Operating Specifications for Box-type Transformers
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1. Transformer Parallel Operation
Transformers may only be operated in parallel if they have the same voltage ratio (allowable deviation: ±0.5%), same impedance voltage (allowable deviation: ±10%), and same connection group.If the voltage ratio and impedance voltage differ, parallel operation is permitted only after verification to ensure no single transformer is overloaded during parallel operation. (Capacity ratio: 3:1)
Before paralleling or disconnecting transformers, check the load distribution, and record the inspection statement in the switching operation ticket.
For newly commissioned or overhauled transformers, phase verification shall be performed before paralleling; parallel operation is allowed only after confirmation of correctness.
2. Transformer Power Shutdown and Restoration
Before shutting down or energizing a transformer, consider the switching of neutral points to ensure the number of directly grounded neutral points remains unchanged after the operation.
- For neutral points directly grounded at 110kV and above: when switching, close the neutral grounding switch of another transformer first, then open the original neutral grounding switch.
- For 35kV arc suppression coil switching: open the originally closed neutral grounding switch first, then close the neutral grounding switch of another transformer.
When de-energizing a transformer, shut down the low-voltage side first, then the medium-voltage side, and finally the high-voltage side.During operation, open the circuit breakers on all sides first, then open the disconnectors in sequence from low to high voltage. For main transformer disconnectors, open the transformer side first, then the busbar side. The sequence is reversed for energization.
For forced-oil air-cooled transformers, put the coolers into service before energization.
3. Operation of On-load Tap Changer (OLTC)
For transformers operating in parallel, voltage regulation shall be performed step by step in rotation. No more than one tap change shall be made consecutively on a single transformer.
Tap changing of the on-load tap changer is prohibited when the main transformer is overloaded.
- Take an oil sample for testing after 6–12 months of operation or 2,000–4,000 tap operations.
- Replace the oil in the OLTC tank after 1–2 years of operation or 5,000 tap operations.
- Perform core-pulling inspection of the tap changer after 5,000–10,000 tap operations.
4. Busbar Operation
When energizing or de-energizing a busbar, switch the secondary side of the voltage transformer to prevent reverse charging of the busbar from the secondary side of the voltage transformer.
When charging a busbar using a bus tie circuit breaker, enable the bus tie circuit breaker charging protection; disable the charging protection after normal charging.
For busbar transfer operations, switch relevant line protections and bus differential protections as required. Set the bus tie circuit breaker to a dead switch before transferring the busbar.
Under normal conditions, busbar charging shall be performed directly with the voltage transformer connected.
5. Disconnector Operation
(1) It is forbidden to switch on or off loaded equipment or loaded lines using disconnectors.(2) It is forbidden to switch on or off no-load transformers using disconnectors.
(3) Permitted operations using disconnectors:a. Opening and closing fault-free voltage transformers and lightning arresters.b. Opening and closing transformer neutral grounding switches when the system is fault-free.c. Opening and closing loop currents without impedance.d. Outdoor three-pole disconnectors may switch load currents of 10kV and below with current not exceeding 9A. Exceeding these limits requires calculation, testing, and approval by the chief engineer of the competent unit.
(5) When de-energizing a voltage transformer, disconnect the secondary air circuit breaker (or remove the secondary fuse) first, then open the primary disconnector. The sequence is reversed for energization.Parallel operation on the secondary side is prohibited for two sets of voltage transformers whose primary sides are not operated in parallel.
