• • • TEST & MEASUREMENT • • •
Partial discharge Testing of HV assets W
hen embarking on a PD testing programme, the challenge lies in tailoring a testing schedule that yields high-quality data and identifies faults promptly, allowing time for intervention to prevent failure. PD is a phenomenon that occurs inside electrical insulation systems when there is a breakdown of dielectric material, causing small electrical sparks or discharges inside the insulation. This can be caused by any variety of factors such as impurities in the insulation or material, leading to voltage stress. PD is a concern for several reasons, relating mainly to failures, safety and maintenance costs. It is often an early indication of insulation breakdown or degradation in high-voltage electrical equipment such as transformers, cables, and switchgear. Detecting PD and identifying the different variants early on, can help prevent catastrophic failures further down the line, as it can can generate localised heat and, in some cases, cause fires within electrical equipment. Naturally, all these issues correlate with poor reliability and performance, and failure to recognise and address PD can result in costly maintenance and major downtimes.
Partial Discharge variants Internal PD
Internal partial discharge refers to a type of PD that occurs within the bulk of an electrical insulation material, rather than on the surface. Partial discharge is a localised electrical discharge that can take place in the presence of voids, defects, or impurities within the insulation material of electrical components, such as transformers, cables, switchgear, and other high-voltage equipment.
Partial discharge (PD) testing, an established condition monitoring test, plays a pivotal role in determining High Voltage asset health. Thomas Whyte from EA Technology, takes a look at the best practice approach to ensure an optimised testing and maintenance regime
Periodic Testing Periodic testing requires a network operator to manually conduct a PD test at intervals throughout year with a PD testing device, such as EA Technology’s UltraTEV® Plus². The risk with periodic testing is that any partial discharge could potentially go undetected for some time, which can cause an asset failure.
In one example, a network operator, conducting periodic PD tests, detected an ultrasonic PD source in a Ring Main Unit (RMU) cable box after five years of testing. Unfortunately, the asset failed in service within the year prior to the next scheduled maintenance or PD test, leading to significant disruptions. The defect site of this failure was found to be in the crutch of the original 11kV PILC cables that were reused when the RMU was replaced. The defect site was likely introduced during the commissioning of the new RMU.
Surface PD Surface partial discharge is a type of PD that occurs at or near the surface of electrical insulation materials or components. It differs from internal partial discharge, which takes place within the bulk of the insulation material. Surface PD typically involves the breakdown of dielectric material on the external surface of conductors, insulators, or other electrical components.
Corona PD
Corona PD, while not inherently harmful in outdoor switchyards, can initiate surface PD if left unchecked in enclosed chambers, leading to the onset of surface PD.
PD Testing Typically, there is two types of testing – periodic testing and permanent monitoring:
Permanent Monitoring Permanent monitoring may be more appropriate for critical or ageing assets to mitigate the risk of potential failures, which could be catastrophic especially when considering outages, replacements and even fines. However, most failures don’t happen in an instant, meaning they can be predicted and prevented with permanent monitoring.
In this example, an aged 11kV switchboard, had 24/7 PD monitoring installed to mitigate risk. Within 18 months, an emerging failure was detected on a voltage transformer, with data showing the rise in PD activity from zero to a high level in a matter of minutes. An outage was scheduled to inspect the VT. The assessment revealed significant damage, indicating the component was nearing failure. Thanks to timely intervention, no customers faced any disruption or interruption to supply, and no other components on the switchboard were negatively impacted. EA Technology’s Astute HV Monitoring® is the ideal fixed monitoring solution, which can alert you to suspicious spikes and pinpoint the parts of your HV system that require investigation. To correctly manage and mitigate PD, preventive measures such as routine monitoring is critical. Detecting and addressing PD early on can extend the lifespan of the equipment and enhance the safety and reliability of electrical systems. To do this effectively and reliably, selecting the correct PD test equipment is crucial.
It is therefore worth looking to reputable manufacturers that can provide full guidance and equipment in this specialist area, ensuring peace of mind and compliance across the board. EA Technology has a wide range of both fixed and remote handheld solutions for PD testing such as the UltraTEV Plus2 and the Astute HV Monitoring, to help keep networks safe, operational and efficient.
38 ELECTRICAL ENGINEERING • NOVEMBER 2023
electricalengineeringmagazine.co.uk
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