R. Candela

Reconfiguration Techniques of Partial Shaded PV Systems for the Maximization of Electrical Energy Production

In this paper, the research of the optimal layout of photovoltaic (PV) modules in a PV array giving the maximum output power under different shaded working conditions is carried out. The particular condition of non uniform solar exposition of the modules is analyzed. The study of the different configurations has been carried out starting from a circuital model used for the design of PV cells and for the simulation of the working behavior of PV arrays. The attained results appear to be interesting although the complexity of the problem in mathematical terms is huge when the number of panels is high. Moreover, the results confirm that this approach often allows to attain a higher electrical energy production compared to that attainable with PV arrays with a static layout.

On PD mechanisms at high temperature in voids included in an epoxy resin

In this paper the effects of temperature on partial discharge (PD) activity taking place inside a spherical void in epoxy resin system are studied. Indeed, some experimental tests previously performed on specimens, having different void shapes, under multi-stress condition of temperature and voltage, have shown very different PD amplitude distributions at temperatures higher than ambient. However, this phenomenon cannot be explained only by taking into account the different thermobaric conditions of the enclosed gas. In consequence of the general physical inaccessibility of such voids, a study is here performed using a numerical model based on an evolutionary optimization algorithm. This is used to evaluate the range values for the physical parameters of the insulating system influencing the observed changes in PD activity. Finally, comments are presented about the adopted criteria by which the comparison between the experimental data and the simulated ones is performed, and about the interpretation of the dependence on temperature of the experimental PD.

A novel HV system for multi specimens aging tests under partial discharges and temperature

In this paper a novel high voltage (HV) system for testing five specimens in parallel under different combinations of voltage and temperature is presented. When partial discharges (PD) are present, the system allows under a computer supervision the cyclic acquisition of PD signals coming out from each specimen by mean of a new partial discharge phase resolved analyser (PDPRA) built in our laboratory. Each specimen is provided by a suitable circuit breaker, ad hoc designed, that disconnects the high voltage (HV) side of the broken-down specimen, without the central HV supply is interested. The specimen temperature regulation (up to 100/spl deg/C) is performed by a pt100 thermo regulator in a silicon oil bath. Finally, some tests have been performed to check the absence of any mutual PD interference between the specimens.

Simplified hybrid PD model in voids

In this paper a novel approach to model partial discharges (PD) activity taking place inside a spherical void in epoxy resin system has been traced. The approach is based on a time varying conductance of the inner void, subjected to multistress conditions: voltage, temperature and pressure. A simple lumped circuit macro-model simulates the global effects of PD activity: the different parameters influencing the discharge phenomenon in the void are taking into account by using a physical approach resulting in a time varying conductance inside the circuit. The evaluated PD activity has been compared with experimental and simulated one for the accessible and inaccessible part of the system. A discussion on the observed changes in PD activity has been reported.

Optimisation of a numerical model for analysis of partial discharge phenomena in a flat cavity

The failure probability of the insulation in HV components can be studied through partial discharge (PD) measurements to detect defects. A numerical model, previously developed by the authors for the simulation of PD activity in a spherical void embedded in epoxy resin, is implemented for a flat cavity under a divergent electric field. This simulates very well the real working conditions encountered in the insulation of electrical machines. The model is developed taking into account all the geometric and electrical parameters, and a fine examination on physical parameters playing a fundamental role on the PD phenomenon is carried out. In particular, a more accurate formulation of the work function and a finite elements analysis (FEA) of the electric field inside the cavity has been applied. A new approach is investigated to reproduce the uncertainty of the discharge phenomenon. A probability function distribution, obtained by Weibull analysis, gains the best results. In order to obtain the best fitting of the model output to the experimental PD data, a strong procedure based on an heuristic algorithm is performed. This algorithm detects the minimum of a quality function that evaluates the distance between the PD patterns of the numerical model and of the experimental data. After a description of the numerical model adopted, a comparison between the experimental and the simulated data are presented together with comments and remarks.

