Incoming Material Inspection
Enameled Copper Wire Inspection
Taking a one-meter core wire as a sample, our technicians measure and calculate the following parameters by using micrometers and a DC resistance tester:
- Bare Copper Dimensions: Width and thickness of the bare wire.
- Overall Dimensions: Width and thickness of the enameled copper wire.
- Enamel Film Thickness: Calculated from overall and bare copper dimensions.
- Electrical Conductivity: Calculated based on measured resistance and conductor area.
Purpose: This inspection ensures a perfect core fit, preventing leakage, arcing, overheating, or burnout at startup.
Insulating Oil Inspection
Equipment: Karl Fischer Moisture Titrator
OMEK Standard: ≤ 30 mg/kg
Purpose: This inspection is to identify any contamination in the water which would cause transformer failure because of its insulation properties, and the unit would be damaged internally by arcs and irreversible damage to the internal paper insulation.
Equipment: Automatic Oil Dissipation Factor & Resistivity Tester
OMEK Standard: tan delta ≤ 0.035% (National Standard: tan delta ≤ 1.0%)
Purpose: This inspection is to detect any incipient oil deterioration, moisture absorption or microscopic oil contamination contaminants, effectively preventing any undetected insulation failure, internal electrical arcing or leakage currents.
Equipment: Automatic Oil Dielectric Strength Tester
OMEK Standard: Raw Oil: ≥ 35 kV; Filtered Transformer Oil: ≥ 70 kV
Purpose: This test determines the highest voltage that the oil can withstand when used under high-voltage conditions, so that there will be no short circuit, violent flashover or dielectric breakdown.
Equipment: Gas Chromatograph
The equipment detects the level of dissolved gases in transformer oil, including hydrocarbons, alkanes, hydrogen (H(2)), carbon monoxide (CO), carbon dioxide (CO(2)), and acetylene (C(2)H(2)).
OMEK Standard: C(2)H(2) must be 0 ppm.
Finished Product Inspection
Routine Tests — 100% Full Inspection
Measurement of Voltage Ratio and Check of Phase Displacement
Each and every single tap position must be measured.
Purpose: This inspection guarantees that actual output voltage is fully compliant with the rated one and ensures that internal coil lead wiring is free of polar or phase errors.
Winding DC Resistance Measurement
OMEK Standard: The three-phase resistance unbalance rate ≤ 2%.
Purpose: Our workers measure the DC resistance for all phases across every tap position on both high-voltage and low-voltage sides. In this way, we can detect defects inside the conductor like broken conductor strands, cold/poorly soldered joints or loose connections.
No-Load Loss and Current Measurement
OMEK Standard:
No-Load Loss (P(0)): Max +15% deviation from guaranteed value (total losses within limit).
No-Load Current (I(0)%): Max +30% positive deviation from design limit per GB/IEC standards.
Purpose: This measurement can verify whether your transformer is energy-efficient or not, and identify any quality defects in the core.
Load Loss and Short-Circuit Impedance Measurement
OMEK Standard:
Load Loss (P(k)): The measured load loss cannot exceed the guaranteed value by more than +15%.
Short-Circuit Impedance (U(k)%): The deviation of the measured impedance from the design value should be within ±7.5%.
Purpose: To test how much power is wasted during full load and clarify the short-circuit impedance to ensure it is safe and reliable under heavy loads or sudden short circuits.
Induced Overvoltage Withstand Test
Duration: 60 seconds at full test voltage.
OMEK Standard:
No current sudden surges.
No internal discharge sounds or arcing.
No degradation of insulation resistance after test.
Purpose: To test inter-turn and inter-layer insulation resistance under extreme high-frequency overvoltage, so as to prevent internal short circuits when transformers suffer lightening stroke, power grid fluctuation, etc..
Applied AC Voltage Withstand Test
Duration: 60 seconds at full power-frequency test voltage.
OMEK Standard:
Zero voltage breakdown or breaker tripping.
Zero flashover or surface arcing.
No abnormal noise or burning odors.
Zero insulation resistance degradation post-test.
Purpose: To test whether the insulation layer is tough enough and whether it will leak current or suffer a breakdown under extreme high AC voltage which exceeds its daily operating voltage.
Type Tests——Sampling Inspection
Lightning Impulse Test
OMEK Standard:
Oscillogram Coincidence: Complete overlap of voltage and current oscillograms between reduced level (50%-70%) and full test voltage (100%).
Integrity: Zero indication of insulation puncture, internal breakdown, or surface arcing.
Purpose: To simulate atmospheric lightning strikes and high-voltage switching surges to see if the transformer can survive without blowing up.
Temperature-rise Test
OMEK Standard: Thermal equilibrium reached (temperature change < 1K/hr) at rated losses with all temperature rises strictly within guaranteed tolerances.
Purpose: To validate cooling system efficiency and thermal dissipation capabilities under continuous rated full-load conditions, ensuring long-term thermal longevity without insulation degradation.
Partial Discharge Measurement
OMEK Standard: Measured PD quantity ≤ 100pC (or strict contract-specified limit, e.g., ≤ 50pC for high-voltage transformers).
Purpose: To detect transformer insulation aging and defects (such as voids, impurities, and moisture).
Sound Level Measurement
OMEK Standard:
| Transformer Range | Dry-Type Distribution Transformers | Oil-Immersed Distribution Transformers |
| 100 – 500 kVA | ≤ 50-55 dBA | ≤ 45-50 dBA |
| 630 – 1,250 kVA | ≤ 55-58 dBA | ≤ 50-53 dBA |
| 1,600 – 3,150 kVA | ≤ 58-62 dBA | ≤ 53-56 dBA |
Purpose: To measure noise levels to avoid disturbance and detect internal defects early to prevent failures during service.
Special Tests
OMEK transformers can also undergo special tests according to customers’ specific requirements and project conditions. Examples include:
Get in Touch
Our professional testing and technical team will be happy to meet your needs.
✅ 11kV Oil-immersed Transformer Type Test Report (85 pages)
✅ 33kV Dry-type Transformer KEMA Style Report
✅ Sound Level & Temperature Rise Test Data
Reliable testing ensures international standard compliance.