Before commissioning a transformer, thorough inspection and testing are crucial to ensure its reliability and safety during operation. The following steps and considerations are essential:
1. Inspection
During inspection, the following steps should be carried out:
- Oil Sampling and Chromatographic Analysis: Obtain oil samples for chromatographic analysis to assess oil condition.
- Valve Status Check: Verify that gate and butterfly valves are open and correctly positioned. Ensure oil levels in storage cabinets and bushings meet specified levels.
- Current Transformer Wiring Check: Verify correct wiring of current transformers, ensuring no secondary coil is grounded or left open-circuited.
- Oil Flow Relay Operation Check: Confirm reliable operation of the oil flow relay.
- Air Insulation Distance Check: Ensure air insulation distances between bushings and ground, and between bushings of different voltage levels, comply with relevant standards.
- Grounding System Check: Ensure proper grounding, including grounding of the oil tank and any grounding bushing leads on the iron core (clamp).
- Protection Device and Control Circuit Check: Verify correct wiring of gas relays, pressure relief valves, oil level gauges, thermometers, and current transformers. Ensure proper operation and reliability of protection devices and circuit breakers.
- Desiccant and Breather Check: Inspect desiccant silica gel condition, oil level, and breather functionality.
- Cooling System and Control Box Operation Check: Verify correct and reliable operation of the cooler and control box systems.
- Oil Circulation Direction Check: Confirm correct oil circulation direction based on the oil pum’s orientation.
2. Transformer Pre-Installation Testing
Before installation, perform the following tests:
- Bushing Current Transformer Test: Measure insulation resistance of the secondary winding to ground, DC resistance of the secondary winding, conduct excitation characteristics test, and check transformer ratio and polarity.
- Bushing Test: Measure insulation resistance of the bushing main insulation and end screen to ground. Measure dielectric loss tangent (tgδ) and capacitance value of non-porous porcelain bushings.
3. Electrical Testing After Transformer Oil Filling
After oil filling, conduct comprehensive electrical tests:
- Measure DC resistance of winding and bushing at all tap positions.
- Measure voltage ratio of all taps.
- Verify transformer connection group.
- Measure insulation resistance and absorption ratio (polarization index) of winding and bushing.
- Measure dielectric loss tangent (tgδ) of winding and bushing.
- Measure DC leakage current of winding and bushing.
- Perform AC withstand voltage test of low-voltage side winding and neutral point winding on the high-voltage side.
- Conduct partial discharge and winding deformation tests on windings and bushings.
4. Grounding Check
Ensure reliable grounding of the transformer’s supply-side neutral point, especially during impact testing.
5. Protection Device Settings
Verify correct settings and synchronization sensitivity of all protection devices and circuit breakers.
6. Gas Relay
During no-load impact testing, connect the gas relay signal contact to the heavy gas action contact for the power trip circuit.
7. On-Site Storage of Transformers
Upon arrival, adhere to the following storage guidelines:
- Check for dampness. Ensure nitrogen pressure is between 0.01-0.03 MPa and oil meets electrical strength, micro water, and tgδ requirements before injecting.
- Store for no more than 6 months under dry air. Conduct twice-daily inspections of box pressure. Promptly address significant leaks to maintain pressure requirements.
- Store disassembled parts (e.g., oil storage cabinets, coolers) with protection against rain, dust, dirt, and rust.
- Store accessories (e.g., measuring instruments, displays, junction boxes) in dry, ventilated rooms post-unpacking.
By meticulously conducting these inspections and tests, the safety, reliability, and long-term performance of the transformer can be ensured before it is put into operation.
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