Partial Discharge (PD) — Transformer Glossary

Localised electrical discharge that does not fully bridge the insulation between conductors. A precursor to insulation failure.

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Partial Discharge (PD)

Localised electrical discharge that does not fully bridge the insulation between conductors. A precursor to insulation failure.

PD is measured in picocoulombs (pC) during routine and type testing. Values below 10 pC are typical for new, high-quality transformers. IEC 60076-3 specifies maximum PD levels: ≤ 100 pC at 1.3 × Um/√3 for 5 minutes, followed by ≤ 10 pC at 1.1 × Um/√3. PD detection methods include electrical (coupling capacitor), acoustic (ultrasonic sensors), and UHF (ultra-high frequency antenna). Online PD monitoring is increasingly used for critical power transformers to provide early warning of insulation degradation.

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Where this term matters in the ETS portfolio.

transformersEHV Power TransformersExtra high voltage power transformers for transmission grids, generation step-up, and interconnection applications at 220–400 kV.

transformersUHV Power TransformersUltra high voltage power transformers for 400–765 kV transmission-class applications, leveraging global JV partnerships for units up to 500 MVA.

switchgearMV Switchgear12kV indoor distribution switchgear with vacuum circuit breakers and SF6-free design. Compatible with GIS and AIS switchgear solutions.

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DGA (Dissolved Gas Analysis)Diagnostic technique that analyses gases dissolved in transformer oil to detect incipient faults.

Tan Delta (Dissipation Factor)Measure of dielectric losses in insulation. A high tan delta value indicates deteriorated insulation.