Partial discharge: An early warning sign

Developments in offshore wind reflect the overall direction of travel for Megger technology for detecting partial discharge in networks of cables. “We want to be able to predict when failure is likely to happen,” explains Daniel Mueller-Bode, diagnostic specialist for cables at Megger in Germany.

Megger products such as the VLF Sine Wave 62 kV and TDM45 already enable the detection of partial discharge in underground cable across a wide range of medium-voltage installations.

These technologies facilitate an intelligent, dynamic approach to maintenance, explains Mueller-Bode. “The most expensive approach to maintenance is one that looks only at cable age and replaces the oldest cable. Rather, we diagnose the condition of the cable, and then find a solution to a particular problem.

“You avoid replacing an older cable with insulation that is fine, or ignoring a newer cable where the insulation is actually very bad. That is where diagnostics play the biggest role.”

Monitoring entails permanent surveillance of the cable in normal operation. Here, the used voltage level is the nominal voltage of the cable (U0). “In condition assessment or measurement, we perform offline measurements of partial discharges with a voltage above U0, since failures develop over time and will most likely start to develop at higher voltages,” explains Mueller-Bode.

Human factors often play a role in partial discharge. Weak points tend to occur at human-made joints or terminations. Locating partial discharge enables preventative maintenance work to be carried out on a joint that might otherwise be destined to fail altogether. This enables engineers to plan maintenance activities before breakdown occurs.

It’s important they understand the significance of the data being generated by the partial discharge monitoring equipment, Mueller-Bode adds. “It’s important to know what to do with the measurement data afterwards. People sometimes get stuck because they don’t know how to handle the data for asset management. That’s why we offer consulting services for interpreting the severity of partial discharge faults.

“That’s where our specialists come into play come into play with their knowledge and experience. Detecting partial discharge is a specialised field. There are a lot of different factors to take into account to understand what is really going on inside the cable.”

Cable faults are likely to have been introduced as part of cable assembly. Or older cables may have suffered damage to the insulation over time due to being in acidic soil and the ingress of moisture or water. “We can measure water molecules and find out if we have a water problem in the cable or not,” says Mueller-Bode.

Megger also works with enterprise asset management partner IPS Systems to help manage large inventories of cables for customers. “We have customers around the world who are saying, ‘we have many tens of thousands of cables, how do we manage them?’” says Dodds. “They need to know the history of the cable, faults, and looking at diagnostics, what is the current health, and which of my 40,000 cables should I be thinking about next?”

IPS helps by creating a detailed cloud-based library of assets. The next step will be to integrate this into a sophisticated overall cable asset management system, Dodds explains.