Partial discharge testing checks for small, localized electrical discharges within or around a power transformer insulation system. Activity may occur in voids, near sharp electric-field concentrations, along contaminated surfaces, or at poorly controlled material interfaces. A discharge does not immediately bridge the insulation, but repeated activity can erode material and reduce long-term reliability.
A dielectric withstand test asks whether insulation survives a specified voltage for a specified time. A partial discharge test asks whether localized activity is present, at what apparent charge level, and at what voltage it starts or stops. The two tests answer different questions.
What the measurement reports
Partial discharge is commonly expressed as apparent charge in picocoulombs. It represents the charge that would create the observed terminal effect in the test circuit; it is not a direct measurement of charge at the physical defect. Useful records include background noise, partial discharge inception voltage, extinction voltage, applied-voltage sequence, duration, bandwidth, calibration method, and connection diagram.
Why the test setup matters
The signals are small. Corona from test leads, nearby switching supplies, poor grounding, radio sources, and inadequate shielding can imitate transformer activity. Before testing, the laboratory should verify the noise floor and inject a known calibration pulse at the transformer terminals. If background noise approaches the acceptance limit, a simple pass/fail statement is not credible.
Construction details that affect discharge risk
- Electric-field concentration at foil edges, leads, terminals, and shield ends
- Air pockets or resin voids in cast or impregnated structures
- Insufficient creepage or poorly graded surface paths
- Loose winding sections that can move under stress
- Moisture, contamination, and damaged enamel or film
- Interfaces between materials with different dielectric properties
Risk reduction begins with controlled geometry, rounded conductive edges, adequate insulation, properly positioned shields, clean winding processes, and repeatable impregnation or casting. Final testing should confirm a sound process, not serve as the only method for sorting defects.
Define acceptance before building samples
The specification should identify terminals under test, grounding, frequency, pre-stress voltage, measurement voltage, hold time, maximum apparent charge, and whether inception or extinction voltage is required. It should reference the applicable customer or product standard. Without these details, two laboratories can test the same transformer and produce results that are not comparable.
Connect results to manufacturing control
A passing prototype does not establish process capability. Link the result to insulation material lots, winding tension, drying, impregnation or casting parameters, cure cycle, cleanliness, and assembly. If a unit shows abnormal activity, preserve it for planned analysis. Electrical signatures, visual inspection, sectioning, and microscopy can help identify the physical source.
BaoHui Tech treats the drawing, insulation specification, and test method as linked documents. Customers evaluating a power transformer should provide the operating voltage, overvoltage category, environment, insulation requirement, and governing standard early enough to influence construction.
Frequently asked questions
Does a low reading guarantee long transformer life?
No single measurement guarantees service life. Thermal aging, surge exposure, vibration, moisture, contamination, and manufacturing consistency also affect insulation reliability.
Can an ordinary hipot tester perform this test?
A withstand tester may supply voltage, but meaningful partial discharge measurement also needs a coupling and measuring circuit, calibration source, controlled bandwidth, low-noise setup, and documented procedure.
Should every power transformer receive the test?
It depends on voltage class, application risk, standards, and the customer specification. It may be used for qualification, sampling, or routine production testing.