George E. Georghiou

Numerical modelling of negative discharges in air with experimental validation

Axisymmetric finite element models have been developed for the simulation of negative discharges in air without and with the presence of dielectrics. The models are based on the hydrodynamic drift-diffusion approximation. A set of continuity equations accounting for the movement, generation and loss of charge carriers (electrons, positive and negative ions) is coupled with Poissons equation to take into account the effect of space and surface charges on the electric field. The model of a negative corona discharge (without dielectric barriers) in a needle-plane geometry is analysed first. The results obtained show good agreement with experimental observations for various Trichel pulse characteristics. With dielectric barriers introduced into the discharge system, the surface discharge exhibits some similarities and differences to the corona case. The model studies the dynamics of volume charge generation, electric field variations and charge accumulation over the dielectric surface. The predicted surface charge density is consistent with experimental results obtained from the Pockels experiment in terms of distribution form and magnitude.

Three-dimensional numerical modelling of gas discharges at atmospheric pressure incorporating photoionization phenomena

A three-dimensional (3D) numerical model for the characterization of gas discharges in air at atmospheric pressure incorporating photoionization through the solution of the Helmholtz equation is presented. Initially, comparisons with a two-dimensional (2D) axi-symmetric model are performed in order to assess the validity of the model. Subsequently several discharge instabilities (plasma spots and low pressure inhomogeneities) are considered in order to study their effect on streamer branching and off-axis propagation. Depending on the magnitude and position of the plasma spot, deformations and off-axis propagation of the main discharge channel were obtained. No tendency for branching in small (of the order of 0.1 cm) overvolted discharge gaps was observed.

Performance of Photovoltaics Under Actual Operating Conditions

Amongst the various renewable energy sources, photovoltaic (PV) technologies that convert sunlight directly to electricity have been gaining ground and popularity, especially in countries with high solar irradiation. Over the past years PV has shown rapid development and a wide variety of new technologies from different manufacturers have emerged. For each PV module type, manufacturers provide typical rated performance parameter information which includes, amongst others, the maximum power point (MPP) power, efficiency and temperature coefficients, all at standard test conditions (STC) of solar irradiance 1000 W/m2, air mass (AM) of 1.5 and cell temperature of 25 °C. As this combination of environmental conditions rarely occurs outdoors, manufacturer data-sheet information is not sufficient to accurately predict PV operation under different climatic conditions and outdoor PV performance monitoring and evaluations are necessary. The objective of this chapter is to provide an overview of different PV technologies ranging from crystalline silicon (c-Si) to thin-film and concentrators. Subsequently, a summary of the main outdoor evaluation performance parameters used to describe PV operation and performance is outlined. An overview of the effects of different environmental and operational factors such as solar irradiance, temperature, spectrum and degradation is also provided along with the results of previously published research efforts in this field. In the last section of the chapter, the installed PV and data acquisition infrastructure of a testing facility in Cyprus is presented and a thorough analysis of the climatic conditions and the performance of different grid-connected PV technologies that have been installed side-by-side and exposed to warm climatic conditions, typical of the Mediterranean region are given.

Outdoor efficiency of different photovoltaic systems installed in Cyprus and Germany

The energy produced by different photovoltaic (PV) systems depends to a great extent on the inverter and photovoltaic module outdoor efficiency which in turn vary according to the irradiance and temperature of the field conditions. The objective of this paper is to describe the investigations performed to evaluate the outdoor efficiencies of thirteen (13) different types of PV modules which have been exposed to real conditions in Stuttgart, Germany and Nicosia, Cyprus. The inverter and PV array efficiencies have been evaluated by using direct outdoor measurements and data analysis on the measured metereological and operational data collected by the installed sensors present at the two test sites. The measured efficiency of the PV modules has been further evaluated on a seasonal basis reflecting the obvious effects of the seasonal variations of incident irradiation, operating temperature and seasonal spectral shift on the outdoor operating efficiency. The average annual inverter efficiency measured in Stuttgart was 89.8 % while the corresponding inverter efficiency in Nicosia was 90.9 % for the same period and for identical inverters that are rated at a European Efficiency of 91.6 %.

New high quality adaptive mesh generator utilized in modelling plasma streamer propagation at atmospheric pressures

A new adaptive mesh generator has been developed and used in the analysis of high-pressure gas discharges, such as avalanches and streamers, reducing computational times and computer memory needs significantly. The new adaptive mesh generator developed, uses normalized error indicators, varying from 0 to 1, to guarantee optimal mesh resolution for all carriers involved in the analysis. Furthermore, it uses h- and r-refinement techniques such as mesh jiggling, edge swapping and node addition/removal to develop an element quality improvement algorithm that improves the mesh quality significantly and a fast and accurate algorithm for interpolation between meshes. Finally, the mesh generator is applied in the characterization of the transition from a single electron to the avalanche and streamer discharges in high-voltage, high-pressure gas discharges for dc 1?mm gaps, RF 1?cm point?plane gaps and parallel-plate 40?MHz configurations, in ambient atmospheric air.

