Testing is the cornerstone of ensuring transformer reliability and a key bridge connecting transformer theoretical design and operational safety. For new transformers, rigorous factory testing ensures that the equipment meets the nameplate specifications and can withstand the test of long-term operation. For refurbished or remanufactured, the focus of testing shifts to diagnostic verification — confirming that the iron core and windings are intact, and that the refurbishment process has successfully restored the equipment's integrity to near-original standards.
Naturally, testing must comply with strict standards. Currently, IEEE C57.12.00 and IEEE C57.12.90 are the main standards for liquid-immersed distribution transformers, power transformers, and voltage-regulating transformers. IEEE C57.12.01 and IEEE C57.12.91, on the other hand, regulate dry-type distribution transformers and power transformers. These standards define "routine tests" (required for each piece of equipment), "design tests" (used to verify model design), and "other tests" (customer-specific tests). Understanding these classifications is crucial for engineers to ensure that the selected transformer — whether new or refurbished — can withstand the expected load without premature failure.
Standard Test Checklist for New and Refurbished Transformers:
Test Items
Remanufactured
(All Types)
New 600V
Dry
New MV
Dry
New Dist.
Liquid
New Class I
Power Sub
Turns Ratio (TTR)
Required
Required
Required
Required
Required
Winding Resistance
Required
≥ 300 kVA
≥ 300 kVA
> 2500 kVA
Required
Impedance & Load Loss
Required
Required
Required
Required
Required
Polarity & Phase Relation
Required
Required
Required
Required
Required
No-Load Loss & Excitation
Required
Required
Required
Required
Required
Applied Voltage (Hipot)
Optional
Required
Required
Required
Required
Induced Voltage
Optional
Required
Required
Required
Required
Insulation Res. (Megger)
Required
≥ 300 kVA
≥ 300 kVA
Required
Required
Insulation Power Factor
Optional
Optional
Optional
Optional
Required
Lightning Impulse
Optional
Design Test
Design Test
Design Test
Required
Pressure & Leak Test
Required
N/A
N/A
Required
Required
Oil Analysis (DGA)
Optional
N/A
N/A
Optional
Required
1. Turns Ratio Test (TTR)
Measures the voltage ratio between transformer windings to verify compliance with nameplate values and design specifications.
Significance: Validates correct winding turns, detects inter-turn short circuits or tap switch faults, and is mandatory for factory acceptance and post-overhaul checks.
Standard Range:
- Power transformers: Ratio deviation typically ≤ ±0.5% (IEC 60076);
- Distribution transformers: Generally ≤ ±1%.
2. Winding Resistance
Measures the DC resistance of each phase winding to reflect conductor material, cross-sectional area, and joint contact conditions.
Significance: Detects broken strands, loose connections, or inter-turn short circuits in windings.
Standard Range:
- Three-phase unbalance: ≤2% for distribution transformers, ≤1% for power transformers;
- Deviation from factory values: Generally ≤ ±2%.
3. Impedance Voltage and Load Loss
Impedance voltage is the voltage applied to the primary winding when the secondary is shorted to achieve rated current; load loss is the active power loss under this condition.
Significance: Evaluates load capacity, efficiency, and short-circuit withstand capability, and is a key indicator for energy efficiency classification.
Standard Range:
- 10kV distribution transformers: Load loss meets IEC60076 energy efficiency requirements;
- Large power transformers: Impedance voltage deviation typically ≤ ±10%.
4. No-Load and Phase Relationship
No-load loss is the active power loss when the secondary is open; no-load current is the excitation current under this condition; phase relationship verifies winding polarity and connection group designation.
Significance: Assesses core quality, detects local overheating, and ensures phase consistency for parallel operation.
Standard Range:
- No-load current: ≤2% of rated current for distribution transformers;
- Connection group designation must match the nameplate.
5. No-Load Loss and Excitation Current
No-load loss refers to hysteresis and eddy current losses in the core under alternating magnetic fields; excitation current is the current required to maintain core magnetization.
Significance: Reflects core material performance and manufacturing quality, and is a core indicator for energy efficiency assessment.
Standard Range:
- Energy efficiency Level 1 distribution transformers: No-load loss reduced by ~30% compared to standard models;
- Excitation current: Generally 1%–3% of rated current.
6. Applied Voltage Test
Applies a specified power frequency AC voltage between windings and ground for a defined duration.
Significance: Verifies the ability of main insulation to withstand power frequency overvoltages and validates insulation strength.
Standard Range:
- 10kV transformers: Applied voltage 35kV/1min (IEC60071);
- 35kV transformers: Applied voltage 80kV/1min(IEC60071).
7. Induced Voltage Test
Applies a voltage higher than the rated value across windings by increasing the power frequency to assess longitudinal insulation (inter-turn, layer-to-layer) strength.
Significance: Detects inter-turn insulation defects to prevent short circuits during operation.
Standard Range:
- Induced voltage: Typically 2× rated voltage;
- Duration: Adjusted by frequency, generally ≤60s.
8. Impulse Voltage Test
Simulates lightning overvoltages by applying standard waveform impulse voltages (e.g., 1.2/50μs) to the transformer.
Significance: Validates the ability of winding insulation to withstand lightning overvoltages, a critical test for high-voltage transformers.
Standard Range:
- 110kV transformers: Full-wave impulse voltage 480kV(IEC60071);
- 220kV transformers: Full-wave impulse voltage 950kV (IEC60071).
9. Insulation Resistance (Megger)
Measures insulation resistance between windings-to-ground and phase-to-phase using a megohmmeter to reflect moisture, contamination, or degradation of insulation materials.
Significance: Quickly assesses overall insulation health and is the primary test in preventive maintenance.
Standard Range:
- Transformers ≤35kV: Insulation resistance ≥300MΩ (20℃);
- Absorption ratio R60/R15 ≥1.3, polarization index ≥1.5.
10. Insulation Power Factor
Measures the dielectric loss tangent (tanδ) of insulation to reflect aging, moisture, or local defects in dielectric materials.
Significance: Evaluates overall dielectric performance and is more sensitive than insulation resistance to early-stage degradation.
Standard Range:
- New transformers: tanδ ≤0.5% (20℃);
- In-service transformers: tanδ ≤1.0%, with no significant increase from historical values.
11. Temp Rise Test
Measures temperatures of windings, core, and oil under rated load to calculate the temperature rise above ambient conditions.
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