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Transformer Insulation Diagnostic Tools — PI, DAR, Step Voltage

2026年05月12日

Three diagnostics — PI, DAR, step-voltage — turn a megger into a real condition-assessment tool.

Why Diagnostics Beat Spot Readings

A 30-year-old transformer can show 1 GΩ insulation today and fail in service next week. Modern transformer insulation diagnostic tools use three time-resolved tests — PI, DAR, step-voltage — to detect early-stage degradation invisible to a single reading.

1. Polarisation Index (PI)

  • PI = R(10 min) / R(1 min).
  • PI > 4: excellent (new winding).
  • PI 2-4: good.
  • PI 1.5-2: marginal — schedule retest in 3 months.
  • PI < 1.5: failing — take out of service.

2. Dielectric Absorption Ratio (DAR)

DAR = R(60 s) / R(30 s). For quick assessment when 10-minute test is impractical. DAR > 1.4 = healthy; < 1.25 = wet or contaminated.

3. Step-Voltage Test

  1. Apply 1 / 2 / 3 / 4 / 5 kV in sequence.
  2. Resistance should be constant ± 10 %.
  3. Drop at higher voltage = weak spot.
  4. Always abort if current spikes — protect the winding.

4. Temperature Correction

Insulation resistance halves for every 10 °C rise. Always log winding temperature; correct readings to 20 °C reference using IEEE 43 Table.

5. Test Voltage Selection by Transformer Class

  • 11 kV transformer: 5 kV test voltage.
  • 33 kV transformer: 5 kV test voltage (instrument limit; higher for factory acceptance).
  • 415 V LV winding: 1 kV test voltage.

6. The EK-C350 Diagnostic Suite

The EK-C350 automates PI, DAR, step-voltage, and temperature-corrected logging in one instrument. CSV export to fleet condition-assessment software. Talk to EK service teams for transformer-fleet rollout.