#1. UHF Partial Discharge Monitoring - Let’s be honest!

I am sometimes asked by partial discharge (PD) monitoring system vendors to comment on procurement specifications for GIS or transformer monitoring installations which they have received as part of a project tendering process. These documents too often contain requirements (or make claims for competitor systems) that defy the laws of physics and contradict the findings of published scientific papers. In this series of articles, I will address several key issues with the aim of clarifying misconceptions and supporting those who describe their system capabilities honestly.

Requirements or claims that UHF PD monitoring equipment can display PD levels in picocoulombs (pC).

Apparent charge measurements to IEC60270 (commonly measured in pC) can only be achieved by a properly calibrated PD measurement system using galvanic electrical coupling to the test object. It has been well known for more than 20 years that ‘unconventional’ methods such as UHF PD detection systems cannot measure apparent charge in pC, so please stop asking for it! Procurement specifications are often at fault, as it seems they have been created with input from individuals with insufficient understanding. Since many are probably only familiar with PD in the context of acceptance test criteria, they seem to insist on using the same units to specify their PD monitoring system requirements. I have some sympathy for vendors who consequently scale their UHF systems’ measurements in pC rather than be ruled out of a tendering process (the customer gets what the customer wants). However, to be clear - these pC scales are MEANINGLESS. Vendors who comply with such requirements only help to further spread the myth that UHF measurements can be calibrated in this way.

Blame must also be attributed to unscrupulous monitoring system developers who claim that their UHF systems can measure PD in pC. This is particularly useful to them if they can influence gullible clients’ procurement documentation to state that this is a requirement. The only valid procedure to relate UHF PD monitoring sensitivity to a pC level is the one set out in the 2016 CIGRE Technical Brochure 654, “UHF Partial Discharge Detection System for GIS: Application Guide for Sensitivity Verification”, which is intended to ensure that a certain minimum level of PD apparent charge is detectable. This is a pragmatic approach which represents the best scheme that could be devised by a group of international PD experts and is based on many years of cumulative experience. Note that CIGRE publications more than 3 years old are freely downloadable via e-cigre.org, including TB 654.

If you need support with UHF PD detection systems, HFDE can offer the following independent and confidential services:

• Evaluation of PD monitoring system specifications to ensure that they are realistic (for customers or vendors),
• Review of tender submissions to ensure the client knows what questions to ask and what validating evidence should be sought to confirm potentially exaggerated specification claims,
• UHF sensor design and sensitivity testing,
• Design and supply of UHF PD sensor calibration systems,
• PD emulation and ‘blind testing’ of installed PD monitoring systems to validate operation and test manufacturer’s claims,
• Preparation of evidence-based reports and literature reviews to rebut unrealistic or unscientific claims relating to UHF PD detection and monitoring.

https://www.hfde.co.uk/Contact.html

Martin Judd is Technical Director of HFDE Ltd, a Scottish company that specializes in condition monitoring and diagnostic techniques for high voltage (HV) electrical insulation systems. His role involves regular working on-site with electrical utility companies, applying advanced techniques for detecting, diagnosing and monitoring partial discharge (PD) activity in HV equipment such as power transformers, open terminal substations, switchgear and GIS. Before founding HFDE in 2014, Martin was Professor of High Voltage Technologies at the University of Strathclyde in Glasgow, where he managed the HV Laboratory and pioneered many aspects of the ultra-high frequency (UHF) technique for PD detection. He has authored more than 250 scientific papers on PD and related topics and is a member of the IEEE DEIS Diagnostics Technical Committee. Dr Judd currently serves as UK member of two CIGRE working groups, “Improvement to PD measurements for factory and site acceptance tests of power transformers” (A2/D1.51) and “PD measurement on insulation systems stressed from HV power electronics” (D1.74).