Power transformers and distribution transformers (collectively referred to as "transformers" below) are critical equipment in power systems, and routine testing is mandatory before factory delivery or on-site commissioning to verify compliance with design standards, safety requirements, and operational reliability. The testing items are standardized by international standards (e.g., IEC 60076 series) and national standards (e.g., GB/T 1094 series), with slight differences between power transformers (higher voltage/capacity, used in transmission networks) and distribution transformers (lower voltage/capacity, used in distribution networks). Below is a structured summary of core routine testing items, categorized by functional objectives:
Electrical Performance Testing (Core Routine Items)
1. Insulation Resistance Test (IR Test)
Purpose: Evaluate the integrity of the transformer's insulation system (windings, core, bushings, etc.) and detect defects such as moisture, contamination, or insulation degradation.
Principle: Use a megohmmeter (500V/1000V/2500V, depending on rated voltage) to measure the resistance between windings, windings to ground (core/ tank), and between different winding phases.
Standards:
No significant drop compared to factory baseline data (typically ≥ 100 MΩ for distribution transformers, ≥ 500 MΩ for power transformers).
The ratio of insulation resistance at 60°C to 20°C should be within 1.5–2.5 (temperature correction required).
Applicability: Mandatory for all transformers; on-site testing often includes pre- and post-installation comparisons.
2. Winding Resistance Measurement
Purpose: Verify winding manufacturing quality (e.g., tightness of turns, correctness of wire gauge), detect poor connections (e.g., loose terminals), and calculate copper loss.
Principle: Use the DC bridge method (Kelvin bridge for low resistance, Wheatstone bridge for medium resistance) to measure the DC resistance of each winding (high-voltage, low-voltage, tap windings) at room temperature.
Standards:
For three-phase transformers: The unbalance rate of phase resistance ≤ 2%; unbalance rate of line resistance ≤ 1%.
For single-phase transformers: Resistance values should be within ±3% of the design value.
Note: Results must be corrected to a reference temperature (e.g., 75°C for copper windings) using the formula: Rt=R20×235+20235+t (235 is the temperature coefficient of copper).
3. Voltage Ratio and Tap Position Verification
Purpose: Ensure the transformer's voltage ratio (high-voltage/low-voltage) matches the design value and that tap changers (on-load or off-load) operate correctly.
Principle: Apply a low AC voltage (typically 10–20% of rated voltage) to one winding (e.g., high-voltage side), measure the induced voltage on the other winding, and calculate the actual voltage ratio: kactual=U2U1.
Standards:
The deviation between the actual voltage ratio and the rated ratio ≤ ±0.5% (for distribution transformers) or ±0.2% (for power transformers).
For transformers with tap changers: Test all tap positions to confirm no missing or incorrect taps.
Method: Use a dedicated voltage ratio tester (with automatic tap position detection for on-load tap changers, OLTC).
4. Polarity and Phase Sequence Check
Purpose: Prevent reverse connection of windings (which would cause short circuits or incorrect power flow) and ensure phase sequence consistency with the power system.
Polarity Test:
Principle: Use the "dot marking" method-apply a DC voltage to the primary winding, and observe the direction of the induced current in the secondary winding using a galvanometer. If the galvanometer deflects positively, the connected terminals are of the same polarity (marked with dots).
Phase Sequence Test:
Principle: Apply three-phase AC voltage to the primary winding, measure the phase relationship between primary and secondary voltages using an oscilloscope or phase sequence meter. Ensure the phase sequence (e.g., ABC) is consistent with the design.
Standard: Polarity and phase sequence must match the nameplate and design drawings (critical for parallel operation of transformers).
5. Short-Circuit Impedance Measurement (Zk Test)
Purpose: Evaluate the transformer's ability to withstand short-circuit currents, calculate short-circuit loss, and verify winding mechanical strength.
Principle: Short-circuit one winding (e.g., low-voltage side), apply a low AC voltage to the other winding (e.g., high-voltage side), and adjust the current to the rated value. Measure the applied voltage (Uk) and input power (Pk). The short-circuit impedance is calculated as: Zk=UratedUk×100%, and short-circuit loss Pk is the power consumed at rated current.
Standards:
Short-circuit impedance deviation ≤ ±5% of the design value (critical for parallel operation, as impedance mismatch causes load imbalance).
Short-circuit loss ≤ ±10% of the design value (for distribution transformers) or ±5% (for power transformers).
