Partial Discharge in Transformer FAT Reports: Why Pass/Fail Is Not Enough

A number that appears in fine print, ignored for years—until it can't be ignored

Most procurement managers reviewing a factory acceptance test report will check the obvious numbers: winding resistance, ratio deviation, impedance. These are the parameters that determine whether the transformer meets its nameplate specification. They are necessary. They are not sufficient.

Further down the report, often buried in a section titled "Dielectric Tests" or "Routine Tests—Supplementary," there is a value expressed in picocoulombs. The unit is unfamiliar. The number is small—perhaps 35, perhaps 85. A note in the margin reads "IEC 60076-3 limit: ≤100 pC." Below 100, so approved. Next page.

That number measures partial discharge. And skipping past it because it passed the standard is like ignoring a small crack in a dam because it hasn't leaked yet.

What partial discharge actually is

Partial discharge (PD) is an electrical discharge that partially bridges the insulation between conductors. Unlike a full breakdown—which destroys the transformer instantly—a partial discharge event is localized. It affects a small volume of insulation. A single event is measured in trillionths of a coulomb, hence the unit: picocoulombs (pC).

What makes PD destructive is repetition. A discharge site inside a transformer does not fire once and stop. Under alternating voltage, it fires twice per cycle—100 or 120 times per second, depending on grid frequency. Over the course of a year, that is more than three billion discharge events at a single site.

Each event does three things:

Erodes insulation material—the discharge plasma, though microscopic, reaches temperatures sufficient to vaporize cellulose fibers and oil molecules at the point of contact.
Creates conductive carbon tracks—the decomposed material leaves behind carbonized channels that extend the reach of the electric field, allowing the discharge to penetrate deeper into the insulation over time.
Generates gases—primarily hydrogen (H₂), which dissolves in the oil and becomes detectable by DGA, providing the first external warning that PD is occurring inside.

The progression is slow—months, then years. And then, one day, the carbon track bridges enough insulation that the partial discharge becomes a complete breakdown. The protection relays trip. The transformer is offline. And a number that was "below 100 pC, so approved" is now a root cause in a failure investigation report.

Why 100 pC is a ceiling, not a target

IEC 60076-3 is commonly used to set a 100 pC maximum partial discharge level for applicable distribution transformer induced-voltage test conditions. This limit was established through decades of industry experience correlating factory PD measurements with field failure rates.

But 100 pC is a compliance threshold, not a quality target. The difference is critical:

A transformer measuring 95 pC at the factory is compliant with IEC 60076-3. It can be shipped. It will be accepted by most procurement specifications.
A transformer measuring 25 pC at the factory is also compliant. It was manufactured with better process control—cleaner winding environment, more thorough drying, tighter assembly tolerances.
The first transformer has a statistically higher probability of developing PD-driven insulation failure within its first decade of service. The standard doesn't say this explicitly, but the physics of discharge propagation makes it inevitable.

A procurement specification that only asks "does PD pass IEC 60076-3?" treats 95 pC and 25 pC as equivalent. They are not equivalent. Ask for the value, not the pass/fail.

What creates partial discharge in the first place

PD does not appear randomly. It initiates at specific types of defects introduced during manufacturing:

Voids in solid insulation. If the cellulose paper wrapping around a conductor contains a gas-filled cavity—from incomplete impregnation with oil, or from a crease that didn't compress during winding—the cavity becomes a discharge site. Gas has lower dielectric strength than oil-impregnated paper. The voltage that the surrounding insulation withstands easily will break down the gas pocket repeatedly.

Conductor surface irregularities. A burr on the edge of a copper conductor, a sharp corner on a connection, or a metallic particle left in the oil creates a point where the electric field concentrates. The field at the tip of a sharp conductor can be orders of magnitude higher than the average field in the insulation. Discharge initiates at the tip even though the bulk of the insulation is operating well within its design limits.

Moisture in the insulation system. Water molecules inside cellulose paper increase its electrical conductivity locally. The increased conductivity distorts the electric field distribution, creating regions of elevated stress. These regions become preferential sites for discharge initiation.

Each of these defects is preventable through manufacturing process control. Each can be detected by PD measurement at the factory. And each, if undetected, will grow.

The scenario: when "passed routine tests" isn't enough

A utility-scale solar project in a tropical region commissioned four pad-mounted transformers from a manufacturer offering competitive pricing. The units passed all routine tests: ratio, impedance, applied voltage, induced voltage. PD was not specified in the procurement documents. The manufacturer did not perform it.

Three years into operation, one unit tripped on differential protection. Inspection revealed carbonized tracking through the layer insulation in the high-voltage winding. The failure analysis concluded that a partial discharge site had been present since manufacturing—likely from a conductor burr that created a localized field enhancement. The discharge had propagated year by year until the carbon track bridged enough insulation to cause a turn-to-turn fault.

The replacement cost, including expedited manufacturing, air freight, site labor, and lost generation, exceeded the initial purchase price of the transformer.

A PD measurement at the factory, costing a few hundred dollars and adding hours to the testing schedule, would have detected the discharge site before shipment. The burr would have been found. The conductor would have been reworked or replaced. The transformer would have been shipped with insulation integrity verified, not assumed.

How to specify PD in your procurement documents

When writing or reviewing transformer specifications, include partial discharge measurement as a routine test—not as an optional type test performed once on a representative design. The reasons:

PD is manufacturing-dependent. Two transformers built to the same design by the same factory can have different PD levels because of variation in winding cleanliness, conductor handling, and assembly conditions. A type test on one unit does not guarantee the PD performance of the next unit off the production line.

PD is the earliest warning. Before DGA shows elevated hydrogen, before the insulation resistance drops, before the transformer trips—partial discharge is the first measurable sign that something is degrading inside. Measuring it at the factory establishes the baseline against which all future measurements will be compared.

PD values create negotiating leverage. A supplier who consistently delivers units measuring 25–40 pC has manufacturing processes under better control than one whose units scatter between 60–95 pC. The PD measurement differentiates suppliers who compete on process quality from those who compete only on price.

Specify: "Partial discharge shall be measured on each unit during the induced voltage test in accordance with IEC 60076-3. The measured value shall be reported in picocoulombs. The acceptance criterion is ≤100 pC; however, any value exceeding 50 pC shall be reviewed with the purchaser prior to shipment."

The last sentence changes the conversation from "did it pass?" to "what does the number actually say about my transformer?"

Next in this series: moisture—why water inside a sealed transformer destroys insulation faster than overload ever will.

→ Contact our technical team about PD testing requirements for your project

References: IEC 60076-3 (Insulation levels, dielectric tests, partial discharge measurement ≤100 pC). Partial discharge physics: discharge plasma temperature, carbon tracking mechanism, gas generation (primarily H₂). Discharge repetition rate under AC voltage: 2× per cycle (100 Hz at 50 Hz, 120 Hz at 60 Hz). PD initiation mechanisms: voids in solid insulation, conductor surface irregularities (field enhancement at sharp points), moisture-induced conductivity changes in cellulose.

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Partial Discharge: The Silent Defect Hiding in Your FAT Report