Photovoltaics (PVs) are promising sustainable energy generators, yet the higher initial cost of PV systems compared to the conventional fossil fuel based energy systems remains a barrier to their large-scale adaptability Concentrating solar radiation onto a smaller area by replacing expensive cell materials with cheaper optical materials can be an alternative way to reduce PV cost, but concentrated photovoltaics (CPV) yield substantially higher cell temperatures reportedly detrimental for CPV life and electrical yield Various thermal management approaches have been adopted to mitigate the adverse effect of temperature on CPV life and performance The potential use of thermal energy discarded in CPV thermal management systems has been traditionally overlooked The aim of this article is to briefly review the progress in PV cells, different CPV systems, and thermal issues specific to concentration techniques and propose potential uses of thermal energy recovered from the CPV The review finds that temperature mitigation and thermal energy recovery & utilization can be a promising pathway to improve CPV performance

Concentrated photovoltaic: A review of thermal aspects, challenges and opportunities

This paper deals with the use of corona ring at the HV end fitting for improving the electric field and potential distributions and then for minimizing the corona discharges on 230 kV AC transmission line composite insulator. A single corona ring is installed at the energized end side. Three-dimensional finite element method (FEM) software is employed to compute the electric field. As the performance of high voltage insulator strings closely depends on designs and locations of corona ring, the effects of the corona ring radius, the ring tube radius and the ring vertical position are examined. The minimization of the electric field necessitates the optimization of corona ring. For this purpose, new nonlinear mathematical objective function linking the electric field strength to the corona ring structure parameters is established. The optimization problem is achieved by minimizing the objective function using a modified particles swarm optimization (PSO) algorithm with a dynamic population size. The algorithm adjusts the size of population for each iteration. Based on the average value and the best solution of the objective function, we propose a new mathematical model to update the population size. This algorithm enables the population size reduction leading to computing time decrease. According to the results, FEM-PSO hybridization technique could be very helpful in optimization of corona ring design.

Optimal design of corona ring on HV composite insulator using PSO approach with dynamic population size

Nonlinear composite dielectrics can function as smart materials for stress control and field grading in all fields of electrical insulations. The percolation process is a significant issue of composite dielectrics. However, the classic percolation theory mainly deals with traditional composites in which the electrical parameters of both insulation matrix and conducting fillers are independent of the applied electric field. This paper measured the nonlinear V-I characteristics of ZnO microvaristors/silicone rubber composites with several filler concentrations around an estimated percolation threshold. For the comparison with the experiment, a new microstructural model is proposed to simulate the nonlinear conducting behavior of the composite dielectrics modified by metal oxide fillers, which is based on the Voronoi network and considers the breakdown feature of the insulation matrix for near percolated composites. Through both experiment and simulation, the interior conducting mechanism and percolation process of the nonlinear composites were presented and a specific percolation threshold was determined as 33%. This work has provided a solution to better understand the characteristics of nonlinear composite dielectrics.

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Understanding the Percolation Characteristics of Nonlinear Composite Dielectrics

Localized overstress due to electric field concentration is a threat to the long-term performance of the insulation in almost all high voltage apparatus. Resistive field grading using field grading materials (FGMs) may be a solution to this problem. Insulation matrixes such as ethylene propylene diene monomer, epoxy, or silicone rubbers, blended with fillers such as ZnO microvaristors, are reported to possess field dependent conductivity with greater and more stable nonlinearity than traditional FGMs [1]. Their application in high voltage apparatus such as bushings [1], cable accessories [2]–[4], insulators [5], [6], and stator coils [7] has been studied recently. It is well known [8] that the most important parameters for FGMs are the nonlinear coefficient α and switching field Eb; the challenge is to tailor these two parameters to a specific application. FGMs can function effectively in most cases when α >10 [9], but the adjustment of Eb is more complex.

Grading electric field in high voltage insulation using composite materials

Silicone composite filled with zinc oxide microvaristors possesses excellent nonlinear conducting behavior as ZnO varistor does. For better adjusting the composite's electrical behavior to satisfy the practical field-grading requirement, this article studied the influence of ZnO filler's property on the nonlinearity of the composite. Several groups of ZnO-silicone composite samples in different filler volume fraction and filler diameter were prepared, the measured J-E characteristics show that the percolation threshold of ZnO-silicone composite is around 35%, above which the composites present reliable nonlinear behavior. The switching voltage of the composite exhibits a considerable decrease as filler's diameter increases or filler's volume fraction increases, while the nonlinear coefficient remains stable. Moreover, filler's size also has a little influence on composite's percolation limit. The conclusion above fits very well with the theory of the conducting composites and percolation process. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42645.

