Commissioning Test Procedures for Oil-Immersed Power Transformers

Transformer Commissioning Tests Procedures

1. Non-Porcelain Bushing Tests

1.1 Insulation Resistance

Suspend the bushing vertically using a crane or support frame. Measure the insulation resistance between the terminal and the tap/flange using a 2500V insulation resistance meter. The measured values should not deviate significantly from factory values under similar environmental conditions. For capacitor-type bushings rated 66kV and above with voltage sampling small bushings, measure the insulation resistance between the small bushing and the flange using a 2500V insulation resistance meter; the value should not be less than 1000MΩ.

1.2 Dissipation Factor Measurement

Measure the dissipation factor (tanδ) and capacitance of the main insulation to the tap using the positive wiring method. Follow the instrument's specified wiring configuration and select a test voltage of 10kV.

The high-voltage test leads for dissipation factor testing must be properly suspended with insulating tape, kept away from other equipment and the ground. Implement appropriate safety measures to prevent unauthorized access to the high-voltage test area. The measured dissipation factor and capacitance values should not significantly differ from factory values and must comply with handover standards.

2. On-Load Tap Changer Inspection and Testing

Check the complete action sequence of the on-load tap changer contacts. Measure the transition resistance value and switching time. The measured transition resistance values, three-phase synchronization deviation, switching time values, and forward-reverse switching time deviation must comply with the manufacturer's technical requirements.

3. DC Resistance Measurement of Windings with Bushings

Measure the DC resistance of the high-voltage winding at each tap position and the low-voltage side. For transformers with neutral points, measure single-phase DC resistance where appropriate. Record the ambient temperature during measurement to facilitate comparison with factory values after temperature conversion. The deviation between line-to-line or phase-to-phase values must comply with handover standards.

4. Voltage Ratio Check for All Tap Positions

Connect the turns ratio tester leads to the high and low voltage sides of the three-phase transformer. Check the voltage ratio for all tap positions. Compared to the manufacturer's nameplate data, there should be no significant difference, and the ratios should follow expected patterns. At the rated tap position, the allowable error is ±0.5%. For three-winding transformers, perform ratio tests for HV-MV, MV-LV separately.

5. Check Three-Phase Connection Group and Single-Phase Transformer Terminal Polarity

The verification results should match the design requirements, nameplate markings, and symbols on the transformer housing.

6. Insulation Oil Sampling and Testing

Oil sampling should only be performed after the transformer has been completely filled with oil and allowed to stand for the specified time. After collecting the oil sample, seal the container properly and promptly deliver it to the relevant department for testing.

7. Insulation Resistance, Absorption Ratio or Polarization Index Measurement

All insulation-related tests should be conducted after the insulating oil has passed inspection and during weather conditions with appropriate humidity levels. For transformers requiring polarization index measurement, verify that the short-circuit current of the insulation resistance meter is not less than 2mA. Record the ambient temperature during testing to facilitate comparison with factory values at equivalent temperatures. The measured values should not be less than 70% of factory values. Test items should include: HV-(MV+LV+ground), MV-(HV+LV+ground), LV-(MV+HV+ground), overall-ground, core-(clamp+ground), and clamp-(core+ground). For example, for HV-(MV+LV+ground), short-circuit all three phases of the high-voltage side and the corresponding neutral point (if present), ground all other parts, connect the high-voltage terminal of the insulation resistance meter to the HV side, and the ground terminal to ground for testing.

8. Measurement of Dissipation Factor (tanδ) for Windings with Bushings

Test using the reverse wiring method, following the instrument's specified wiring configuration. Test items include: HV-(MV+LV+ground), MV-(HV+LV+ground), LV-(MV+HV+ground), and overall-ground, performed sequentially. During testing, suspend the high-voltage test leads of the dissipation factor tester with insulating tape to prevent contact with the transformer tank. Record the ambient temperature during testing. When comparing with factory values at equivalent temperatures, the measured values should not exceed 1.3 times the factory values. If measurements deviate significantly from factory values, clean the bushings or use conductive shielding on the bushings to reduce surface leakage current. Testing should preferably be conducted during weather with relatively low humidity.

9. DC Leakage Current Measurement for Windings with Bushings

Leakage current measurement should preferably be taken at the high-voltage terminal. Test items include: HV-(MV+LV+ground), MV-(HV+LV+ground), LV-(MV+HV+ground). Testing should be conducted during weather with low humidity, and the ambient temperature should be recorded. The leakage current values must not exceed the specifications in the handover standards.

10. Electrical Tests

10.1 Winding Deformation Test

For transformers rated 35kV and below, the low-voltage short-circuit impedance method is recommended. For transformers rated 66kV and above, the frequency response analysis (FRA) method is recommended for measuring winding characteristic spectra.

10.2 AC Withstand Voltage Test

Perform AC withstand voltage tests at the transformer terminals using either externally applied power frequency voltage or induced voltage methods. When possible, use series resonance induced voltage testing to reduce the required test equipment capacity. For transformers rated 110kV and above, the neutral point should preferably undergo separate AC withstand voltage testing. Test voltage values should follow handover standards.

10.3 Long-Time Induced Voltage Test with Partial Discharge Measurement

For transformers rated 220kV and above, long-time induced voltage tests with partial discharge measurement must be performed on-site during new installation. For 110kV transformers, partial discharge testing is recommended when insulation quality is in doubt. These tests detect non-penetrating internal insulation defects in transformers.

10.4 Full-Voltage Impulse Closing Test at Rated Voltage

Perform according to startup plan requirements.

10.5 Phase Verification

Verify the transformer phase sequence, which must match the grid phase sequence.

Special attention should be paid to oil characteristics at negative temperatures for each oil system. For example, the oil inside the main tank has higher viscosity at negative temperatures, resulting in poor fluidity and heat dissipation. The oil in the on-load tap changer switching compartment at negative temperatures can prolong the switching process and increase the temperature rise of transition resistors.

For the main tank oil system of EHV oil-immersed transformers, attention should also be paid to oil flow electrification phenomena. Prevent the transition from oil flow electrification to oil flow discharge by controlling oil resistivity, oil flow velocity in various parts, and providing sufficient space for releasing electrical charges in the oil.