Internal Discharges

Small gaps or defects present in the electrical element. It is often caused by low insulation quality, improper curing, trapped moisture or gas, and aging under the head.

 

Surface Discharges

These occur at the surface of the insulation. Common triggers causing surface discharges are dust, oil films, humidity, and weathered or damaged insulation.

 

Treeing

It is a small breakdown that spreads in branch-like patterns. It is caused by continuous overvoltage, impurities, and long-term electrical stress.

 

Corona in Air

It is a type of partial discharge formed in air around high voltage conductors, where the electric field is concentrated. High voltage levels, poor geometry, and tight clearances are the typical causes.

Partial discharge happens when high voltage creates very strong electric stress inside or around an insulator. Even though an insulation looks smooth and solid, it contains tiny air voids or defects where the stress becomes concentrated. Locally, these tiny gaps break down, producing electrical sparks without causing a full short circuit. These small discharges, called the partial discharges, slowly damage the insulation and can lead to failure.

Partial discharges can be surface, internal, or corona discharges. Partial discharge activity is detected using conventional and non-conventional test methods. Conventional testing uses a coupling capacitor and measuring circuit to capture discharge pulses and measure them in picocoulombs (pC).

Non-conventional methods use sensors to pick up the effects of discharges instead of measuring charge directly. The most common electromagnetic sensors that detect high-frequency signals and acoustic sensors that detect sound waves produced by discharges.

PD activity is determined by observing PD pulses or signals above noise. It is either measured as charged pulses with PRPD analysis or high-frequency electromagnetic or acoustic emissions, indicating ongoing discharge within the insulation system.

Both on-line and off-line PD measurements are important, but they serve different purposes. On-line testing is more commonly employed for ongoing health monitoring and early warning during service. While off-line testing is preferred for detailed diagnostics and benchmarking. So using both methods together gives the best assessment of machine health rather than relying on one.

The differences are discussed in relation to power frequency PD testing v/s low frequency (VLF) Partial Discharge testing. But the difference is not only limited to sinusoidal VLF testing but also to Damped AC and cosine rectangular testing. This is because partial discharge depends on how fast the voltage changes. Sinusoidal VLF changes slowly, while DAC and cosine rectangular waveforms change faster. Faster voltage changes make it easier to spot the insulation defects.

In the applied voltage PD test, the winding under test is energized directly at power frequency, while the other windings are grounded. The test voltage is gradually increased to the specific level, held for a short time, and then reduced. This checks the insulation to ground and between windings.

In the induced voltage PD testing, a high-frequency supply is applied to the low-voltage terminals to induce voltage in other windings. With only the neutral and tank grounded, other terminals are left open. Now, the PD is monitored while the voltage is raised, held, and recorded over time.

The partial discharge (PD) testing method is dependent on whether the neutral point of the winding is accessible or not. This difference changes how voltage is applied and how clearly we can identify the insulation problems.

• PD test with an accessible neutral

When the neutral point is available, PD testing becomes straightforward. We can energise the whole winding and also check each phase one by one. The measurement is taken at the line terminals A1, B1, or C1, while the other phases are kept grounded.
The voltage is applied from the neutral side, giving a cleaner and more reliable signal. This makes it easier to see exactly which phase has a problem.

• PD test without an accessible neutral

When the neutral is not available, the test needs a bit more care. If all phases are tested together, the result shows only an average value, and a defect in one winding may go unnoticed. So each phase is checked indirectly by applying voltage to one phase and measuring PD at another.
This method still helps us assess each winding separately and reduces the risk of missing insulation faults.

Partial discharge is checked by applying high voltage to insulation and measuring the resulting discharge signals using electrical, acoustic, or electromagnetic detection methods.

What is the standard for partial discharge testing?

Partial discharge (PD) testing is mainly carried out according to IEC 60270, which explains how PD should be measured and evaluated in high-voltage equipment.

What is the main cause of partial discharge?

Partial discharge (PD) is caused when local electric stress exceeds the dielectric strength of a small part of the insulation, often due to defects such as voids, air pockets, impurities, cracks, or inclusions within the insulation material. These defects create high-field regions that trigger small electrical discharges before full insulation breakdown.