How to determine the insulation performance of a transformer？

Step 1: Conduct an initial visual inspection 1. Inspect the appearance of the insulation components: Check if The Transformer's bushing has cracks, damage, or stains. 2. Inspect the transformer oil: Observe the color of the oil - whether it is clear, transparent, cloudy, or black. 3. Inspect the leads and grounding: Check if the insulation sheath of the leads is intact, and whether the distance between the leads and the box body, as well as the iron core, meets the requirements. Avoid discharge caused by insufficient insulation distance.

Step2:Core Electrical Test and Inspection
1. Insulation Resistance Test (Most Common and Basic Method)
Test Equipment: Megohmmeter (Multimeter), select the appropriate range based on the transformer voltage level:
Low-Voltage Transformer (≤1kV): Select 500V Megohmmeter
Medium and high-voltage transformer (6kV and above): Select 2500V or 5000V Megohmmeter
Test Items and Standards
Insulation Resistance to Ground: Connect the test winding terminal to the “L” terminal of the Megohmmeter, connect the transformer core and oil tank shell to the “E” terminal, short-circuit and ground the non-tested winding, rotate the Megohmmeter at a constant speed, and read the insulation resistance value at 1 minute.
Insulation Resistance Between Windings: Connect two tested windings to the “L” and “E” terminals respectively, short-circuit and ground the remaining windings, and read the insulation resistance value at 1 minute.
Judgment Criteria
The insulation resistance value should not be lower than the product technical requirements or industry standards.
More importantly, compare with historical data: at the same temperature, the current insulation resistance should not be lower than the previous test value by 70%, otherwise it indicates a decline in insulation performance.
After the test, record the ambient temperature, as the insulation resistance decreases with temperature increase, and convert it to the same reference temperature (usually 20℃) for comparison.
2. Absorption Ratio and Polarization Index Test (To Determine Insulation Moisture)
This test is an extension of the insulation resistance test and is mainly used to determine whether the insulation is moist:
Absorption Ratio (K₁₀₋₆₀): The ratio of the insulation resistance value at 60s to the insulation resistance value at 15s.
For transformers with a voltage level ≥ 6kV, the absorption ratio ≥ 1.3 is qualified, and values lower than this indicate severe insulation moisture.
Polarization Index (PI): The ratio of the insulation resistance value at 10 minutes to the insulation resistance value at 1 minute.
A PI ≥ 1.5 is qualified and is applicable for judging the insulation state of large-capacity and high-voltage transformers.
3. Dielectric Loss Tangent Value (tanδ) Test (To Determine Insulation Aging)
Test Principle: By measuring the energy loss of the insulating medium under AC voltage, determine whether the insulation is aging, moist, or has local defects.
Judgment Criteria
The smaller the tanδ value, the better, and it should comply with the standard requirements for the corresponding voltage level and capacity.
The tanδ values of different windings of the same transformer and the same batch of products should be basically consistent. If there is a significant difference, it indicates a local insulation defect.
4. Oil Dissolved Gas Analysis
By analyzing the gas components dissolved in transformer oil (such as hydrogen, methane, acetylene, etc.) using a gas chromatograph, it can determine whether the insulation has latent faults such as overheating or discharge, which is an important means for predicting insulation failure.

Step3:Supplementary Tests in Special Circumstances
1. Insulation Oil Breakdown Voltage Test: This test assesses the oil's ability to withstand voltage. If the breakdown voltage is below the standard, it indicates that the oil quality has deteriorated or the moisture content is too high. In such cases, the oil needs to be replaced or filtered.
2. Partial Discharge Test: For high-voltage and large-capacity transformers, this test examines whether there is any partial discharge inside the insulation. Such discharge accelerates the aging of the insulation.