Degradation of different photovoltaic technologies under field conditions

Over the past years a number of testing facilities have been monitoring the performance and degradation of PV systems according to the established standards of indoor and outdoor testing. The objective of this paper is to present the initial first year and longer-term rate of degradation of different PV technologies installed at the testing facility of the University of Cyprus, based on outdoor field measurements and methodologies. The first year degradation of the technologies was obtained using a data filtering technique of DC generated power at Maximum Power Point (MPP) at irradiation points of higher than 800 W/m2 and normalising the measured power to Standard Test Conditions (STC). Over the first year, mono-crystalline silicon technologies showed degradations in the range 2.12 % – 4.73 % while for multi-crystalline technologies the range was 1.47 % – 2.40 %. The amorphous silicon system demonstrated the highest first year decrease in power with an average degradation of 13.82 %. For validation purposes the first year degradation was also obtained using a second technique by evaluating outdoor measured data-sets under Air Mass (AM) 1.5 (morning and afternoon) conditions and during noon (high irradiance and temperature). In this case the evaluated results showed deviations of up to 6 % and 3 % for mono-crystalline and multi-crystalline technologies respectively whereas for thin-film this was 5 %. Finally, the longer-term degradation rates were evaluated by using the least-square fit method on average monthly data-set blocks of (i) Performance Ratio (PR), (ii) PR evaluated by filtering outage data-sets and restricting to high irradiance conditions and (iii) the Photovoltaic for Utility Systems Applications (PVUSA) rating methods, for the period June 2007 – June 2009.

Rating of annual energy yield more sensitive to reference power than module technology

At present, many different photovoltaic (PV) technologies share the market. Especially investors want to know how much energy each of the PV technologies produces. This paper discusses the measured annual energy yield EAC of twelve PV technologies under different climatic conditions in Germany and Cyprus over three years of operation. In order to compare the annual yield of different PV technologies, the EAC data are normalized to the rated power PN, to the flasher power Pflash, and to the measured field power Pfield. An error analysis is done for both, the energy measurement EAC and the nominal power PSTC. It is found that the typical uncertainty for an energy yield comparison is ±5 %. This means that a difference of 10 % in the annual energy yield between PV technologies can not be traced back to the technologies themselves. The performance analysis of all PV systems shows that the differences in the energy yield are smaller than the error bars on the reference power. Therefore it is not yet possible to decide which PV technology is the best. Moreover, despite obvious trends on the data, we can not unambiguously conclude that PV modules with a better temperature or low light behavior will ensure a higher energy yield in general, since the propagation of state-of-the-art nominal power rating errors outbalances the well recognized effects of low light and temperature dependencies.

Comparison of two partial discharge classification methods

Two signal classification methods have been examined to discover their suitability for the task of partial discharge (PD) identification. An experiment has been designed to artificially mimic signals produced by a range of PD sources that are known to occur within high voltage (HV) items of plant. The bushing tap point of a large Auto-transformer has been highlighted as a possible point on which to attach PD sensing equipment and is utilized in this experiment. Artificial PD signals are injected into the HV electrode of the bushing itself and a high frequency current transformer (HFCT) is used to monitor the current between the tap-point and earth. The experimentally produced data was analyzed using two different signal processing algorithms and their classification performance compared. The signals produced by four different artificial PD sources (surface discharge in air, corona discharge in air, floating discharge in oil and internal discharge in oil) have been processed, then classified using two machine learning techniques, namely the support vector machine (SVM) and probabilistic neural network (PNN). The feature extraction algorithms involve performing wavelet packet analysis on the PD signals recorded over a single power cycle. The dimensionality of the data has been reduced by finding the first four moments of the probability density function (Mean, Standard deviation, Skew and Kurtosis) of the wavelet packet coefficients to produce a suitable feature vector. Initial results indicate that very high identification rates are possible with the SVM able to classify PD signals with a slightly higher accuracy than a PNN.

Partial discharges and associated transients: the induced charge concept versus capacitive modeling

Ensuring a reliable electric service to industrial, commercial and residential customers is becoming an important parameter for every electricity company due to the increased demand from the customers for less downtime, high productivity and reduced maintenance cost of machinery caused by unscheduled frequent interruptions due to underground power cable failures. The presence of partial discharges (PD) is one of the most prominent indicators of defects and ongoing degradation processes of electrical insulation. The capacitive network representation of the void (model) has long been used for the study of the transients generated by a breakdown void. As an alternative to the capacitive representation of the void (model) the induced charge concept has been introduced expressing strong criticism against the capacitance modeling of voids. This paper discusses and argues this criticism and exploits the potential and weakness of the classical capacitive representation of the voids from a theoretical point of view. Finally, a new capacitive model for voids is proposed based on the induced charge concept.

Assessing the power quality behaviour of high photovoltaic (PV) penetration levels inside the distribution network

In recent years, increasing concerns about climate change and the liberalisation of energy market have provided the necessary impetus for a revolutionary restructuring of the electricity network at every level, namely production, transmission and distribution. Increased electricity production from renewable energy sources (RES) coupled with energy efficiency lie at the heart of the ambitious targets set by Europe in the quest to curb greenhouse gas emissions and to reach energy sustainability. Therefore the security and stability of the power system should be considered carefully to identify possible impacts due to uncontrolled deployment of RES. This paper focuses on the study of varying concentrations of photovoltaic (PV) systems on a proposed electricity grid in an attempt to assess the power quality response of the power system. The study has been performed using a detailed PV system model. The model is initially validated using data from the output of PV systems and then this is used for the study of varying PV penetrations on common distribution system topologies. The results are compared to international power quality standards.