What Is Partial Discharge (PD) in a Transformer?

Published 27 Aug 2026Updated 27 Aug 20267 min readAdvancedDiagnostics

Quick answer — Partial discharge (PD) is a localised electrical discharge that partially bridges the insulation between two conductors and is confined to a small volume. It does not instantly fail the unit, but it erodes the insulation over time and is a strong early indicator of an incipient fault — detected by PD measurement and often by hydrogen in the oil.

Contents

Where it occurs
How it is detected
Why it matters

What is partial discharge and why does it matter?

Where it occurs

PD happens at sites of stress concentration: in voids or gas bubbles in the insulation, at sharp edges, on contaminated surfaces, or at a delamination. The discharge repetitively erodes the insulation.

How it is detected

PD can be measured electrically (transient pulses), acoustically (ultrasonic sensors on the tank), or via dissolved-gas analysis — PD produces hydrogen (with methane) in the oil.

Why it matters

Persistent PD degrades the insulation to the point of a flashover or failure, so early detection with PD and DGA monitoring lets you plan intervention rather than suffer an unplanned outage.

References & standards

IEC 60270 (PD measurement)
IEC 60599 (DGA interpretation)

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Related manufacturers

Acme Electric (Hubbell) · Central Moloney · Controlled Magnetics, Inc. · CORE Transformers · Daelim Transformer (US) · Delta Star

Related components

Protection, monitoring & diagnostics Insulation materials

Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.

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