Tailoring the nonlinear conducting behavior of silicone composites by ZnO microvaristor fillers

An approach to high field control, particularly in the areas near the high voltage (HV) and ground terminals of an outdoor insulator, is proposed using a nonlinear grading material; Zinc Oxide (ZnO) microvaristors compounded with other polymeric materials to obtain the required properties and allow easy application. The electrical properties of the microvaristor compounds are characterised by a nonlinear field-dependent conductivity. This paper describes the principles of the proposed field-control solution and demonstrates the effectiveness of the proposed approach in controlling the electric field along insulator profiles. A case study is carried out for a typical 11 kV polymeric insulator design to highlight the merits of the grading approach. Analysis of electric potential and field distributions on the insulator surface is described under dry clean and uniformly contaminated surface conditions for both standard and microvaristor-graded insulators. The grading and optimisation principles to allow better performance are investigated to improve the performance of the insulator both under steady state operation and under surge conditions. Furthermore, the dissipated power and associated heat are derived to examine surface heating and losses in the grading regions and for the complete insulator. Preliminary tests on inhouse prototype insulators have confirmed better flashover performance of the proposed graded insulator with a 21 % increase in flashover voltage.

Stress control on polymeric outdoor insulators using Zinc oxide microvaristor composites

The current paper presents an alternative and innovative technique to predict the severity of pollution of high voltage insulator using a higher harmonics component with up to the 7th component of leakage current. The leakage current was measured using a current transformer and a shunt resistor. Next, laboratory tests were conducted on glass and porcelain insulators with artificial pollution under salt-fog pollution state which is further represented by three levels, namely light, medium, and high contamination. In this case, the formulation of a new severity of harmonic index refers to a ratio of the sum of 5th and 7th to the 3rd harmonic component. More importantly, the new index managed to provide more accurate results when used as a diagnostic tool for the levels of pollution, compared to the ratio of the total harmonic distortion (THD) to the number of odd harmonics components (n) as the boundaries. In this case, the insulators were found to be in a clean and normal condition when the $K_{(5+7)/3}$ value was greater than 3%. Contrastingly, the insulators were in an extreme condition when the $K_{(5+7)/3}$ was lower than 3%. Nevertheless, there is a high probability of a flashover in glass and porcelain insulators if the $K\,\,_{(5+7)/3}$ value is less than 2%. The present study shows the possibility of utilizing the value of strange harmonics up to the 7th component of leakage current as the parameter for the monitoring of leakage current in overhead insulators in the presence of contamination. Overall, it can be concluded that the 3rd, 5th, and 7th harmonics details extracted from the leakage current act as a good indicator for the level of contamination.

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The Leakage Current Components as a Diagnostic Tool to Estimate Contamination Level on High Voltage Insulators

In this article, an innovative approach to assess the risk of uniform/non-uniform pollution and wet high voltage glass insulator is proposed. The assessment is based on a new index, named as $R_{hi}$ , which is derived from the $3^{\mathrm {rd}}$ , $5^{\mathrm {th}}$ and $7^{\mathrm {th}}$ harmonics of the leakage current. Two 33 kV strings of cap and pin glass insulators having three units with different profiles are employed to evaluate the risk level, based on measured critical voltage gradient ( Ec ). Critical voltage stress tests are executed on the contaminated insulators under different ratios of lower to upper surface ( $J$ ) and wetting rates. Furthermore, the effects of ratio of soluble deposit density ( ESDD ) to non-soluble deposit density ( NSDD ) on the flashover occurrence are analyzed. It was found that the insulator profile has significant influences on the likelihood of flashover which is estimated as 38% and 24% for type A and B insulators, respectively. From the results, it can be concluded that the proposed $R_{hi}$ index is able to predict the risk level of the insulator and the probability of flashover occurrence more accurately than the 5th/3rd index, which is widely published in literature.

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Risk Assessment of Polluted Glass Insulator Using Leakage Current Index Under Different Operating Conditions

This work aimed to study the influence of contamination profiles and humidity on flashover electrical characteristics of polluted insulators. Firstly, the flashover tests on cap and pin glass insulators under four pollution levels represented by salinity were conducted. Eight artificial contamination profiles based on the solid layer method have been modeled for the selected insulators. The numerical analysis has been used to determine the insulator electrical characteristics such as potential, electric field, and power dissipation under proposed contamination profiles using finite element methods (FEM). Next, the power dissipation has been simulated with consideration of thermal stress propagation in locations with high power. Finally, flashover voltage gradient tests have been conducted under various humidity and contamination profiles. The values of the flashover voltage gradient due to pollution were determined as the percentage of the value of the flashover voltage gradient in the clean condition which was identified as the reference point. The numerical model indicated that the initiation of arc generally occurs at area in which the electric field and power dissipation is maximum. It was also observed from experimental results that the flashover voltage gradient under different contamination profiles has different values depends on the location and dimension of the pollution region.

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R. Abd-Rahman

Papers

Stress control on polymeric outdoor insulators using Zinc oxide microvaristor composites

The Leakage Current Components as a Diagnostic Tool to Estimate Contamination Level on High Voltage Insulators

Risk Assessment of Polluted Glass Insulator Using Leakage Current Index Under Different Operating Conditions

Pollution Flashover Under Different Contamination Profiles on High Voltage Insulator: Numerical and Experiment Investigation

A Review of the Dynamic Modelling of Pollution Flashover on High Voltage Outdoor Insulators