Power transformers are critical assets in electrical networks. To ensure reliability, safety, and performance, a series of tests must be conducted in accordance with international standards such as IEC 60076. These tests are categorized into Routine Tests, Type Tests, Special Tests, and Extra Tests. This guide provides a comprehensive overview of each test, its purpose, and the general methodology involved.
Routine Tests
Routine tests are conducted on every transformer to verify manufacturing quality, electrical integrity, and operational readiness.
2.1 Measurement of dissolved gasses in dielectric liquid from each separate oil compartment except diverter switch compartment. (Before test and After test)
Purpose: To analyze the condition of the insulating oil and detect potential internal faults (e.g., arcing, overheating) before and after other tests.
Method: Oil samples are taken from each separate oil compartment (excluding the diverter switch compartment) and analyzed using gas chromatography.
Key Parameters: Hydrogen (H₂), Acetylene (C₂H₂), Methane (CH₄), Carbon Monoxide (CO), and moisture content.
2.2 Check of core and frame insulation for liquid immersed transformers with core or frame insulation
Purpose: To verify that the core and clamping structures are properly insulated from the tank to prevent circulating currents.
Method: Insulation resistance is measured between the core and tank, core and clamping unit, and clamping unit and tank using a megohmmeter.
2.3 Measurement of Voltage Ratio and Phase Displacement
Purpose: To confirm that the voltage ratio between windings and the vector group (e.g., YNd1) match the design specifications.
Method: The voltage ratio is measured on all taps, and the phase displacement is checked against the specified vector group.
2.4 Measurement of Winding Resistance
Purpose: To detect loose connections, broken strands, or incorrect tap-changer settings.
Method: DC current is injected into the windings, and the resistance is measured for each phase and tap position. Results are corrected to a reference temperature (usually 75°C).
2.5 Check of Ratio and Polarity of Built-in Current Transformers (CTs)
- Purpose: To ensure that the CTs embedded in the bushings or turrets have the correct ratio and subtractive polarity.
- Method: The turns ratio is verified using a ratio meter, and polarity is checked against the marking (P1/P2, S1/S2).
2.6 Measurement of DC Insulation Resistance between each winding to earth and between windings
Purpose: To assess the overall dryness and cleanliness of the insulation system.
Method: Insulation resistance is measured between windings and earth, and between windings, using a DC voltage (typically 5000V). The absorption ratio (R60/R15) and polarization index (PI) are calculated.
2.7 Measurement of dissipation factor (tanθ)of the insulation system capacitances
Purpose: To evaluate the dielectric losses in the insulation system, indicating moisture or aging.
Method: Capacitance and tan δ are measured for the main body and bushings at a specified voltage (e.g., 10 kV).
2.8 Determination of capacitances windings to earth and between windings
Purpose: To determine the capacitance values between windings and earth and between windings.
Method: This is often derived from the same measurement as the dissipation factor (Tan δ).
2.9 On-Load Tap-Changer (OLTC) Operation Test
Purpose: To verify the mechanical and electrical operation of the OLTC.
Method: The OLTC is operated through its full range under various conditions: de-energized, with reduced control voltage (85%), energized at no-load, and with rated current applied.
2.10 & 2.11 No-Load Loss and Current Measurement
Purpose: To measure the magnetizing losses (core losses) and excitation current.
Method: The transformer is energized at rated voltage and frequency on one winding while the other is open-circuited. Measurements are typically taken at 90%, 100%, and 110% of rated voltage.
2.12 Measurement of Short-Circuit Impedance and Load Loss
Purpose: To verify the impedance voltage and copper losses (load losses).
Method: One winding is short-circuited, and a reduced voltage is applied to the other winding to circulate rated current. Measurements are taken on the principal tap and extreme taps.
2.13 & 2.14 Dielectric Tests (LI, LIC, AC, AV, IVPD)
These tests verify the insulation strength of the transformer.
Full Wave Lightning Impulse Test for the line terminals (LI)
- Purpose: To verify the insulation strength of the transformer windings against high-voltage surges simulating a direct lightning strike. This test ensures that the insulation coordination between turns, layers, and to earth is adequate.
- Method: A high-voltage impulse wave (standard 1.2/50 µs) is applied to each line terminal. The test sequence typically includes one reduced-level impulse, one rated-level impulse, and two additional rated-level impulses. The oscillogram of the applied voltage and neutral current is recorded. Any abnormal collapse or distortion in the waveform indicates insulation failure.
Chopped Wave Lightning Impulse Test (LIC)
- Purpose: To simulate the stress on the transformer insulation when a lightning surge is interrupted by a protective device (e.g., arrester), creating a steep-fronted voltage collapse. This test is critical for verifying inter-turn and inter-layer insulation.
