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Highlights • Using the experimental tests, the effects of longitudinal and fan-shaped non-uniform pollutions on the ac flashover voltage of SiR insulators are investigated and analyzed. • The experimental results indicate that the flashover voltage increases under longitudinal non-uniform pollution and decreases under fan-shaped non-uniform pollution, compared to uniform pollution. • A new flashover voltage correction factor under combined longitudinal and fan-shaped non-uniform pollutions is proposed based on the test results. Abstract The pollution non-uniformity in the axial and circumferential directions of insulator surface has a great impact on the ac flashover voltage. So far, few studies have been conducted about the flashover voltage performance of Silicone Rubber (SiR) insulators under fan-shaped and longitudinal non-uniform pollutions. Therefore, this paper aimed to investigate the effects of longitudinal non-uniform, fan-shaped non-uniform, and combined longitudinal and fan-shaped non-uniform pollutions on the ac flashover voltage of four different types of SiR insulators. The analysis of experimental results indicated that longitudinal and fan-shaped non-uniform pollutions compared to uniform pollution contributed to an increase and a decrease in the ac flashover voltage, respectively. The ac flashover voltage increases with the increment of longitudinal non-uniformity degree and decreases with the increment of fan-shaped non-uniformity degree. Furthermore, a new correction factor of flashover voltage under combined longitudinal and fan-shaped non-uniform pollutions has been proposed. For all types of insulators, the correction factor under different non-uniform pollution conditions ranges from 0.85 to 1.13. [ABSTRACT FROM AUTHOR]

Highlight • Under DC voltage, surface charge indicates polarity effect. • Increasing the voltage amplitudes led to an increase of the surface charge density. • Corona voltage and the charging time for both polarities rise, the amount of charges increases linearly. • Relationship between amount of surface charges and number of photons have been carried out using corona camera. Abstract The objective of the present study is to investigate the surface charge measurement on the surface of polymeric insulator under AC and DC voltages experimentally. According to the test results, increasing the voltage amplitudes led to an increase of the surface charge density, the voltage application duration was also extended. During the tests, it was observed that as the corona voltage and the charging time for both polarities rise, the amount of charges increases linearly. The results show that more surface charge was deposited by negative corona charging when compared with positive corona charging. Also in this paper, the effect of the corona variations on the amount of surface charge have been investigated. For this purpose, relationship between amount of surface charges and number of photons have been carried out using corona camera. Moreover, attempt was made to investigate the effects of surface charges on potential and electric field distribution along polymeric insulator under AC and DC voltages using computer simulations based on finite element method (FEM). The present paper was conducted to study the effects of magnitude, polarity, and location of surface charges on surface potential distribution along the insulator. [ABSTRACT FROM AUTHOR]

This paper presents proper indicators for evaluation of the surge arrester condition based on the leakage current analysis. Maximum amplitude of fundamental harmonic of the resistive leakage current ( I mr1 ), maximum amplitude of third harmonic of the resistive leakage current ( I mr3 ) and maximum amplitude of fundamental harmonic of the capacitive leakage current have been used as indicators for surge arrester condition monitoring. The effects of operating voltage fluctuation, third order of voltage harmonic, over-voltage and varistor aging have been investigated to show the ability of introduced indicators for accurate diagnostic of arrester’s conditions. In order to have accurate values of stray capacitors, 3D-FEM has been used. Moreover, surge arrester model for calculating leakage current has been performed in ATP–EMTP. In addition, the signal processing has been done using MATLAB software. This study shows that the introduced indicators are proper criteria for surge arrester condition monitoring. [ABSTRACT FROM AUTHOR]

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]

