1. Introduction
The insulation resistance test is among the most important routine tests for power transformers and shunt reactors. Specifically, these tests measure:
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Insulation resistance between each winding and ground (tank).
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Insulation resistance between different windings.
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Where accessible, insulation resistance between the tank, core frame, and core.
These values characterize the condition of the main insulation system. Consequently, they reveal developing problems such as moisture ingress, surface contamination, insulation ageing, and incorrect core grounding.
Furthermore, engineers use the same measurements—taken as a function of time—to calculate the Polarization Index (PI) and Absorption Index (AI). These indices provide a much deeper view of the overall insulation quality.
2. Test Principles, Preparation, and Depolarization
During the insulation resistance test, you apply a DC voltage and measure the resulting leakage current using an insulation resistance meter (a “megger”). You read the resistance at different times after energization, typically at 15 seconds, 60 seconds, and (when required) up to 10 minutes.
You must perform this test with great care because polarization and surface leakage strongly influence the final result.
2.1 General Preparation
Before starting any measurements, strictly follow these steps:
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Isolate the equipment: Disconnect the transformer from the system and remove all external connections.
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Create a common reference: Bond the tank, core, and frame together. Ground them to form a common reference during winding-to-ground tests.
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Clean the surfaces: Clean and dry the bushings. Contamination and moisture on bushing surfaces frequently cause misleadingly low readings.
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Check the environment: Perform the test when ambient humidity is reasonably low, and record the humidity for later interpretation.
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Safety warning: Never perform insulation resistance tests while the transformer is under vacuum.
For each winding under test, you must short-circuit all its terminals at the bushing together. Normally, you connect windings not under test to the tank/ground or to the Guard terminal of your test device.
2.2 Depolarization Between Tests
Dielectric absorption and residual charges in the insulation can heavily influence subsequent measurements. Therefore, you must use a depolarization step to obtain stable and reproducible results:
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After finishing a set of measurements, connect all windings together and bond them to the tank (ground).
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Maintain this connection for at least twice the duration of the preceding voltage application, or until the resistance reaches its steady value (whichever is shorter).
This interval allows the insulation to discharge and depolarize adequately before the next test. Additionally, using the guard terminal of the meter (according to the tables in Section 3) further reduces surface leakage influence and decreases the number of required depolarization cycles.
3. Typical Test Connections and Sequences
You must perform the insulation resistance test in a defined order. Below are the practical connection schemes for various transformer types.
3.1 Two-Winding Power Transformer (HV / LV)
For a three-phase, two-winding transformer, follow this main test sequence:
| Test No. | Measured Insulation | Megger “+” Lead | Megger “–” Lead | Guard Lead | Notes |
| 1 | HV – Ground | HV terminals (all phases shorted) | Tank / ground | LV terminals | |
| 2 | LV – Ground | LV terminals | Tank / ground | HV terminals | |
| 3 | Depolarization | – | – | – | Connect HV, LV & tank together |
| 4 | HV – LV | HV terminals | LV terminals | Tank / ground |
(Figure 1: Test configuration for insulation resistance measurement of the HV–GND, LV–GND, and HV–LV)
3.2 Three-Winding Power Transformer (HV / LV / TV)
For a three-winding transformer, use these typical tests and guard connections:
| Test No. | Measured Insulation | Megger “+” Lead | Megger “–” Lead | Guard Lead |
| 1 | HV – Ground | HV | Tank / ground | LV + TV |
| 2 | LV – Ground | LV | Tank / ground | HV + TV |
| 3 | TV – Ground | TV | Tank / ground | HV + LV |
| 4 | Depolarization | – | – | – |
| 5 | HV – LV | HV | LV | TV + tank |
| 6 | TV – LV | LV | TV | HV + tank |
These connections ensure that when you measure one winding to ground, the other windings act as a guard to effectively bypass surface leakage currents.
3.3 Three-Winding Autotransformer (HV / LV common winding + TV)
For an autotransformer, treat the common HV/LV winding as a single body for specific tests:
| Test No. | Measured Insulation | Megger “+” Lead | Megger “–” Lead | Guard Lead |
| 1 | (HV + LV) – Ground | (HV + LV) | Tank / ground | TV |
| 2 | TV – Ground | TV | Tank / ground | (HV + LV) |
| 3 | Depolarization | – | – | – |
| 4 | (HV + LV) – TV | (HV + LV) | TV | Tank / ground |
While the exact sequence can follow utility practice, the core principle remains: measure all relevant insulation paths using suitable guard connections and adequate depolarization intervals.
3.4 Core and Frame Insulation Resistance
Where core and frame connections are accessible, you can perform a separate core insulation test (distinct from the main winding tests):
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Temporarily disconnect the core and frame from earth.
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Measure the insulation resistance between:
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Core – frame
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Core – earth
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Frame – earth
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Apply 500 V DC for 1 minute.
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After the test, immediately restore the permanent core grounding.
4. Test Voltage, Timing, and Temperature Correction
4.1 Test Voltage Guidelines
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The preferred DC test voltage is the value recommended by the transformer manufacturer.
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If this information is missing, a 5 kV DC test voltage is commonly used for main insulation resistance tests.
