PARTIAL DISCHARGE SECRETS, TIPS, AND TRICKS - Feature - FALL 2020

Sep 30, 2026

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Partial discharge in medium-voltage power cable systems is addressed through practical, client-learned insights focused on commissioning reliability. The discussion explains how cable termination issues can create higher field stresses in low-permittivity regions and how common installation mistakes may lead to voids, field concentration, and partial discharge even without a visible void. It also covers safety and signal considerations for RFCT placement in online testing, and clarifies how contact discharge and floating metal discharge differ in characteristic phase-resolved partial discharge behavior.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages7

Reading Time18 min

AccessFree to download

CAN CABLE TERMINATION MISTAKES CAUSE PD?

Voltage stress, voids, and common termination errors

CAN INCORRECT RFCT PLACEMENT INDUCE PD?

Safety vs signal and field effects near the termination

WHAT ARE CONTACT DISCHARGE AND FLOATING METAL DISCHARGE?

PRPD patterns and how to distinguish them

PARTIAL DISCHARGE TESTS FOR MEDIUM-VOLTAGE POWER CABLE SYSTEMS ISSN 2167-3594 NETA WORLD JOURNAL PRINT ISSN 2167-3586 NETA WORLD JOURNAL ONLINE USING VLF AND PD TESTING TO IMPROVE COMMISSIONING RELIABILITY PAGE 64 PARTIAL DISCHARGE SECRETS, TIPS, AND TRICKS PAGE 48 FEATURE PARTIAL DISCHARGE SECRETS, TIPS, AND TRICKS BY WILLIAM G . HIGINBOTHAM, EA Technology LLC Most of us started our electrical careers with fairly good knowledge about the practical aspects of our jobs. We understood voltage, current, power, resistance, etc. We mastered the more esoteric areas such as electrical fields and stress control and only revisited these topics if our day-to-day job required it. Partial discharge (PD) is one of those areas where we might have learned how to identify it or how to use a test set, but most of our roles have not exposed us to the ins and outs ofthis topic. In this article, I share some unexpected things I have learned while working with clients on partial discharge issues. I hope this includes the secrets, tips, and tricks you always wanted to know about partial discharge but were afraid to ask. 2 • FALL 2020 PARTIAL DISCHARGE SECRETS, TIPS, AND TRICKS CAN CABLE TERMINATION MISTAKES CAUSE PD? When a termination containing multiple layers and different materials is exposed to an electric field, the voltage stresses are larger in the areas of lesser permittivity. Air has a permittivity about 2.3 times lower than insulation, so a void in the termination will have approximately 2.3-times-higher field stresses. Combined with a 10-times lower dielectric strength, this leads to discharge across the void. Cable terminations are specifically engineered to ensure voltage stresses do not exceed the FEATURE insulation capability at any point. This is accomplished by ensuring the voltage gradients across the insulation are even (Figure 1) . Voids can be caused by common mistakes in terminations. Failure to completely fill gaps between insulation and conductors with mastic is very common. Imprecise trimming of semiconductor layers can leave a void where the stress tube cannot fill the poorly trimmed section. Failing to fully clear the semiconductor layer off the insulation causes a concentration of stress and results in PD even if there is no void. Figure 2 shows that electrical stress is greater in and around the void. Figure 1: Voltage Gradients in a Cable Termination Figure 2: Voltage Gradients in a Poor Termination PHOTO: © ISTOCK PHOTO COM/PORTFOLIO/ANIGOWEB PARTIAL DISCHARGE SECRETS, TIPS, AND TRICKS NETAWORLD • 3 FEATURE Figure 3: Defective Cable Termination Cable termination procedures are precisely engineered, and any failure to follow them closely can lead to voids, field concentration, and partial discharge. An example of this can be seen in Figure 3, where an overly large gap that was not filled with mastic led to failure. CAN INCORRECT RFCT PLACEMENT INDUCE PD? When performing an online partial discharge test on a cable, a radio frequency current transformer (RFCT) must be attached to the cable ground strap. In placing the RFCT, think 95% about safety and 5% about getting a good signal. Improper placement of an RFCT can affect both safety and signal. For safety reasons, ensure you have a solid ground capable of withstanding any phase-to-shield fault current. Also ensure the RFCT is away from the termination and close to the ground bar so the RFCT is still safe if the ground does separate. Figure 4 shows an RFCT placed on an adequate ground close to the main ground bar and away from the termination. Is that the only reason you put the RFCT away from the termination? Remember that the termination is where the electric field goes from nice and contained inside the shield to free and unrestrained after the shield cut-back. The termination is designed to allow the field to spread out gradually so there is a lesser field but evenly distributed around the termination. The

FAQs

How can cable termination mistakes cause partial discharge?

Voltage stresses become higher in areas with lesser permittivity, so voids or poor stress-field control can trigger discharge across weak regions. Errors such as not filling gaps with mastic, imprecise trimming, or incomplete semiconductor removal can lead to PD through voids or stress concentration.

Why does RFCT placement affect both safety and PD test results?

The RFCT must be attached to the cable ground strap, with placement prioritizing safe grounding and correct positioning. If placed too close to the termination field region, it can induce PD interference and make it difficult for the RFCT to discern real partial discharge.

What distinguishes contact discharge from floating metal discharge in PRPD plots?

Both produce characteristic phase-resolved PD plots where discharge level remains consistent as voltage increases. The discharge phase angle shifts with each event due to underlying physics, creating flat, broad PRPD lines that can help identify the offending component despite similar overall patterns.

Sep 30, 2026

Upload by jay