Partial discharge is really small electrical sparks or breakdowns that take place inside the insulation of electric equipment. The key is that these discharges do not break through all the way between conductors — they’re just small, local electrical “micro-sparks.” We can’t usually see them, but they do take place inside cracks, voids, or imperfections in insulation.

We think to ourselves, “No big deal, right?” But the trick is — over time, with the passing of time, all those small discharges will exhaust the insulation. It’s like a small leak in a dam that bursts later with a flood. Ignoring partial discharge can lead to total insulation failure, and that’s where equipment breakdown happens — sometimes gigantic and costly ones.

Why We Should Care About Partial Discharge Testing

Regardless of whether we’re dealing with transformers, cables, switchgear, or generators, good insulation is absolutely paramount. Partial discharge testing assists us in detecting those small faults before they become huge issues. It’s one of the most effective methods to avoid surprise breakdowns, blackouts, and costly emergency repairs.

Imagine it as a check-up for our electrical equipment. Catching PD early allows us to schedule maintenance and repair problems before they become significant hazards. Predictive maintenance like this saves us time and dollars. And it keeps our equipment operating longer and more safely — a benefit for all of us, particularly when working with assets distributed over large and sometimes distant networks.

How in Practice Do We Test for Partial Discharge?

Partial discharge generates all sorts of signals — electrical pulses, ultrasonic sounds, and electromagnetic waves — and there are plenty of techniques for detecting them. Here’s a brief overview.

One common method utilizes ultrasonic sensors. PD generates ultrasonic sound that the human ear cannot hear but which are picked up using special sensors. This method is great for detecting discharges on surfaces or in air gaps.

And then there is the Transient Earth Voltage (TEV) method. If PD is occurring inside metal-clad switchgear, it induces voltage pulses on the metal housing. TEV detectors can be used to sense the pulses without needing to open the equipment — handy for safe, non-invasive testing.

In the case of gas-insulated switchgear (GIS), we use Ultra High Frequency (UHF) sensors. They sense electromagnetic waves produced by PD in sealed equipment and are suitable for monitoring online.

The second technique uses High-Frequency Current Transformers (HFCTs), which are located around grounding conductors or cables to detect PD pulses through the system. They are normally used for continuous monitoring in cables and transformers.

There is also Acoustic Emission (AE) monitoring, which detects stress waves generated by PD in solid insulation materials — useful in identifying exactly where the problem is.

New technology like acoustic imaging cameras and fiber optic sensors becomes increasingly precise for PD detection, especially under electrically noisy conditions or in inaccessible areas.

When Should We Do PD Testing?

PD testing is flexible. Recently bought equipment usually will be tested before it leaves the factory to verify insulation quality is great. Installation commissioning tests, conducted subsequently, confirm no shipping or installation damage occurred. However, true value comes from periodic testing over the life of the equipment to track insulation condition.

For high-critical assets, on-line PD monitoring systems provide us with real-time information, so we’re immediately informed the moment something shifts. And if something’s going wrong — e.g., during a fault investigation — PD testing enables us to trace the problem and guide repair exactly.

What Equipment Do We Need for PD Testing?

There’s something to suit every need. For field inspections, portable PD detectors are fantastic — usually they contain more than one type of sensor so we capture the complete picture. Where equipment is really important, for permanent online PD monitoring systems that keep a constant eye on our assets 24/7.

If we require top-level analysis, laboratory-grade analyzers examine the PD signals in-depth. Acoustic imaging cameras enable us to see the origin of discharges by translating ultrasonic sounds into images, and that can turn it into a game-changer as far as pinpointing uncooperative faults is concerned. And fiber-optic PD sensors are picking up support because they are not affected by electromagnetic noise, so they are ideal for vexing environments.

All companies offer and keep this equipment in some conditions and standards.

How Do Standards Work for PD Testing?

Though every region has its own electrical standards, PD testing largely follows international best practice — mainly IEC standards. The most prominent are IEC 60270, explaining how to measure partial discharges correctly, and IEC 60060-2, covering high-voltage test techniques.

Standards for power transformers, switchgear, and other equipment frequently include PD testing as a required part of insulation diagnosis. These standards ensure that PD testing adheres to international standards and caters to local system requirements.

Partial Discharge Testing in the Real World

Let’s put it into perspective with some real-world examples.

A utility found partial discharge in the bushing of a 66kV transformer during planned testing. The PD measurements were low but repetitive, reflecting incipient insulation deterioration that other tests did not reveal. The utility planned a replacement at the next scheduled outage, preventing what could have been a costly failure and extended power outage.

An 11kV cable feeder hosted online HFCT sensors in a manufacturing facility. The system picked up increasing PD activity on one of the feeders, which resulted from water ingress in a cable joint. Thanks to early indication, they managed to resolve the problem in time by drying and resealing the joint — rather than emergency cable replacement and unplanned outages.

Power plant engineers used UHF sensors in gas-insulated switchgear. If the sensors picked up an abrupt rise in PD activity near an insulating spacer, they initiated a planned shutdown to swap out the faulty component. This avoided the PD escalating to a dangerous fault, allowing for safe operation.

Handheld ultrasonic detectors helped detect corona discharge at a cable termination after heavy rain caused moisture to accumulate. The plant cleaned and resealed the connection, preventing insulation failure and possible outages.

Why PD Testing Is a Game-Changer for the Power Sector

Power grids are huge and complex, with scores of assets spread over large and sometimes isolated areas. PD testing helps keep these grids at a dependable level by detecting faults before they happen, preventing shock outages which impact residences, industry, and commerce.

It’s also a massive money saver. By spending maintenance only where it is needed, we cut down on unnecessary, high-cost repairs and delay the day when expensive assets have to be replaced. Also, it enhances safety — fewer chances of electrical fires or catastrophes.

With the rapid growth of smart grid technology and renewable energy installation, tracking insulation health more than ever is imperative. PD testing allows one to cope with the complex insulation systems in new inverter-based equipment and advanced grid components.

Some Things to Consider

PD testing works, but it does require some expertise. Interpreting the signals is not always a simple task, and there can be interference from the environment that makes a mess of things. Satisfactory results rely on correct sensor placement and compliance with testing protocol.

If we’re thinking about PD testing for our assets, it’s important to work with trained professionals and use quality equipment. That way, we’ll get the most accurate insights and avoid wasting time or money on false alarms.