Electromagnetic radiated field measurements for partial discharge diagnostic

Recently, improvements have been made in the diagnostic of electrical insulation systems by means of electric Partial Discharge (PD) standard measurements through a stochastic approach in the treatment of the acquired data: in this way, the identification of the defects as well as the separation of multiple defects simultaneously active, have been successfully performed. However, the standard PD measurement process can be affected by the well-known parasitic effects due to conducted noise whose spectrum can have characteristics similar to those of the measured signal. Moreover, the classic PD measurement system have to be suitable assembled on the Device Under Test (DUT) so determining a complex erection on site operations, which can force the temporary outage of the same DUT and the related equipment. In the aim of overcoming the previous limitations of the classic PD measurement system, in this paper a novel approach for the characterization of the defects in a dielectric material is proposed. It is based on the remote measurement of the ElectroMagnetic (EM) transient field radiated from PD. In order to validate the approach, the standard CIGRE I test cell is used. Different test measurements with different spatial sensors location enable to obtain the PD diagnostic. A comparison with standard electric current-voltage PD measurements has been carried out with a good agreement in terms of diagnostic results.

Analysis of partial discharge activity at different temperatures through an heuristic algorithm

Partial discharge, PD, activity is strongly influenced by the temperature. The analysis of PD phenomenon in a void embedded in an insulating material is a difficult problem because such voids are physically inaccessible. On the basis of a numerical model for the simulation of PD activity, it is possible to give a good interpretation of the physical parameters values. The developed model is stochastic and it depends on these parameters, which need to be optimised as the temperature changes. On this purpose, an Evolutionary Algorithm has been used for the parameters optimisation. This algorithm is a Differential Evolution strategy for non-linear optimisation with mixed continuous and discrete variables. For this application, it has proved to be more efficient as compared to other enhanced evolutionary algorithms. In the search algorithm, the objective function has been calculated by means of the Weibull distribution, which is used to extract valuable information from a set of stochastic data. In this way, it is possible to compare the cumulative probability distribution of the experimental data and of the calculated data.

Space Charges and Partial Discharges Simultaneous Measurements under DC Stress

In the field of HVDC, the main causes of insulation aging are due to the space charge and PD phenomena. In particular, the purpose of the present work is to verify a possibility to measure simultaneously the space charge and the PDs under DC stress in order to evaluate the correlation between both phenomena. The space charge was measured by using a modified PEA cell and PDs were measured by using a novel wireless sensor system. The space charge profile and the PD pulses carried out simultaneously have been reported and discussed.

An improved MSD-based method for PD pattern recognition

In this paper a new method for multi source partial discharge (PD) pattern recognition by discrete wavelet transform, is presented. The proposed method is based on the application of multi-resolution signal decomposition (MSD) technique applied to a 3D PD pattern image. The multi-resolution technique has shown that some detail images (horizontal, vertical and diagonal) at different decomposition levels have frequencial characteristics typical of a single discharge phenomenon. By applying this method to a double-source PD pattern it is observed that one source prevails on the other. Taking advantage from the linearity of the biorthogonal wavelet functions by using a removal and reconstruction process, it is possible to eliminate the first characterized discharge phenomenon, so obtaining the pattern of the second source. Tests have been performed on specimens having single and multiple defects. The obtained results prove that the proposed improved recognition method is quite efficient and accurate.

A novel flexible approach for prediction and on line diagnostic of partial discharge [electrical insulation applications]

Partial discharges (PD), due to unavoidable local defects produced by the industrial manufacturing process, can determine local erosion of the material that may cause the electrical breakdown of the component in which the dielectric material represents the fundamental part. Recently, improved diagnostics of electrical insulation systems by means of electric PD measurements through a stochastic processing of the acquired data have been carried out. In this way, the identification of the defects as well, as the separation of multiple defects simultaneously active, have been performed. In this paper, a novel flexible approach for the on-line characterization of the defects in a dielectric material is proposed. The approach is based on the measurement of the electromagnetic transient field radiated from the PD.