Note: Tests are performed at rated frequency (50/60 Hz), and results are corrected to the reference temperature.
6. No-Load Loss and Excitation Current Measurement (Iron Loss Test)
Purpose: Evaluate the core's magnetic performance (e.g., hysteresis loss, eddy current loss) and detect core defects (e.g., loose laminations, short-circuited core bolts).
Principle: Open-circuit one winding (e.g., high-voltage side), apply the rated voltage (and rated frequency) to the other winding (e.g., low-voltage side), measure the input power (P0, no-load loss) and the excitation current (I0).
Standards:
No-load loss ≤ ±10% of the design value (distribution transformers) or ±5% (power transformers).
Excitation current ≤ 5% of rated current (distribution transformers) or ≤ 3% (power transformers, for large capacity units).
Key Insight: No-load loss is mainly caused by core material quality-excessive loss indicates poor core laminations or insulation damage between laminations.
7. Dielectric Withstand Test (Hi-Pot Test)
Purpose: Verify the insulation system's ability to withstand transient overvoltages (e.g., lightning, switching surges) without breakdown.
Types:
AC Dielectric Test: Apply a sinusoidal AC voltage (1.5–2.5 times the rated voltage) to windings for 1 minute (e.g., 3kV for 10kV distribution transformers, 20kV for 110kV power transformers).
DC Dielectric Test: Used for large power transformers (to avoid capacitive current overload); apply a DC voltage (2.5–3 times the rated AC voltage) for 1 minute.
Standard: No insulation breakdown (arcing, flashover) or significant leakage current increase during the test.
Note: Perform after insulation resistance testing (if IR is too low, Hi-Pot test may cause insulation damage).
Oil-Filled Transformer-Specific Tests (Most Power/Distribution Transformers Are Oil-Immersed)
1. Transformer Oil Quality Test
Purpose: Ensure the insulating oil's dielectric strength, moisture content, and purity (oil acts as insulation and coolant).
Core Items:
Breakdown Voltage Test: Measure the voltage at which oil breaks down (≥ 35 kV/2.5mm gap for new oil, ≥ 25 kV for in-service oil).
Moisture Content: ≤ 10 ppm (parts per million) for new oil, ≤ 30 ppm for in-service oil (moisture reduces insulation performance).
Acidity: ≤ 0.03 mg KOH/g (acidity accelerates insulation aging).
Dissolved Gas Analysis (DGA): For power transformers-detect gases (e.g., H₂, CH₄, C₂H₂) generated by insulation degradation or overheating (optional for routine testing but mandatory for large units).
2. Oil Leakage Test
Purpose: Prevent oil leakage (which causes insulation failure and environmental pollution).
Methods:
Pressure Test: Seal the transformer tank, fill it with dry air or nitrogen to 0.03–0.05 MPa, and maintain for 24 hours-no pressure drop or visible leakage.
Oil Immersion Test: Submerge the tank (or key joints) in water; no air bubbles indicate no leakage.
Standard: No oil leakage at joints (flanges, bushings, tap changers) or welds.
Mechanical and Auxiliary System Testing
1. Tap Changer Operation Test (For Transformers with Tap Changers)
Purpose: Verify the reliability of on-load tap changers (OLTC) or off-load tap changers (OLTC).
Items:
Mechanical Operation: Manually or electrically operate the tap changer through all positions-no jamming, and the tap position indicator is accurate.
Electrical Continuity: Check that the winding circuit is continuous at each tap position (no open circuits).
OLTC Specific: Test transition resistance (to avoid arcing during tap change) and mechanical endurance (≥ 10,000 operations for OLTC).
2. Core Grounding Test
Purpose: Prevent floating potential in the core (which causes insulation damage) while avoiding core short circuits.
Principle: Measure the resistance of the core grounding lead (typically 1–100 Ω). A resistance of 0 Ω indicates a short-circuited core (hazardous), while infinite resistance indicates poor grounding.
Standard: Core must be grounded at one point (single-point grounding) with resistance within the specified range.
3. Temperature Indicator and Cooling System Test
Temperature Indicator: Calibrate the oil temperature and winding temperature indicators to ensure accuracy (deviation ≤ ±2°C).
Cooling System:
For natural cooling (ONAN): Verify no obstructions to heat dissipation.
For forced cooling (ONAF/OFAF): Test fans/pumps-start/stop automatically based on temperature, and no abnormal noise or vibration.