The paper suggests a method for effects evaluation of a few activities and measures, undertaken as a part of preventive maintenance of power transformers. The method enables calculation of expected failures repair cost and load curtailment cost. The method identifies minor and major failures. Power transformer is a complex system, consisting of five components (functional parts). It is assumed that each component has two independent, competing failure modes: wear-out failure mode, modeled by two-parameter Weibull distribution, and a chance failure mode, characterized by an exponential distribution. The application of the method is demonstrated for one transformer station (TS) 110/x kV/kV with 2 × 31.5 MVA transformers. Also, by applying the method, influence of system for condition monitoring of transformer windings and oil on failures repair cost and load curtailment cost is evaluated.
With regard that equipment in substations is growing older, maintenance organization in each electric power company has one of the most important functions.
The goal of effective preventive maintenance is minimizing the unplanned outages, i.e. reduction of long and expensive failures repair and load curtailment cost. Having in mind their functions, purchase costs, number of installed units and techno-economical consequences of unplanned outages, preventive maintenance of power transformers has an essential importance.
The aim of this paper is to propose a model enabling the evaluation of effects of a few activities and measures, undertaken as a part of power transformers preventive maintenance.
In the following sections, a model for calculation of expected failure repair cost and a model for calculation of load curtailment cost are formulated.
The paper suggests a method for evaluation effects of different forms of power transformer preventive maintenance.
First, let us compare an expected failure repair cost with an expected cost for each of analyzed forms of preventive maintenance. Comparison will be made between:
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CET(T, N) (4) and CET(m, N) (5),
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CET(T, N) (4) and CET(Toil, N) (7),
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CET(T, N) (4) and CET(Treg, N) (11),
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CET(T, N) (4) and CET(Tref, N) (13).
After calculation of the following ratios: (C ET(T , N )/C ET(m , N ))|T = m, (CET(T,N)/CET(Toil,N))|T=Toil(CET(T,N)/CET(Toil,N))|T=Toil, (CET(T,N)/CET(Treg,N))|T=Treg(CET(T,N)/CET(Treg,N))|T=Treg, and (CET(T,N)/CET(Tref,N))|T=Tref(CET(T,N)/CET(Tref,N))|T=Tref, Fig. 15, Fig. 16 and Fig. 17, we can conclude as follows:
From Fig. 15 it is obvious that, from economic point of view, performing of 1-day preventive maintenance and/or oil regeneration is not justified.
An expected yearly cost will be reduced if insulation system regeneration, Fig. 16, or transformer refurbishment, Fig. 17, be performed after 11 years of exploitation.
Also by comparing (Fig. 3 and Fig. 5) and (Fig. 4 and Fig. 6), we can conclude as follows:
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during time interval (0, 5) reduction of expected load curtailment cost is equal to increasing of expected failure repair cost. If we perform 1-day preventive maintenance after 6 years of operation without failures, total reduction of expected cost and load curtailment cost during time interval (6, 40) will be €3/year;
If we compare expected cost and load curtailment cost for the case of operation with non-preventive maintenance with expected cost and load curtailment cost for the case of operation with performing of oil regeneration we can conclude as follows:
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Performing of oil regeneration during time interval (0, 19) is not economically justified.
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If we perform oil regeneration:
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after 19 years of operation, total reduction of expected cost and load curtailment cost during time interval (20, 40) will be €95/year;
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after 20 years of operation, total reduction of expected cost and load curtailment cost during time interval (21, 40) will be €257/year;
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after 21 years of operation, total reduction of expected cost and load curtailment cost during time interval (22, 40) will be €477/year;
According to the criterion of expected load curtailment cost, installing of system for condition monitoring of windings and oil is economical solution from the first year of transformer operation.
According to the criterion of expected failure repair cost, installing of system for condition monitoring of windings and oil is economical solution after 20 years of transformer operation.
