2026-09-11
What partial discharge testing is, and when to run it
A small internal discharge that erodes transformer insulation over years, with almost no visible warning before it fails.
Photo: EU-Ukraine Cooperation · CC BY-SA 2.0 · openverse.org
What partial discharge actually is
Partial discharge (PD) is a localised electrical discharge inside a small void or surface defect in insulation. It does not cause immediate breakdown, but it steadily erodes the material around it. IEC 60270, the governing international standard, measures it as apparent charge in picocoulombs (pC). Any single discharge is tiny — the risk is that it keeps happening.
What it damages
CIGRE Working Group A2.37 surveyed 964 major transformer failures across 56 utilities in 21 countries between 1996 and 2010. In the 69–100kV class, dielectric failure was the largest single cause, at 70% of failures. Partial discharge is one of the earliest and most measurable signs of that insulation breakdown — usually developing for years before the transformer actually fails.
Photo: EU-Ukraine Cooperation · CC BY-SA 2.0 · openverse.org
Online monitoring vs offline testing
Offline testing is done with the transformer de-energised — a clean measurement environment and a clear baseline, typically run at factory acceptance or scheduled maintenance. Online monitoring runs continuously while the transformer is in service, catching discharge patterns that only appear under load or thermal changes — at the cost of a noisier signal environment that needs more careful processing. The two are complementary, not a choice between one or the other.
What testing costs against what ignoring it costs
A single partial discharge test costs a small fraction of replacing a transformer that fails early from insulation breakdown — and far less than the downtime from an unplanned outage. The exact cost depends on the equipment class and site conditions, and should be quoted against the actual transformer specification rather than a flat number.
When to move to permanent monitoring
For critical transformers, or ones already showing signs of deterioration, a single test only gives a snapshot at one point in time. Where the rate of deterioration itself needs tracking, permanent online monitoring is the more suitable approach. We represent Messko's transformer monitoring and diagnostics line and can help scope the right setup.
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Tell us your transformer's specification and current condition, and we'll advise on the right testing or monitoring approach.
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