- Method: A lightning impulse is applied but “chopped” (abruptly terminated) after a short duration (typically 2–5 µs) by a chopping gap. The test is performed at a higher peak voltage (e.g., +10% of the LI level) to account for the increased stress. The oscillogram is analyzed for voltage overshoot and current behavior.
Line Terminal AC Withstand Voltage Test (LTAC)
Purpose: To verify the insulation strength between line terminals and between line terminals and earth under power frequency voltage.
Method: A single-phase AC voltage is applied between line terminals of the same winding, with the transformer in air or oil. The test is performed for a specified duration (e.g., 30–60 seconds) without sudden voltage drop or breakdown.
Applied Voltage Test (AV)
Purpose: To check the main insulation between each winding and earth, and between separate windings, under power frequency voltage.
Method: AC voltage is applied between the winding under test and all other windings connected to earth (or tank) for 60 seconds. The test voltage is specified based on the insulation level (e.g., 95 kV for HV side). A successful test is indicated by the absence of sudden voltage drop or flashover.
Induced Voltage Withstand Test with PD Measurement (IVPD)
- Purpose: To verify the inter-turn, inter-layer, and main insulation, as well as to detect partial discharge (PD) activity that could lead to long-term degradation.
- Method: A voltage exceeding the rated voltage (typically
) is applied to one winding to induce a proportional voltage in the other winding. The test is performed at a frequency higher than rated (to avoid core saturation) and includes a long-duration phase (e.g., 30–60 minutes) where partial discharge levels are continuously monitored. A typical acceptance criterion is PD ≤ 100 pC at .
2.15 Leak Testing with Pressure (Tightness Test)
- Purpose: To ensure the tank and gaskets are sealed against oil leakage.
- Method: The transformer is filled with oil, and a positive pressure (e.g., 30 kPa) is applied for a specified duration (e.g., 24 hours).
Type Tests
3.1 Temperature-Rise Type Test
Purpose: To verify that the cooling system is adequate and that temperature limits (top oil, average winding, hot-spot) are not exceeded.
Method: The transformer is subjected to total losses (load loss + no-load loss) using the short-circuit method until temperatures stabilize. Temperatures are measured using resistance methods (winding) and thermometers (oil).
3.2 Determination of Sound Level
Purpose: To verify that the noise level meets the guaranteed values.
Method: The transformer is energized at rated voltage and frequency. Sound pressure levels are measured at specified points (usually at 0.3m and 2m from the surface) and calculated as sound power level (dBA) per IEC 60076-10.
3.3 Measurement of Power Taken by Fan and Liquid Pump Motors
Purpose: To measure the auxiliary power consumption of the cooling equipment.
Method: The power input to the cooling fans and oil pumps is measured under rated operating conditions.
Special Tests
Special tests are performed by agreement between the manufacturer and customer to provide additional data on specific characteristics.
4.1 Measurement of Zero-Sequence Impedance(s) on three-phase transformers
Purpose: Required for system protection coordination and earth fault calculations.
Method: For three-phase transformers (e.g., YNd1), the three phases are connected in parallel, and a single-phase voltage is applied.
4.2 Winding Hot-Spot Temperature-Rise Measurement
Purpose: To directly measure the hottest temperature within the winding using fiber optic sensors.
Method: Fiber optic probes are inserted into the winding during assembly. This is typically done during the temperature-rise test to validate the calculated hot-spot.
4.3 Determination of Transient Voltage Transfer Characteristics
Purpose: To understand how high-voltage surges are transferred from the HV side to the LV side (relevant for insulation coordination of connected equipment).
Method: Impulse voltages are applied to the HV terminals, and the induced voltages on the LV terminals are recorded.
4.4 Measurement of Frequency Response (Frequency Response Analysis or FRA)
Purpose: To create a “fingerprint” of the transformer’s mechanical integrity for future comparison (detects winding deformation).
Method: A sweep frequency voltage is injected, and the transfer function (magnitude/phase) is measured across a wide frequency range (e.g., 10 Hz to 1 MHz).
4.5 Pressure and Vacuum Deflection Tests on liquid immersed Transformers
Purpose: To verify the mechanical strength of the tank.
Method: The tank is subjected to positive pressure (e.g., 100 kPa) or vacuum (e.g., 133 Pa). Permanent deflection at critical points must not exceed specified limits (typically 1 mm).
Extra Tests
Extra tests are additional checks not strictly mandatory but performed for specific quality or operational requirements.
5.1 Measurement of No-Load Current Harmonics
Purpose: To analyze the harmonic content of the magnetizing current, which is important for power quality studies.
Method: During the no-load test, the current waveform is analyzed to determine the magnitude of the 3rd, 5th, 7th, etc., harmonic components.
5.2 Determination of Dew Point / Moisture Content
Purpose: To measure the absolute moisture content in the insulating oil or solid insulation.
Method: Dew point meters or Karl Fischer titration (for oil) are used. In the provided report, moisture content was found to be less than 0.5%.
