Condition assessment and diagnostic methods to support asset management of MV cable networks

Working Group (WG) B1.58 has produced Technical Brochure (TB) 924 which provides a comprehensive overview and insight on Medium Voltage (MV) cable asset management based on criticality-based maintenance. In Australia, there has been an increasing demand for these techniques as many of the early cables installed in central areas are now approaching their end-of-life stage.  The Australian member of the WG was Dong-Churl Lee.

The TB also revisits most of the commonly used on-line and off-line diagnostic methods and provides a toolbox that asset owners can use to easily correlate between various failure modes and corresponding diagnostic methods.

In recent years the complexity, size and demand profile of distribution power systems have witnessed huge change.  This is mainly due to the advent of more numerous and smaller scale independent power producers connecting to the Grid (at the MV level). This is coupled with the change in electrical consumer technologies (heat pumps, EV chargers etc.). This is driving a change in the focus on and the details of Asset Management Strategies.

Despite the fact that most underground cables are in MV networks and there are also growing demands on more effective and efficient asset management strategies as the population of aged cables are continually increasing, only a few guidelines are available internationally. Furthermore, MV cable networks are increasingly becoming complex due to the different material and ageing stages as well as the increasing number of repairs and modifications. Sometimes, a lack of records on cable maintenance history or age profile makes it even more difficult to understand the cable condition without proper use of diagnostic methods. Subsequently, WG B1.58 was formed which was the first ever CIGRE WG to focus only on MV cables. The majority of the WG B1.58 members were from asset management providers within distribution utilities, equipment manufacturers or testing service providers.

TB 924 starts by reviewing the life cycles of MV cables and illustrates how proactive maintenance, based on proper condition assessment, could extend the service life of MV cables and consequently maximise the economic benefit. Unlike other guidelines, the brochure was written from the viewpoint of asset owners, so that it focuses more on the cable failure modes rather than different diagnostic methods. Once asset owners have the cable network details and can define the failure modes, the brochure guides the readers to a specific set of diagnostic methods.  These methods were agreed by WG members, based on experiences from their countries, to be the optimal combination of testing that serves the purpose.

The TB also introduces new concepts of “Level” and “Grade”. Each cable network can be given a “Level” depending on its criticality to the system reliability or economic impact, which also determines the level of cost and effort required to perform the corresponding diagnostic tests. Then the TB details each individual diagnostic method to provide further in-depth technical background on each individual testing. In the same way, “Grade” was introduced in Partial Discharge (PD) measurement.  This is one of the most important diagnostic methods which differentiates different types of PD testing in terms of their measurement accuracy and associated costs and efforts to perform. The brochure also discusses the use of different excitation voltages such as power frequency, Damped AC and Very Low Frequency based on the latest studies and publications. At the end of the brochure, several case studies based on real field experiences were presented as practical examples of the diagnostic methods used for condition assessment of MV cables.

It is expected that the work of WG B1.58 may continue in the future with further examinations of asset management of sub-sea cables.

The TB is free for members and €300 for non-members.