Partial Discharges (PD): A Discreet Enemy to Watch Out for .

12/10/2021

Partial discharges, few people know what it really is. But even if they seem like minor incidents, they can have devastating consequences. With repetitive frequencies, they deteriorate the insulation of cables and electrical equipment, eventually causing malfunctions, production interruptions, safety issues and even incidents that can lead to serious injury or death. According to extensive research, partial discharges are the cause of 85% of the failures in medium and high voltage equipment. How to protect yourself against it? Contrary to what one might think, strategies to prevent such disasters are relatively simple and inexpensive to implement.

How and where does this phenomenon occur?

But before we talk about solutions, it is helpful to understand what is meant by Partial Discharge or Partial Discharge PD. For a variety of reasons (poor design, poor installation, defective materials, contamination, or obsolescence), an insulation system may at some point be unable to withstand the electric field applied to it. The result is a short-lived, energy-efficient current pulse (in the order of a microsecond) that flows through or on the surface of the insulation. If this is an internal partial discharge, it erodes the insulation from within with its heat and ionization. In the case of partial discharge at the surface, this causes the breakdown of air molecules into oxygen (ozone) and nitrogen (nitrogen monoxide). Nitric oxide forms nitric acid that affects the insulation from the outside. Partial discharge usually only occurs with equipment operating at 3.3 kV phase-to-phase or higher: switchgear, transformers, stator windings and power cables.

Taken separately, partial discharge has no serious consequences. She is content with, so to speak, letting the insulation overheat. In contrast, its repeated appearance over time will have a cumulative effect that can lead to damage from chemical decomposition.

How to monitor medium and high voltage equipment?

Thanks to the technical evolution of sensors, electronics and memory, partial discharge monitoring has become more affordable. It has also become a lot easier. A single, easy-to-use instrument, completed with accessories to facilitate measurement, is sufficient to detect and analyze partial discharge without interrupting the power supply, while protecting maintenance personnel.

There are various measuring methods to detect irregularities in medium and high voltage equipment. The first is to use a TEV (Transient Earth Voltages) sensor. Partial discharge generates high frequency electromagnetic pulses called transient ground voltages. These signals travel on the inner surfaces of the switchgear and find their way out through the openings in the metal frame. An externally placed TEV sensor can measure these pulses. It is particularly effective for internal partial discharge that generally does not contain any visual, olfactory or audio cues.

Another method is to measure the ultrasonic sound produced by the partial discharge. When a partial discharge occurs, the electrons are displaced and rub against each other to generate ultrasound. These have a frequency that is too high for the human ear and must be converted within an audible range. Different devices can detect this ultrasonic sound, depending on the configuration of the property: flexible sensors for hard-to-reach places, contact probes for confined spaces and directional microphones or antennas for higher equipment.

It is also possible to test the partial discharges of cables thanks to RFCT (Radio Frequency Current Transformer) loops installed around the earth connection or the bond between the cable sheath and the earthed equipment. When a partial discharge occurs in the cable insulation, a voltage spike escapes through the earth braid and the RFCT loop identifies it quickly.

All of these live inspections allow operators to understand the current condition of the equipment and make the right decisions to maintain maximum operation. No, the network currently does not require any additional inspection. Yes, further inspection is required and the network must be de-energized.

What to choose: permanent monitoring or periodic tests?

Everything will depend on the degree of criticality of the installations. A hospital complex, a data center, an airport, a metro company or a power grid operator needs a monitoring system that is active 24 hours a day, 7 days a week. All continuously collected data is sent via GPRS to a dedicated server for storage. In the event of deviations, specialists advise on the procedure to be followed, depending on the seriousness of the situation.

To learn more about our partial discharge detection solutions, check out our dedicated UltraTev Plus2 and Astute HV Monitoring pages or contact us.

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