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Crucial Rule: The test voltage must never exceed the rated RMS voltage of the winding under test.
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For core-insulation tests, a typical application is 500 V DC for 1 minute.
4.2 Measurement Times
During each specific test cycle:
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Apply the DC voltage and read the insulation resistance at exactly 15 seconds and 60 seconds.
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If you require a more detailed assessment, continue the test for up to 10 minutes, recording the resistance every single minute.
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The DC voltage should remain approximately constant. However, if the leakage current rises continuously without stabilizing, you must stop the test immediately.
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Ultimately, these multiple readings allow you to compute the PI and AI (see Section 5.3).
4.3 Temperature Correction to 20 °C
Insulation resistance is strongly temperature-dependent. To ensure meaningful data comparison:
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Preferably, carry out tests close to 20 °C to minimize correction requirements, or repeat them at similar temperatures during future maintenance intervals.
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Convert the measured resistance (R) to its standardized value at 20 °C ($R_{20}$) using a temperature correction factor ($a$). You can obtain this factor from a standard table or curve based on oil temperature.
The formula is:
$R_{20} = R \times a$
Where:
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$R_{20}$ = Insulation resistance referred to 20 °C
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R = Measured insulation resistance
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$a$ = Temperature correction factor (derived from oil temperature; use interpolation for intermediate temperatures)
Note: All limits and comparisons in the next section refer to 10-minute values corrected to 20 °C.
5. Interpretation of Results
5.1 Main Insulation (Windings to Ground / Between Windings)
Desired insulation resistance values at 20 °C heavily depend on the equipment’s voltage class and historical test results:
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The 1-minute corrected value should not be more than 20% lower than the previous test result for the same transformer. However, if the value is very high (above 20 GΩ), small variations are less significant.
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For transformers with a rated voltage ≥ 145 kV, the resistance at 20 °C should be at least 1 GΩ.
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For transformers with a rated voltage ≤ 72.5 kV, the resistance at 20 °C should not fall below 500 MΩ.
If your asset fails to satisfy these conditions, you should perform further tests, such as dielectric dissipation factor (power factor), oil quality testing, and moisture analysis.
Trending results over time is highly useful. A gradual decrease in resistance or a noticeable change in the resistance-versus-time curve may indicate:
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Moisture ingress
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Ageing or cracking of solid insulation
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Contamination on internal surfaces
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Incomplete oil drying following maintenance
5.2 When to Extend Tests to 10 Minutes
Measuring resistance up to 10 minutes provides valuable diagnostic data. We recommend this extended test:
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During initial commissioning tests.
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During detailed periodic maintenance or active fault investigations, provided the extra time (roughly one additional hour total) is acceptable.
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Whenever previous historical tests showed that the resistance took longer than 6 minutes to reach a steady value.
5.3 Polarization Index (PI) and Absorption Index (AI)
When a 10-minute value is available, calculate the Polarization Index:
PI = $R_{10\text{ min}} / R_{1\text{ min}}$
If the test is only conducted for 1 minute, use the Absorption Index instead:
AI = $R_{60\text{ s}} / R_{15\text{ s}}$
These indices hold meaning primarily when the absolute resistance is below 20 GΩ. For extremely high resistance values, the ratios tend to saturate and become less useful.
Typical PI Values:
| PI Value | Condition |
| < 1.0 | Dangerous |
| 1.0 – 1.1 | Poor |
| 1.1 – 1.25 | Questionable |
| 1.25 – 2.0 | Fair |
| > 2.0 | Good |
Typical AI Values:
| AI Value | Condition |
| 1.0 – 1.25 | Questionable |
| 1.25 – 1.6 | Good |
| > 1.6 | Excellent |
Important Note: Always interpret PI and AI alongside the absolute insulation resistance and your knowledge of the insulation liquid. For example, a new transformer filled with highly clean mineral oil may show a low PI simply because its leakage current is extremely small and dominated entirely by capacitive effects.
5.4 Core Insulation Resistance
For the core and core-frame insulation test (conducted at 500 V DC, using 1-minute values corrected to 20 °C), use this typical interpretation guide:
| Transformer / Reactor Condition | Core Insulation Resistance at 20 °C | Assessment |
| New unit | > 500 MΩ | Normal |
| In-service unit | > 100 MΩ | Normal |
| In-service unit | 10 – 100 MΩ | Insulation ageing – monitor trend |
| In-service unit | < 10 MΩ | Requires detailed investigation |
Abnormally low core insulation resistance strongly indicates damaged core support insulation, incorrect core grounding arrangements, or severe contamination and moisture at the core and frame.
6. Conclusions
Insulation resistance testing is a powerful yet relatively simple diagnostic tool for power transformers. You should carry it out systematically between windings and ground, between the windings themselves, and—where physically possible—between the core, frame, and tank.
By strictly following defined connection schemes with appropriate guard leads, you can bypass unwanted currents. Furthermore, by observing mandatory depolarization intervals, applying suitable test voltages, and accurately correcting all readings to 20 °C, engineers obtain highly reliable data. Ultimately, this accurate data clearly reflects the true operational condition of the entire insulation system.
