectionalizer switch) or the high-voltage disconnect switch is opened (especially if the auxiliary contact fails), the energized VT's secondary power can back-feed and step up voltage to the de-energized VT's high-voltage side. The capacitive charging current to ground on the de-energized side may cause the secondary miniature circuit breaker of the energized VT to trip. If the bus carries connected equipment, this current is larger, potentially causing protective relays or automatic devices on the energized bus to lose AC voltage. This could lead to misoperation and tripping, resulting in equipment or grid accidents.

Real-World Incidents

Such accidents have occurred. In one case, failing to open the VT's secondary miniature circuit breaker first allowed the secondary voltage to back-feed through a voltage switching relay contact in a transformer protection relay (which should have opened but remained closed), energizing the de-energized bus. This caused the voltage switching relay in the transformer protection to burn out, forcing an unplanned transformer outage.

VT.jpg

Two Common VT Operation Scenarios

  • Independent VT De-energizing/Energizing:

    • De-energizing: Open the VT secondary miniature circuit breaker first, then open the high-voltage disconnect switch.

    • Energizing: Reverse the sequence.

  • VT De-energizing/Energizing with the Bus:

    • De-energizing: With the bus already de-energized, open the VT secondary miniature circuit breaker, open the bus-tie or sectionalizer switch to de-energize the bus, then open the VT high-voltage disconnect switch.

    • Energizing: Reverse the sequence.

500 kV Line VT Operations

500 kV lines are equipped with line-side VTs directly connected to the line, with no other secondary sources connected. During line outage for maintenance:

  • De-energize the line breakers and disconnect switches on both ends.

  • Confirm no voltage by checking the absence of secondary voltage indication from the line VT (indirect voltage detection, common for 500 kV systems).

  • Close the line-side grounding switch.

  • Finally, open the line VT's secondary miniature circuit breaker.

  • Energizing follows the reverse order.

Commissioning New Equipment

During initial energizing of new equipment, back-charging is generally not a concern. Since the primary sides of two buses are not paralleled during charging, the VT secondaries cannot be paralleled. Therefore, the "high-voltage first, then low-voltage" rule need not apply. Instead, the secondary miniature circuit breaker can be closed first, followed by energizing the high-voltage side.

For new busbar VTs, charging is typically done together with the bus:

  • With the bus de-energized, close the VT high-voltage disconnect switch.

  • Close the VT secondary miniature circuit breaker.

  • Energize the bus and VT together using a breaker (line, bus-tie, or sectionalizer).

This sequence allows immediate verification of voltage on the VT secondary side to confirm successful operation. Delaying the closing of the secondary breaker until after energizing would delay verification and expose personnel to risk when checking a newly energized system.

Modern Developments

With technological advances, optical-signal VTs are now used in substations, eliminating the risk of secondary back-feeding. In smart substations, VT signals are transmitted via networks, avoiding direct secondary wiring. In these cases, strict operational sequence rules between high and low voltage sides are no longer technically necessary. Procedures can be defined based on operational convention.

A recommended approach is

  • Energizing: Close the low-voltage (secondary) side first, then the high-voltage side.

  • De-energizing: Open the high-voltage side first, then the low-voltage side.

This allows direct voltage presence verification on the secondary side, making operation checks more intuitive and convenient.

Conclusion

In switching operations, follow the principle of "choosing the lesser of two benefits and the lighter of two harms." Arrange the operation sequence safely and logically based on actual site conditions to achieve safe and smooth execution.

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