Asset management of power transformers is based on statistical analysis of transformer’s failures and forced outages. The statistical analysis results are useful. They can be used to determine the design of the transformer, also enhance utility. More so, the results can be used to improve the care of the transformers as well as monitor its actual conditions. It is essential to understand the transformer’s outage rates as well as outage and repair durations.
The Egyptian Electricity Transmission Company (EETC) provides outage information in this thesis. This work presents outage data analysis within eight years, from 2002 to 2009, for 1922 average number of transformers in voltage populations ranging from 33 kV to 500 kV and MVA rating from 5 MVA to 500 MVA. Forced outages due to correct and false action of transformer’s protection systems are carefully considered.
There are two phases of conducting Outage data analysis. The first phase performs failure and repair analysis of transformers while the second phase assesses the impact of transformer outages on customers. Percentage average number of failures (%AANF), and annual average repair time (AART) per transformer represent the failure and repair data of power transformers. Two indicators represent the impact of transformer outages on customer interruptions. These indicators are the annual average interrupted MW (AAIMW) and annual average customer-interruption duration (AACID).
This thesis presents analysis of study on the reliability, availability, and maintainability of power transformers manufactured by EETC. Additionally, failures of transformers are also estimated in the study. The thesis also addresses different maintenance practices that can make a system reliable and efficient.
1.0 Thesis outline
This thesis constitutes four chapters as outlined below:
1.1 Chapter one: Covers theory and operation of transformers. It focuses on the transformers construction and other installation equipments necessary for the transformer operation. It also covers the transformer’s maintenance process. Additionally, the chapter covers the Egypt’s transmission system together with the load development for the past five years.
1.2 Chapter two: Covers in detail the maintenance definition and category as well as the scope of responsibilities of transformers. More so, the chapter addresses the maintenance policies, reliability as well as the causes for transformers failures. It also mentions the previous international survey and studies concerning transformers failures.
1.3 Chapter four: Focuses on the RAM analysis for voltage subpopulation in Egypt and its comparison to IEEE survey in 1979. The chapter investigates calculations of hazard functions, true and estimated failures rates for transformers in all voltage subpopulation against 1979 IEEE survey.
1.4 Chapter five: Provides summary and conclusion of the work already covered in the previous chapters. Moreover, it provides recommendations and suggestions for the future research work meant to reduce the forced outages of transformers.
The outage data analysis carried out on 1922 (average number) transformers in voltage populations ranging from 33kV to 500kV and MVA rating from 5MVA to 500 MVA presents the following results:
Outages of incomers and over current protection are the leading causes of outages. They are responsible for about 57 percent of outages falling within the range of 6kV to 33kV subpopulation, and of outages within 132kV subpopulation. In the 220 kV subpopulation, the leading cause of outages is the over current protection which contributes to the cause by 19 percent followed by others equivalent to 14 percent of the outage cause. Additionally, the major cause of outages in the 500kV subpopulation is the other over current protection of 23 percent followed by fire systems which translates to 19 percent of the cause.
More so, outages within the range of 66kV to 33kV voltage subpopulation caused the highest annual average of percentage number of failures (%AANF) per transformer followed respectively by the 220kV, 500kV, and 132kV subpopulations. In addition to that, the 500kV recorded the longest annual average repair time (AART) followed by the 220kV, 132kVand 66-33kV subpopulations respectively.
The longest annual average customer interruption duration (AACID) per transformer associates with the 132 kV voltage subpopulation the 66-33kV, 500kV, and 220kV respectively. It was noted that highest annual average interrupted MW (AAIMW) per transformer is associated with the 500kV voltage subpopulation followed by 220kV, 132kVand 66-33kV subpopulations respectively.
Additionally, the fire systems are responsible for the highest number of false trips in all voltage subpopulations except for 220kV subpopulation, where the leading cause of false trips is the busbar protection. Busbar protection is also a significant cause of unauthentic outages in both the 132kV and 66-33kV subpopulations. In the 500kV population, a large number of unauthentic outages arise from over current protection, buchholz and pressure relief. More so, the 220kV subpopulation has the highest failure rate followed by the 66-33kV, 500kVand 132kV subpopulations respectively.
It is clear that both estimated failures and true failures rates are extraordinarily in comparison with the failure rates of 1979 IEEE power transformers survey. Consequently, the reliability of the considered transformer is extremely low. Calculated hazard functions show that failure rate for transformers in most voltage subpopulation increases significantly with time. Therefore, the considered transformers operate in the wear-out phase.
Ideally, the availability of EETC transformers is higher than the average of the 1979 IEEE survey. Although EETC transformers have an unusually high failure rate, their availability is high. This is due to the extremely small MTTR. As matter of fact, MTTR is not the most significant factor that can efficiently and effectively impose a positive or negative impact on the availability.
EETC should consider replacing most of their transformers, and instead increase the number of transformers to face rapid growth of loads. This will serve to limit over current outages. More so, EETC should provide sufficient stock of spare parts in each power substation. Additionally, the body should also apply new maintenance for the transformers like online monitoring of DGA and RCM. Ideally, false trips of protection system can be reduced significantly by improving maintenance procedures, system monitoring, and strategies as well as revising the design of protection systems.
EETTC should train technicians and Engineers in charge of the maintenance of transformers for to enable them identify and analyze arising problems. Finally, EETC should invest in improving engineers’ educational curriculum in both regional and national control centers to make engineers competent enough to deal with cases of outages, and also understand the behavior of the disconnected network components.
(1). W H Tang & Q H Wu, Condition Monitoring and Assessment of Power Transformers Using Computational Intelligence Power Systems, Springer, Liverpool, 2011