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Surge arresters are the most critical equipment for protecting high voltage substations. They play an important role in substations for limiting switching and lightning surges and diverting these surges to ground. On the other hand, surge arrester number and placement for high voltage substations can be determined based on some evaluations in the designing process of substations. Surge arresters can be placed on the both ends of substations, transformers, circuit breakers, reactors, capacitors and also high long bus-bars and etc. Therefore, failure of arresters during overvoltage can put substations in risk condition. Moreover, surge arresters may be inclined to be short circuit during normal operation condition due to ageing process and/or improper quality. This paper attempts to assess reliability of three common substation configurations namely: (1) one breaker and a half; (2) double-bus double-breaker; and (3) ring bus-bar in different placement of surge arresters. At first, maximum voltages on equipment are calculated in different lightning stroke locations through simulation in EMTP-RV. Studies without surge arrester and the presence of surge arrester in different locations are analyzed and compared. Then surge arrester’s placement impacts on the substations reliability indices are calculated in normal operating condition and overvoltage condition by minimal cut set method and simulation results. Analytical studies reveal that surge arrester can increase substations reliability. But for low annual number of lightning stroke, substation reliability may decrease. Also increasing surge arrester number more than substation need reduces reliability. [ABSTRACT FROM AUTHOR]

Abstract: This paper extracts a comprehensive reliability model for a transformer with oil natural air forced (ONAF) cooling system, which is the most common type of power transformers. The transformer is first classified into three subsystems and a reliability model is thereafter developed corresponding to each subsystem. Markov process and frequency/duration approaches are applied to analyze the extracted models and achieve their equalized representations. Merging subsystem models leads to the transformer entire reliability model consisting 17 states. The model is then reduced to an 11-state one which is more computationally manageable. Numerical analyses and sensitivity studies are conducted to demonstrate the performance of the proposed model and the results are thoroughly discussed. [Copyright &y& Elsevier]

• Evaluate the effect of different insulator profiles on Flashover Voltage (FOV). • Analyze the risk of the insulators utilization under different ambient conditions. • Effect of different pollutions and humidities variations on the reliability of the aged Silicone Rubber (SiR) insulators. • Reliability of the aged Silicone Rubber (SiR) insulators has been calculated. In this paper, the effect of different pollution and humidity variations on the reliability of aged Silicone Rubber (SiR) insulators are investigated. Firstly, four different types of 20 kV SiR insulators have been selected to evaluate the effect of different insulator profiles on Flashover Voltage (FOV). Then artificially accelerated aging tests with 1000 h duration have been conducted for the aging of insulators. Numerous FOV tests have been carried out on the aged SiR insulators under different humidity and pollutions. Next, the risk of insulators utilization under different ambient conditions have been calculated by using the experimental test's data. Finally, the reliability of insulators under different conditions have been calculated and a new mathematical model for the reliability in different humidity has been proposed. The effect of SiR insulator profiles on FOV has been studied and the results have been shown that lower FOV is related to insulators with thicker profiles as well. [ABSTRACT FROM AUTHOR]