SUMMARY Nowadays, the requirement on diminishing operating and maintenance costs is highly concerned for utilities in competitive electricity market. Power transformer is first focused due to its acquisition costs and failure consequences (FCs). Traditional preventive maintenance for power transformer is generally applied. However, it is very costly and does not take into account actual transformer condition. Hence, risk‐based maintenance is introduced in this paper to facilitate the maintenance works of power transformers. The conditions of power transformers are evaluated from electrical test, insulating oil test and visual inspection by using the ranking and weighting techniques. In addition, the Analytic Hierarchy Process (AHP) is proposed to determine the important weighting factor of each transformer component. The three main criteria of the AHP are maintenance difficulty, FC and failure history. Moreover, the Weibull distribution techniques are applied to analyze failure causes. The computerized web‐application program is developed and implemented for practical use in a utility in Thailand. The power transformers installed in 115 kV and 230 kV transmission systems in Thailand are assessed due to availability and quality of data. From statistical analysis of failure records, the conditions of power transformer can be divided into three different zones as risk, moderate and healthy zone. The defective components in risk zone with higher proportion of failure should be carefully focused. The components and overall condition of the transformers are perceived from the analysis of test results. Subsequently, the risk‐based maintenance is properly planned according to the actual condition. Therefore, maintenance of power transformer fleet can be effectively managed resulting in higher availability and reliability, lower risk of failure and lower cost of maintenance. Copyright © 2013 John Wiley & Sons, Ltd.

Simulations of the switching process of an SF6-selfblast test circuit breaker have been performed and compared with experimental results by measuring voltage, current and pressure build-up. Arc radiation has been considered with the P1-radiation model. To include turbulence effects the shear stress transport model was applied. A comparison of the measured voltage, pressure and nozzle ablation in the test circuit breaker with the calculations is used to show the precision of the simulation tool.

This research focuses on problem identification due to faults in power transformers during operation by using dissolved gas analysis such as key gas, IEC ratio, Duval triangle techniques, and fuzzy logic approaches. Then, the condition of the power transformer is evaluated in terms of the percentage of failure index and internal fault determination. Fuzzy logic with the key gas approach was used to calculate the failure index and identify problems inside the power transformer. At the same time, the IEC three-gas ratio and Duval triangle are subsequently applied to confirm the problems in different failure types covering all possibilities inside the power transformer. After that, the fuzzy logic system was applied and validated with DGA results of 244 transformers as reference cases with satisfactory accuracy. Two transformers were evaluated and practically confirmed by the investigation results of an un-tanked power transformer. Finally, the DGA results of a total of 224 transformers were further evaluated by the fuzzy logic system. This fuzzy logic is a smart, accurate tool for automatically identifying faults occurring within transformers. Finally, the recommendation of maintenance strategy and time interval is proposed for effective planning to minimize the catastrophic damage, which could occur with the power transformer and its network.

Nowadays, the goals of electrical supply utilities are to reduce equipment failures, extend service life, increase equipment reliability, and reduce their related operating and maintenance costs. The high-voltage circuit breaker is an important element in the electrical network. In order to determine or to detect abnormal conditions inside a circuit breaker, powerful vibration analytical techniques have been proposed. In this paper, a vibrational analysis is carried out by analyzing the signal in the time-frequency domain under no-load switching operations with a commercially available high-voltage puffer-type circuit breaker without opening its major parts. Vibration of the circuit breaker poles, operating mechanism, and various monitored parameters were recorded under normal and variable operating conditions. Moreover, a synthetic mechanical damage introduced deliberately is also investigated. The experimental result indicates that mechanical defects can be detected by analyzing the vibration signal. 

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