How to Operate & Maintain Voltage Transformers Safely?
I. Normal Operation of Voltage Transformers
A voltage transformer (VT) may operate long-term at its rated capacity, but under no circumstances should it exceed its maximum capacity.
The secondary winding of a VT supplies high-impedance instruments, resulting in a very small secondary current, nearly equal to the magnetizing current. The voltage drops across the leakage impedances of both primary and secondary windings are therefore very small, meaning the VT operates close to no-load under normal conditions.
During operation, the secondary side of a voltage transformer must never be short-circuited.
For VTs rated at 60 kV and below, the primary side must be equipped with fuses to prevent fault escalation. For VTs rated 110 kV and above, primary-side fuses are generally not installed, as the likelihood of failure is lower and the required interrupting capacity for fuses at these voltage levels is difficult to achieve.
The operating voltage of a voltage transformer should not exceed 110% of its rated voltage.
For safety, one terminal of the secondary winding or the neutral point of the VT must be solidly grounded to prevent high voltage from the primary side from entering the secondary circuit in case of primary insulation failure, which could endanger personnel and equipment. When working on the VT body or its base, not only must the primary side be disconnected, but there must also be a visible disconnection point on the secondary side to prevent back-charging from other VTs through the secondary circuit, which could induce high voltage on the primary side.
When commissioning a VT, check that the insulation is sound, phasing is correct, oil level is normal, and connections are secure. When de-energizing a VT, first withdraw associated protective relays and automatic devices, open the secondary automatic circuit breaker or remove the secondary fuses, then open the primary disconnect switch to prevent back-charging. Record the time period during which energy metering circuits are disabled.
II. Operation of Voltage Transformers
After completing preparations, operators may perform energizing operations: install high- and low-voltage fuses, close the output disconnect switch to bring the VT online, then energize the relays and automatic devices supplied by the VT.
Paralleling VTs in Double Busbar Systems: In a double busbar configuration, each busbar has one VT. If the loads require the two VTs to be paralleled on the low-voltage side, first confirm the bus tie breaker is closed. If not, close it before paralleling the secondary sides. Otherwise, voltage imbalance on the primary side will cause large circulating currents in the secondary circuit, likely blowing low-voltage fuses and causing loss of power to protection devices.
De-energizing a Voltage Transformer: In a double busbar system (in other configurations, the VT is de-energized with the bus), when maintenance is required on the VT’s output disconnect switch, the VT body, or its secondary circuit, follow this procedure:
First, disable the protective relays and automatic devices supplied by the VT (unless an automatic or manual transfer device is installed, allowing these devices to remain in service).
Remove the secondary fuses to prevent back-charging, which could energize the primary side.
Open the VT’s output disconnect switch and remove the primary-side fuses.
Perform voltage testing using a properly rated and qualified voltage detector to confirm no voltage is present on each phase of the VT’s incoming lines. After confirming de-energization, install grounding sets, hang warning signs, and proceed with maintenance only after obtaining proper work permits.
III. Precautions When Replacing a Voltage Transformer or Secondary Coil in Service
When replacing an individual VT damaged in service, select a VT with a voltage rating matching the system voltage, identical ratio, correct polarity, similar excitation characteristics, and which has passed all required tests.
When replacing a group of VTs, also check the connection group and phase sequence of the VTs intended for parallel operation.
After replacing a VT secondary coil, verify the wiring to prevent incorrect connections and avoid secondary circuit short circuits.
After replacing a VT or its secondary coil, the polarity must be tested and confirmed.
IV. Routine Inspection of Voltage Transformers in Service
Inspect insulators for cleanliness, absence of damage, cracks, or discharge phenomena.
Check that oil level is normal, oil color is clear and not darkened, and no oil leakage or seepage is present.
Check the color of the desiccant in the breather; it should be normal and not saturated. Replace the desiccant if more than 1/2 has changed color.
Listen for normal internal sounds; no discharging, severe electromagnetic vibration, or burnt odors should be present.
Check that the sealing system is intact, all bolts are tight, and no loosening exists.
Inspect primary lead connections for good contact, no looseness or overheating. Ensure the current-limiting resistor for the high-voltage fuse and the capacitor for open-circuit protection are intact. Check that secondary circuit cables and wires are free from corrosion and damage, and that secondary wiring has no short circuits.
Verify that the primary neutral point grounding and secondary winding grounding are in good condition.
Check that the terminal box is clean and free from moisture.