• Proposing reliability models for four HVDC transmission network configurations. • Considering Lines Capacity Restrictions in all DC transmission lines and investigating several reliability indices. • Considering system states with the highest probability of failure. • Detailed investigation of each system state for all of the configurations by calculating probability, frequency, available capacity, P kj , L kj , and D kj for each system state. • Investigating the effect of load level on reliability indices and carrying out a numerical analysis on the obtained results. The ongoing increasing demand for electrical energy and the reliable access to remote renewable energy generation points such as off-shore wind power stations or solar power generation, have revived the interest in HVDC multi-terminal systems. With the expansion of HVDC systems, their reliability assessment has become continuously important. One of the most important aspects of the reliability assessment of electrical systems is to have an accurate understanding of the engineering implications of the system and its associated components. The next step is to appropriately model the system components to represent their characteristics and functions in the overall system. In order to deliver a specified amount of electrical power, different methods can be considered. There are two general methods: 1) using several electrical power sources with low capacity, or 2) using fewer electrical power sources with high capacity. The decision to choose a certain method for transmitting electrical power depends on various parameters, and one of the most important parameters is reliability studies. This paper presents reliability modeling and analysis of HVDC transmission systems incorporating hybrid HVDC breakers to interrupt HVDC line faults. In this paper, comprehensive detailed reliability models of point-to-point, three-terminal, and four-terminal configurations are developed. Using this equivalent reliability model, various reliability indices are calculated at the load point of the system. Also, the effects of line constraints and incorporating DC circuit breakers of the DC transmission lines on the load point reliability indices have been evaluated. In the end, the effect of the load level on the reliability indices of the load point of each configuration has been assessed. [ABSTRACT FROM AUTHOR]

• Artificial pollution flashover tests have been utilized under ac voltage to investigate the impact of uniform and FSNU pollution on flashover voltage of composite insulators. • Morphological effects of multi-stress aging have been carried out by the SEM analysis. • The effects of contamination roughness and extension on flashover voltage of the polymeric insulators have been conducted. • Simultaneous presence effect of non-uniform pollution and multi-stress aging on the flashover voltage of polymeric insulators has been analyzed. Pollution deposition along with ultraviolet (UV) radiation and electro-thermal stress cause the composite insulators to face with critical conditions. The type of pollution which settles on insulators is not only uniform but also it can be non-uniform and uneven. In this study, the results of the uniform and fan-shaped non-uniform (FSNU) pollution tests on four various types of virgin and aged polymeric insulators under AC voltage are presented. To find the effects of accelerated aging on polymer chains of composite insulators, scanning electron microscopy (SEM) is carried out. Based on IEC 60507, the solid layer method is applied to settle pollution on the insulator surface. The results demonstrate that the flashover voltage of virgin and also aged samples decreased when pollution non-uniformity degree (J) increases from 1 to 13. The flashover voltage of studied insulators is reduced by increasing the occupation ratio (K). However, the impact of K was lesser than J on flashover voltage of the polymeric insulators. Investigated polymeric insulators have been aged up to six weeks (1000 h). Although the influence of two weeks (333 h) accelerated aging on flashover voltage and morphology of the composite insulators was not tangible, therefore flashover strength of the samples is decreased while the aging period time increases. Statistical analysis has been done to estimate the flashover voltage of accelerated aged composite insulators under uniform and FSNU pollution based on virgin insulators. The research aimed to select insulators in windy and sunny areas. [ABSTRACT FROM AUTHOR]

• 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]

• A dynamic theoretical model is proposed for AC flashover under pollution condition. • It calculates flashover parameters for the fan-shaped non-uniformly degrees. • The experimental tests are performed on the sample insulator to validate model. • With increasing non-uniformly degrees, the critical flashover voltage decreases. • Using experimental results, the propoer insulation level is selected for a sample system. The geometry of fan-shaped pollution on the surface of an insulator accounts for two different values of Salt Deposit Density (SDD), conductivity, and resistance on the leeward and windward sides of the insulator. This paper proposes a novel dynamic theory model of AC flashover based on equivalent resistance in the presence of a fan-shaped pollution layer. The proposed model can calculate flashover parameters for different values of the fan-shaped pollution density degree (T) and extension degree (J) considering instantaneous variables in arc progression. In this model, the arc current is formulated using the arc equivalent circuit for a constant arc voltage in terms of arc length propagation. Also, the Form Factor (F) is defined as a function of the arc length by sampling from the desired insulator and curve fitting. To validate the analytical model, some experiments are performed on an insulator unit of suspension porcelain by applying artificial pollution at three levels: light, medium, and heavy. Test results show that the critical flashover voltage (CFO) is reduced by 19% by varying J from 1 (uniform pollution) to 1/15. Moreover, by varying T from 10% to 30%, the CFO decreases by 9%. As the non-uniformity degree increases, the correction factor varies in the range of 0.798 ~ 0.938. The proposed modeling of flashover provided by the paper could be used to perform a dimensioning method on insulators under fan-shaped pollution conditions. Also, the proposed model can be included in the EMTP-type platforms to determine the minimum insulator flashover characteristic that will provide an adequate flashover performance under pollution conditions. [ABSTRACT FROM AUTHOR]