Asset Management of Underground Cables Factoring Resilience

An electrical power system comprises both primary and secondary equipment. The primary equipment is the network's backbone, which supplies electricity from generation to the customers. At the same time, secondary equipment provides the capability for monitoring, protection, and control of the primary equipment. This equipment, or assets, can be distributed on the ground and underground across a wide area. The health and age of the equipment in different parts of the system will vary depending on many factors, including the environment in which it is installed. To keep these assets operational and to maintain power quality, it is essential to manage the equipment efficiently. The techniques currently used for asset management are based on various mathematical models, historical data, and probability of failure. The focus of current asset management techniques practised by the power systems industry is reliability based. The asset management technique(s) used will also vary depending on the type of asset or equipment being maintained. However, with the climate crisis becoming a reality, electrical power system companies face an ever-growing threat to their equipment with the increase in high-impact, low-probability events such as storms and flooding. The damage to equipment and outage of supply caused by such events is on a larger scale than that due to equipment failure or routine maintenance. The traditional asset management techniques practised by the power system industry have helped maintain the network's reliability, but to cope with climate change and extreme weather events, the industry now needs a new approach to improve the resilience and reliability of its power systems network. Overhead lines and assets on the ground are relatively easy to maintain since the faults can be visibly located and repaired, whereas underground cables buried in ducts and trenches are more challenging to maintain, repair and replace. Generally, faults in underground cables are hard to locate until they gradually develop into large faults. In recent years, underground cables in New Zealand have suffered significantly from damage due to earthquakes. For example, the 2010-2011 Canterbury earthquakes caused widespread damage to underground cables, resulting in a massive power outage in the region. The damage and impact of seismic events on the life of underground cables are still unknown. Thus, there is a need for a thorough study to not only improve the resilience of underground cables but also strengthen the entire power systems network to withstand future events. The underground cable network in New Zealand is unique then the rest of the world in the sense that New Zealand has HVDC, HV and MV cables in the transmission network. The distribution network consists of sub-transmission and distribution cables, and the LV network has three-phase and single-phase cables. This research covers the underground cables in transmission, distribution and LV networks and presents practical use case studies for New Zealand’s power system network. The first section of the thesis, therefore, presents a case study on the underground cables damaged during the 2010 and 2011 Canterbury earthquakes. For this study, a detailed geospatial map of an 11 kV underground cable with repaired joints is built to calculate the repair rate and develop fragility curves with respect to the peak ground velocity of the Canterbury earthquakes. The liquefaction from the earthquake is also factored into the repair rates because the preliminary results showed that most of the damage to the underground cables was due to liquefaction. The dataset is further filtered by removing the cables and joints on the hill and in the lateral spreading zone. Moreover, fragility curves are developed with the filtered dataset to analyse in detail the performance of underground cables during the Canterbury earthquake sequence. Water ingress into the insulation of underground cables is one of the major causes of cable failure. Moisture can enter the cable insulation, and over time, this will result in partial discharge and then, eventually, cable failure. Therefore, techniques for moisture testing in underground cables were examined. The traditional chemical method used for moisture content measurement in underground cables by the New Zealand transmission company is the Karl Fischer titration test. This method is time-consuming and requires a fresh reagent for every test, and electrical power system companies, in most cases, have to send the test sample overseas to cable manufacturers or independent consulting companies to conduct the test. In recent times, the shipping of test samples has become a challenge due to current covid-19 restrictions. Therefore, two novel and efficient tests for measuring the moisture content in underground cables are proposed in this research: the thermogravimetric analyser and the differential scanning calorimeter. A 220-kV XLPE cable from a transmission network is taken as a test sample, and both thermogravimetric analyser and differential scanning calorimeter tests are performed to measure the moisture content in seven different layers of the cable. Based on the test results, a critical section of underground cable was repaired, which otherwise would have taken months. The third section of the thesis describes the use of new technology to improve asset management plans and decision-making by collecting real-time data using the Internet of Things (IoT). IoT technology will not only improve the reliability of the network but also give an insight into how assets and equipment respond to climate change and extreme weather, hence improving the resilience of the power systems network. Low-cost, high-volume assets like pedestals are run till failure; however, the trend of pedestals catching on fire has increased in New Zealand during the past few years. In the case study, IoT sensors are installed in plastic pedestals housing fuses and underground cables to find out the incipient fault leading to fire in pedestals. To summarise, this thesis contributes to improving the resilience of underground cable networks by introducing a unique and detailed geospatial map of underground cables to develop fragility curves and repair rates to study the impact of earthquakes. Two novel methods are also proposed that allow quick and accurate moisture test results. And finally, the utilisation of new technologies is discussed as a way to improve the resilience and reliability of underground cables.

Name thesis:degree_name

PhD

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Electrical and Electronic Engineering

Grantor dc:publisher

ResearchSpace@Auckland

Year dc:date.issued

2021

Author dc:creator

• Rehman, Ebad Ur

Advisor dc:contributor.advisor

• Nair, Nirmal

Statement dc:rights

• Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Licence dc:rights.uri

• https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm

Handle dc:identifier.uri

https://hdl.handle.net/2292/61701

OAI identifier oai:identifier

oai:researchspace.auckland.ac.nz:2292/61701