Learn the step-by-step procedure for Megger testing for transformer insulation resistance. Ensure safety, accuracy, and reliability with proper testing methods and precautions.
Purpose of Megger Testing
The insulation resistance (IR) measurement is the most fundamental and widely used test for assessing transformer insulation strength. This test not only helps detect major internal defects—such as an unremoved temporary transportation mount on the live part of the tap changer—but also evaluates the insulation quality and the dryness level of the paper insulation, which directly impacts the transformer’s efficiency, reliability, and overall lifespan.
Regular IR testing ensures early detection of potential failures, preventing costly breakdowns and enhancing operational safety in electrical systems.
Test Equipment
Megger testing for transformer insulation resistance is performed using Meggers available in 500V, 1000V, 2500V, and 5000V ratings. A 2.5 kV Megger is required for transformer windings with voltage ratings of 430V, 500V, and above. For windings rated at 1000V and higher, a 1000V Megger is used. For transformers rated at 11 kV and above, a 5 kV motorized or digital Megger is preferred to ensure accurate insulation resistance (IR) measurements, especially for extra high voltage (EHV) transformers.
Testing Procedures
Megger testing for transformer insulation resistance must be performed across all windings collectively (with all windings connected together) and the earthed tank (earth). Additionally, readings should be recorded between each winding and the tank while keeping the remaining windings earthed.
Precautions Before Measurement
Before taking IR measurements, always disconnect the neutral from the earth to avoid incorrect readings.
IR Measurement Procedure
1) HV Windings to Earth
- Connect the Megger’s line terminal (5000V) to the transformer’s HV winding.
- Connect the Megger’s earth terminal to the earth pin provided at the electrical equipment.
- Turn on the Megger and record the insulation resistance (IR) reading in MΩ.
2) LV Windings to Earth
- Connect the Megger’s line terminal (500V) to the transformer’s LV winding.
- Connect the Megger’s earth terminal to the earth pin provided at the electrical equipment.
- Turn on the Megger and record the insulation resistance (IR) reading in MΩ.
3) HV Windings to LV Windings
- Connect the Megger’s line terminal (5000V) to one of the transformer’s windings.
- Connect the Megger’s earth terminal to the transformer’s other winding.
- Turn on the Megger and record the insulation resistance (IR) reading in MΩ.
Minimum IR Requirements
The following are the minimum insulation resistance (IR) values required for different voltage classes, measured for 1 minute:
- Below 6.6 kV Class: 200 MΩ
- 6.6 kV to 11 kV Class: 400 MΩ
- Up to 33 kV Class: 500 MΩ
- Up to 132 kV Class: 600 MΩ
- Up to 220 kV Class: 650 MΩ
IR Measurement Combinations for Different Transformer Types
| Two-Winding Transformer | Three-Winding Transformer | Auto Transformer | Shunt Reactor |
| HV to Earth | HV + IV + LV to Earth | HV + IV to Earth | HV to Earth |
| HV to LV | HV + IV to LV | HV + IV to LV | – |
| LV to Earth | HV + LV to IV | LV to Earth | – |
Where:
- HV = High Voltage Winding
- IV = Intermediate Voltage Winding
- LV = Low Voltage Winding
- E = Earth
These measurements ensure the transformer’s insulation is in optimal condition, helping to prevent electrical failures and improving overall reliability.
Core Insulation Check Using a 2500V Megger
To ensure proper insulation of the transformer core, insulation resistance (IR) measurements must be taken between the following components using a 2500V Megger:
| Test Points | Procedure | Required IR Value (for 60 seconds) |
| Core to Frame | Connect the Megger’s line terminal to the core and the earth terminal to the frame. | To be recorded in MΩ |
| Core to Tank | Connect the Megger’s line terminal to the core and the earth terminal to the tank. | To be recorded in MΩ |
| Frame to Tank | Connect the Megger’s line terminal to the frame and the earth terminal to the tank. | To be recorded in MΩ |
These measurements help ensure that the core insulation is intact and prevent potential faults due to insulation breakdown.
Connection Diagram- Megger Testing for Transformer
Measurement Table
Insulation Resistance (IR) Measurement Record
The following table is used to record insulation resistance (IR) values over different time intervals and calculate Dielectric Absorption Index (DAI) and Polarization Index (PI).
Test Conditions:
- Temperature: °C (To be recorded at the time of measurement)
- Megger Voltage Rating: As per test requirement
| Measurement Between | 15 Seconds (MΩ) | 60 Seconds (MΩ) | 600 Seconds (MΩ) | DAI Value (60 sec / 15 sec) | PI Value (600 sec / 60 sec) |
| HV to Earth | |||||
| HV to LV | |||||
| LV to Earth |
Formulas for Calculation:
- Dielectric Absorption Index (DAI) = IR (60 sec) / IR (15 sec)
- Polarization Index (PI) = IR (600 sec) / IR (60 sec)
These values help in assessing the condition of insulation. Typically, a PI value of 2.0 or higher indicates good insulation, while lower values may suggest moisture or insulation deterioration.
Precautions for Insulation Resistance (IR) Testing
- Document Temperature Before Testing: The ambient and equipment temperature must be recorded before starting the test, as insulation resistance values vary with temperature.
- Use Continuous Testing Cables: Ensure the test cables do not have any joints to avoid inaccurate readings or high resistance connections.
- Avoid Contact with Live Parts: Test leads must not touch any energized components to prevent electrical hazards.
- Properly Earth the Megger Body: If the megger has a separate earth terminal, it must be connected to the ground before testing.
- Clean Testing Surfaces and Terminals: Ensure that all terminals and surfaces are free from dirt, oil, or moisture, as contaminants can affect readings.
- Perform IR Testing in Dry, Sunny Conditions: Preferably, insulation resistance measurements should be taken in dry conditions to avoid moisture-related discrepancies.
- Do Not Connect Energized Equipment: Always verify that the equipment under test is de-energized before connecting it to the megger.
- Follow Proper Grounding Procedure: The ground terminal should be connected first before testing and should be the last to be disconnected.
- Ground Test Terminals After High Voltage Testing: Once high-voltage testing is completed, all test terminals should be properly grounded before personnel handle them.
- Use Shielded and Screened Test Leads: Ensure test leads are properly shielded to prevent leakage currents and interference, ensuring accurate IR measurements.
Following these precautions ensures safe and accurate insulation resistance testing while minimizing risks of electrical hazards.
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