Maintenance and retirement of ageing power system assets and reinforcement of transmission systems with seismic risk

Vasquez Guerrero, Wilson Andres (2022). Maintenance and retirement of ageing power system assets and reinforcement of transmission systems with seismic risk. University of Birmingham. Ph.D.

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Abstract

Power system components age over time, due to thermal, electrical, mechanical, and chemical stresses. Ageing components tend to fail more frequently, increasing the risk of power supply interruptions during normal system operation and under extreme events such as earthquakes. Maintenance and retirement activities should be targeted at critical components that pose the highest risks of interruptions. However, developing failure models of ageing medium voltage underground cables and old power transformers, used for quantitative risk assessment, is a challenging task for utilities due to the lack of failure data. Using non-traditional failure models, i.e., models that incorporate components' condition data and loading levels, is an interesting alternative that would allow utilities to enhance their risk-based maintenance and retirement approaches. This thesis addresses the modelling challenges for cable maintenance prioritisation, power transformer retirement, and transmission system reinforcement considering earthquakes.

This thesis presents an approach to prioritise the maintenance of ageing medium voltage underground cables. The approach combines, for the first time, the index known as maintenance potential index with a new aged-related repairable failure model to identify cables whose maintenance would yield the greatest benefits to the reliability of a power distribution system. The case study in this thesis suggested that cable maintenance prioritisation significantly depends on the impacts of age-related repairable failures on the expected system interruption cost, which emphasises the importance of considering the effects of cable ageing. The proposed model for aged-related repairable failures was compared with the IEC-Arrhenius-Weibull model through computer simulations, which consist of failure rate and cable ranking calculations as well as sensitivity analysis. The results revealed that using Arrhenius relationship may negatively affect the calculation of the failure rate of ageing cables, reducing its value and making it very dependent on the shape parameter of the Weibull distribution. This thesis also presents a risk-based approach for planning the retirement of power transformers. To overcome the problem of limited end-of-life failure data, the approach calculates the apparent age of power transformers using health index data, and estimates the scale parameter of the Weibull distribution using the Arrhenius relationship, the IEC thermal model, and an equivalent load model. The case study results suggested that the improved ageing failure model yields better predictions of the unavailability due to ageing failure, increasing its value as power transformers get older and thus enhancing retirement decisions.

Finally, a two-stage adaptive robust optimisation problem to determine reinforcement decisions of transmission systems with seismic risk and aged equipment is presented. The availability of generating units, lines, and transformers is modelled by classifying these components into several groups (based on seismic hazard zone, ageing status, and risk of being damaged by a tsunami), and by using a polyhedral uncertainty set for each group. Unlike traditional optimisation problems, the proposed problem calculates the expected capacity of all aged components by using models of repairable and ageing failures, helping system planners to obtain more accurate reinforcement decisions. The assessment proposed in the thesis suggested that transmission systems with aged components require more reinforcements (i.e., more transmission lines and power transformers) to mitigate the impacts of earthquakes.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):