Abstract
Due to the large number of power transformers (ETs) in the distribution system, there is a need for a relatively simple representation of the status of each unit in order to more easily determine where and how to allocate the budget for preventive and corrective maintenance. In recent years, the concept of the transformer health index (HI) as an integral part of resource management was adopted for the condition assessment and ranking of ETs. HI algorithms take different forms and can be determined based on a large number of specific parameters. However, the main problem in HI methodology or any modern diagnostic technique is the existence of regular measurements and inspections and accurate test results. The paper proposes a solution in the form of the upgraded HI and the novel methodology for ET ranking including the value of available information to describe ET current state. The confidence to the measurement results is calculated using evidential reasoning (ER) algorithm based on Dempster–Shafer theory. The contribution to the ER methodology is the calculation of the initial degrees of belief using Markov chains. The aging process of an ET and transition probabilities from state to state are modelled using the statistical data for the population of 300 ETs and 20 years monitoring data. The proposed methodology is tested on the real data for 110/35 kV transformer, and in the second case, compared to the sample of 30 110/x kV transformers with traditional HI calculation.
Highlights
Determining the health index (HI) of a transformer is a suitable tool that should provide in a reproducible and consistent way information about its condition, the operational correctness, and availability
HI is a tool that combines the results of in-service electrical testing, laboratory testing of transformer oil, maintenance data, and work history data to manage basic resources and build priorities when designing maintenance plans using a numerical grade of transformer status. e choice of methodology depends on the technical capabilities of a particular ET in terms of the number and types of diagnostic tests used to evaluate the condition of transformer elements, as well as on the business and organizational arrangements for storing and updating data and expert teams involved in testing and diagnostics of transformer conditions
Some methods such as the measuring of partial discharges, in spite of their great importance for the assessment of the condition, are not usually used in determining the HI. e reason is possible financial losses resulting from the interruption of electricity supply that can be caused by unnecessary inspections. e advanced methods are used to assess the condition of ETs with developing defects or aged units, such as frequency domain
Summary
Determining the health index (HI) of a transformer is a suitable tool that should provide in a reproducible and consistent way information about its condition, the operational correctness, and availability. The concept of the transformer health index (HI) as an integral part of resource management was adopted for the condition assessment and ranking of ETs. HI calculation method that combines the impact of all available data and criteria based on the industry’s common practices and technical standards is presented in [7], while Gorgan et al [8] calculated HI based on the standard model of twentyfour diagnostic factors expanding the list with three factors (loss factor at very low frequency, conductivity factor, and polarization index). E integrated fuzzy and evidential reasoning model is presented in [25], with previous operation history, results of the latest inspection, and states of the on-load tap changer taken as evidence to assess the working state of the transformer.
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