Transformer Insulating Testing

How to Ensure Precise Results in Transformer Insulation Testing?

Why this test matters

Insulation resistance (IR) testing helps confirm that a transformer’s insulation system limits leakage current and resists moisture, contamination, and aging. While IR is only one part of a complete dielectric assessment, it is quick, safe, and highly repeatable when performed correctly—with appropriate voltage, guarding against surface leakage, proper timing, and temperature normalization. IEC 60076 series defines dielectric tests for power transformers, and IR is widely used as a routine/diagnostic check alongside winding resistance, PF/tan δ, and applied/induced tests.

What to measure on site

Winding-to-ground IR (HV-to-ground, LV-to-ground, tertiary-to-ground) and between windings if accessible.
Core and structure IR to detect inadvertent multiple ground points in the core/press-frames. These parts use simpler insulation and normally sit at (or near) ground potential, so low IR is a strong indicator of a faulted ground path. (Your original point was correct.)

Test setup for accuracy

Instrument & leads
Use a calibrated IR tester (≥5 kV capability for MV/HV transformers) with a dedicated guard terminal and high-resistance measurement range. The guard terminal is essential to shunt surface leakage so you read the true bulk insulation resistance—not the parallel path over dirty/ damp surfaces. Wrap a bare guard wire around porcelain/bushing skirts or use a guard strap on test fixtures.
Test voltage selection
As a field rule (from vendor application notes and common utility practice):

≤1 kV equipment → 500 V to 1 kV
1–35 kV class → 2.5–5 kV
35 kV class → 5–10 kV
Use the lowest voltage that yields stable readings without stressing aged insulation; for acceptance or type tests, defer to the transformer’s specification/IEC 60076 test plan.

Preparation

De-energize, isolate, discharge windings, and apply grounds.
Clean and dry external insulation (bushing skirts) to reduce surface leakage.
Record ambient and top-oil temperatures; stabilize if possible. Temperature strongly influences IR.

Connections

For HV winding to ground: “LINE” to HV terminal, “EARTH” to tank ground, GUARD to a guard band around insulator surfaces. Mirror for LV, tertiary, and inter-winding measurements.

Timing matters: get IR, DAR, and PI right

After applying the selected DC voltage, log resistance vs time:

1-minute IR (IR₁min): commonly trended value.
DAR (Dielectric Absorption Ratio): IR(60 s) / IR(30 s).
PI (Polarization Index): IR(10 min) / IR(1 min). PI is more robust for large insulation systems with longer absorption time constants (typical of oil-paper systems).

Interpreting ratios (rules of thumb)

PI ≥ 2.0 generally indicates dry, clean insulation; 1.0–2.0 suggests moisture/contamination or insufficient time constants; <1.0 is poor—investigate. Track trends over time more than single thresholds, and corroborate with PF/tan δ and oil tests. (PI criteria are widely taught in Megger/IEEE guidance for high-voltage insulation.)

Temperature correction & reporting

IR varies strongly with temperature (roughly halves for each ~10 °C rise for many systems). Always normalize to a reference temperature (commonly 20 °C) using manufacturer curves or standard charts, and report both measured and corrected values alongside ambient/top-oil temperatures. This makes results comparable across seasons and sites.

Your test record should include: asset ID, tap position, test voltage, timing points (30 s, 60 s, 10 min), IR values, DAR/PI, temperatures, connection diagram, guard use, surface condition, and remarks. Align with the unit’s IEC 60076 test dossier where applicable. (Iteh

Limits and when IR is not enough

On large, high-voltage transformers, IR alone may not reveal localized wet spots; complement it with PF/tan δ (dissipation factor) for bulk dielectric loss, and DGA for incipient faults.
As voltage rating and size increase, PF/tan δ’s diagnostic value rises relative to IR, so use both where possible. (Your original observation aligning with this trend is correct.)

Step-by-step field procedure (checklist)

Verify clearances, LOTO, and grounds; discharge windings.
Clean/dry bushing surfaces; install guard wrap/strap.
Select test voltage (2.5–10 kV for MV/HV) and set timer logging.
Measure IR vs time for each object (HV-G, LV-G, Tertiary-G, inter-winding).
Record 30 s, 60 s, 10 min; compute DAR and PI.
Normalize results to 20 °C using correction charts; document temperatures.
Compare to historical trends and companion tests (PF/tan δ, oil quality).
If abnormal: re-clean, re-test with guard confirmed; if still low, investigate moisture, surface contamination, or incorrect tap/connection.

Key takeaways

Use guarding to eliminate surface leakage; otherwise IR can be deceptively low.
Log time-resistance and compute DAR/PI; PI is the more reliable health index for large oil-paper systems.
Temperature-correct to 20 °C for apples-to-apples comparisons.
IR is a screening tool; corroborate with PF/tan δ and oil diagnostics per IEC 60076 test programs.

Sources

IEC 60076-3 Dielectric tests for power transformers; example IEC 60076-1 test report structure.
Megger application notes on IR testing, guard terminal, and best practices; “A Stitch in Time” and “Seven Tips.”
PI/DAR timing definitions and practice (Megger/TestGuy/IEEE 43 tutorial material).
Context on transformer test suites per IEC/IEEE (overview articles).