Condition evaluation and lifetime strategy of HV cable systems

Introduction

However, there may be economic or strategic reasons that justify the end of life of a system long before its technical condition requires replacement.

In addition, political and regulatory issues need to be considered to avoid heavy penalties or forced decommissioning.

A system that takes into account all of the above factors is unique to each country and utility. A generic approach was first introduced by CIGRE WG B1.09 in 2008 and published in TB 358, "Remaining Life Management of existing AC Underground Lines". Based on a two-step questionnaire, a score is calculated for each asset, allowing it to be categorized as requiring immediate attention ("red"), requiring close monitoring ("orange") or being uncritical ("green").

In 2018, it was decided to revise and, if necessary, update the system proposed in TB 358. Therefore, the Working Group B1.68 was formed. The objective was to collect past experiences on remaining life management in order to develop an updated approach suitable for extra high voltage and high voltage cable systems, both at AC and DC voltage. This update should also take into account new technologies that have emerged during the past years, which allow for more comprehensive and accurate condition assessment of power cable systems.

The aim of this Technical Brochure is therefore to reflect current practices for remaining life management and to evaluate the usage of the system presented in TB 358, to outline the various aspects that need to be considered when designing a remaining life management strategy, and to propose an exemplary method for determining the condition of a cable system and deriving the actions necessary to allow for timely repair or replacement actions.

Methodology

Based on feedback from 61 utilities on their approach to lifetime management and the use of the methodology proposed in TB 358, an updated concept is being developed. The proposed lifetime management system is derived from the reliability-centered maintenance (RCM) concept, i.e. both the condition and the criticality of a cable or cable system are taken into account when assessing the remaining lifetime. The method is fully customizable to the specific requirements and information available for each cable system and utility. It also conforms to the ISO 55000 series.

The condition can be expressed by a Health Index (HI), which assigns a number between 1 (very good condition) and 5 (very poor condition) to each asset under consideration. The HI can be further subdivided into specific indicators relating to measurement results obtained during on-line or off-line testing, cable age, maintenance aspects and failure rate. These sub-HIs are then combined in a weighted sum to calculate the overall HI. The choice of weighting factors is left to the reader of the TB, as this not only requires expertise and experience, but should also take into account the amount and reliability of available information and reflect the utilities' philosophy of asset management. It is recommended that the HI calculation is repeated periodically to track changes and ageing of the cable system so that a relationship of HI evolution over time can be established. It is also expected that the prediction of end of life will become more accurate over shorter periods of time.

Criticality is expressed by a Criticality Index (CI), which assigns a score to the consequences of a potential cable failure in terms of safety, finance, reliability and environment. As this has already been introduced, the TB refers to TB 422, 541 and 734 as appropriate examples. Again, the weighting used to calculate the overall CI should reflect the utility's policy. It is also advisable to review the CI periodically in the event of changing operating conditions that may require an updated calculation.

The Risk Index (RI) is calculated as the product of HI and CI and is a direct indication of the action required for each cable or cable system. A low RI therefore indicates that the system is in good condition, as reflected by a low HI, and that the consequences of failure are low, i.e. a low CI means that no action is required by the asset manager, whereas a high RI indicates that action should be taken. The RI should be reassessed each time the HI and RI are recalculated.

In addition to the lifetime management methodology, the TB presents and discusses life extension options and contingency strategies. The general structure for life extension is to first analyze the ongoing degradation mechanisms and to make a prediction for the case of undisturbed degradation compared to taking action, where the required expenditure and the resulting benefit in cable performance should be considered. Based on the outcome of the analysis, the chosen action can then be implemented. Examples and best practices are given in the TB. Emergency strategies are designed to maintain an acceptable level of service in the event of a cable failure, and a recommended approach is also described in the TB.

Structure and Content of the Technical Brochure

The TB contains seven main chapters which are structured as follows:

In chapter 2, a short overview on different approaches to lifetime management are described based on a literature review, with a focus on reliability-centered maintenance.

In chapter 3, current practices in remaining life management are assessed based on the outcome of a questionnaire that was answered by 61 utilities. It is found that awareness of the importance of lifetime management is growing, and the trend is moving towards condition-based and reliability-centered maintenance. Due to large variations in grid structure, there is no one-fits-all solution, but the need for a customizable generic approach.

In chapter 4, possible reasons for the end of life of a cable or cable systems are explained. Technical reasons are illustrated by means of a Failure Mode Effect Analysis (FMEA). Non-technical reasons for decommissioning a cable system comprising environmental, economic and strategical reasons are briefly explained.

Chapter 5 gives an overview of the most commonly used tests used as a basis for determining the health index of a high voltage cable system with reference to the FMEA established in chapter 4.

Chapter 6 contains the health index method as proposed by the working group, taking into account technical as well as non-technical aspects to determine the remaining life of a high voltage cable system as a result of the asset's health and risk index.

Options for life extension and emergency strategies are presented in chapter 7, with a focus on practical examples and best practices.

Conclusion

The proposed method for lifetime management includes a comprehensive guideline for calculating an asset’s Risk Index as a product of its Health Index and Criticality Index, and practical options for life extension and emergency strategies. The approach can and should be adapted to fit the needs of the reader, taking into account the already existing infrastructure and information available. Setting up a lifetime strategy still requires expertise and experience, and it is recommended that the strategy be periodically re-evaluated to keep up with the state of the art and to implement new methods of condition assessment.