Partial Discharge Analysis

Partial Discharge Measurement

A partial discharge is a microscopic electrical breakdown confined to a defect inside insulation: a void, a crack, a delamination or a sharp interface. Each individual discharge is tiny, but they repeat continuously and they erode the insulation around them. Because that damage accumulates over a long period before any conventional dielectric test fails, partial discharge measurement finds ageing that a standard test will miss entirely.

This matters most on the equipment that cannot be taken out of service. A transformer or a long cable run that is quietly deteriorating has no second life available to it, so the value of knowing is the ability to act while the decision is still open. Testing is carried out with the test set energised and the equipment isolated, so the background noise level can be established and external interference is never reported as a defect, and the test voltage is applied in defined steps with the reading allowed to stabilise at each level.

Partial discharge analysis detects:

• Current and future decay of internal voids within solid or liquid insulation
• Surface discharge along a dielectric interface or an insulator surface
• Corona discharge at sharp or poorly screened conductors
• Floating electrodes and poorly formed terminations
• Ageing of transformer insulation long before a power factor or ratio test becomes abnormal
• Damage to cable insulation, joints and terminations

Test Methods and Applications

Partial discharge testing is applied to transformers, cables, switchgear, motors and generators, either offline with the equipment isolated or in service with portable or permanently installed sensors and couplers. Offline testing employs very low frequency, damped AC and 50/60 Hz dielectric test sets, selected to suit the asset and its age, with capacitance and power factor or tan delta measurement taken as a supporting test.

Each result reports the partial discharge magnitude in picocoulombs at every test voltage, the phase resolved discharge pattern interpreted against the known void, surface and corona signatures, and the background noise level that demonstrates the signal is not ambient interference. The diagnosis is compared with any previous test on the same asset to establish whether the activity is stable or increasing, and a severity assessment is given with the consequence of allowing the condition to continue and the recommended corrective action or replacement planning.