Although they are an increasingly important component of modern distribution networks, experience has shown that local degradation, as well as joints and terminations often constitute the weak spots in medium voltage cable systems. Key relevant factors include ageing, water ingress, inadequate workmanship and utilization of either inappropriate or defective cable accessories. Given this, there is need to deploy testing and diagnostics techniques on both new and service-aged MV cable systems to minimize downtime because of incorrect workmanship or critical ageing. These are outlined in Guidelines and Standards, such as the IEEE 400 series and IEC 60502-2.
However the standards as of today are written mainly for cable withstand testing. While there is guidance to some extent for diagnostic methods (e.g. for TanDelta and partially for partial discharge), this is limited, outdated or applicable only on newly installed cables. Within CIGRE, several working group activities have taken place to address diagnostic methods for both MV and HV cables. For example, a new brochure, TB841, has been published that discusses alternative test methods for after laying testing of HV and EHV cables and also includes some guidance for PD measurements. In the case of MV networks, there is an active working group (WG B1.58) dealing with MV cable asset management. This working group, closing this year, will propose condition assessment strategies for commissioning, after-repair and maintenance testing for several type of cables such as polymer, paper and mixed. This proposal will discuss PD measurements extensively.
Focusing on diagnostic for MV cables, specific methods can differ both in core and context. Core methods include VLF TD, TDR (Time Domain Reflectometer), PD testing and oversheath testing and are applicable for almost every type of cable. In spite of a lack of agreement on acceptance criteria for most of these methods, they are nonetheless well known and documented. Modern devices are easy to operate and provide the user with fast and reliable results. Core methods should be considered as the basic toolbox for cable testing and diagnostics
The purposes of context specific methods are multiple. These can be recommended during a commissioning test for purposes of baselining, or at higher level of diagnostics or troubleshooting to increase accuracy of measurements. Typically, these context specific measurements are applied on critical system infrastructure such as nuclear power plants and should be considered as an added value to core methods. Due to their specificity some can require experienced operators to obtain exploitable results. Context specific measurement methods include FDR (Frequency Domain Reflectometry), neutral resistance measurements and FDS (Frequency Domain Spectroscopy). The two main core methods being widely applied are VLF TD and PD testing.
Attend the 2022 INMR WORLD CONGRESS in Berlin this October to listen to a lecture by cable testing expert, Hein Putter, with Megger in Germany. His presentation will recommend and detail PD testing application strategies, given the wide availability of equipment, ranging from online to offline methods and also the variety of excitation voltages and coupling sensors.