Expanding the number of photovoltaic (PV) systems integrated into a grid raises many concerns regarding protection, system safety, and power quality. In order to monitor the effects of the current harmonics generated by PV systems, this paper presents long-term current harmonic distortion prediction models. The proposed models use a multilayer perceptron neural network, a type of artificial neural network (ANN), with input parameters that are easy to measure in order to predict current harmonics. The models were trained with one-year worth of measurements of power quality at the point of common coupling of the PV system with the distribution network and the meteorological parameters measured at the test site. A total of six different models were developed, tested, and validated regarding a number of hidden layers and input parameters. The results show that the model with three input parameters and two hidden layers generates the best prediction performance.

The prediction of the time-dependent failure rate has been studied, taking into account the operational history of a component used in applications such as system modeling in a probabilistic safety analysis in order to evaluate the impact of equipment aging and maintenance strategies on the risk measures considered. We have selected a time-dependent model for the failure rate which is based on the Weibull distribution and the principles of proportional age reduction by equipment overhauls. Estimation of the parameters that determine the failure rate is considered, including the definition of the operational history model and likelihood function for the Bayesian analysis of parameters for normally operating repairable components. The operational history is provided as a time axis with defined times of overhauls and failures. An example for demonstration is described with prediction of the future behavior for seven different operational histories.

Summary Considering the recent trend in energy sector transformation towards high share of renewable energy sources, it has become very hard to imagine modern power system without the integration of power electronic devices. A grid‐connected converter will be on the forefront of future energy trading, while simultaneously striving to offer good dynamic behaviour and operation in full accordance with the relevant grid requirements. The control algorithm of the grid‐connected converter has to be capable of achieving the stable steady state operation even during the most severe faults in the system. More importantly, the power quality of the injected currents (and thus the power) needs to be kept at the maximum possible level. This paper presents the control strategy for the grid‐connected converter that offers the possibility of symmetrical grid current injection at the point of common coupling even during unbalanced grid conditions. Proposed control strategy uses delay signal cancellation in the negative sequence synchronous rotating reference frame for the mitigation of the respective current components. The negative influence of asymmetrical grid voltages, present at the point of common coupling as a result of unbalanced grid loads or faults, will be shown within the paper. The key features of the improved control method are outlined, with a special reference to basic theoretical background. The proposed method is experimentally verified using sophisticated research and development station for control of grid‐connected converter.

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Infrared thermography, in the analysis of photovoltaic (PV) power plants, is a mature technical discipline. In the event of a hailstorm that leaves the PV system without the support of the power grid (and a significant portion of the generation potential), thermography is the easiest way to determine the condition of the modules and revive the existing system with the available resources. This paper presents research conducted on a 30 kW part of a 420 kW PV power plant, and demonstrates the procedure for inspecting visually correct modules that have suffered from a major natural disaster. The severity of the disaster is shown by the fact that only 14% of the PV modules at the test site remained intact. Following the recommendations of the standard IEC TS 62446-3, a thermographic analysis was performed. The thermographic analysis was preceded by an analysis of the I-V curve, which was presented in detail using two characteristic modules as examples. I-V curve measurements are necessary to relate the measured values of the radiation and the measured contact temperature of the module to the thermal patterns. The analysis concluded that soiled modules must be cleaned, regardless of the degree of soiling. The test results clearly indicated defective module elements that would result in a safety violation if reused. The research shows that the validity criterion defined on the basis of the analysis of the reference module can be supplemented, but can also be replaced by a statistical analysis of several modules. The comparison between the thermographic analysis and the visual inspection clearly confirmed thermography as a complementary method for testing PV-s.

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Original scientific paper Recently, there has been made a great effort to include electricity generation from the renewable energy sources into the power system. Random renewable generation creates the imbalance between electricity production and consumption, which requires power plants with fast response or energy storage systems. Generally accepted solution for load balancing is the concept of smart grids and one of the elements of smart grid efficiency is the ability of real-time demand-supply balancing. In this paper, the model of the part of power distribution network of the city of Osijek has been created based on results of the power measurements of total electricity consumption in a family house in Osijek, air conditioning system consumption and PV power plant production. Also, algorithm for real-time load management is proposed. It assumes coordinated control of air conditioning system units depending on the production of PV power plants and electricity consumption of distribution network, in order to reduce peak demand in the distribution network.Keywords: demand side load management; distribution system; photovoltaic generation; real-time demand-supply balancing; smart grid Upravljanje potražnjom u distribucijskom sustavu s fotonaponskim elektranamaIzvorni znanstveni članak Posljednjih godina učinjen je znatan napor na uključivanju obnovljivih izvora električne energije u elektroenergetski sustav. Nepredvidiva prozvodnja obnovljivih izvora električne energije stvara neravnotežu između proizvodnje i potrošnje, što onda zahtijeva elektrane s brzim odzivom, ili sustave skladištenja električne energije. Opće prihvaćeno rješenje za uravnoteženje potrošnje je koncept naprednih mreža, a jedan od elemenata učinkovitosti naprednih mreža je sposobnost za uravnoteženje potražnje i opskrbe u stvarnom vremenu. U ovom radu razvijen je model dijela distribucijske mreže grada Osijeka i to na temelju rezultata mjerenja ukupne potrošnje obiteljske kuće u Osijeku, potrošnje klimatizacijskog uređaja te proizvodnje fotonaponske elektrane. Također, predložen je algoritam za upravljanje potražnjom u stvarnom vremenu. Algoritam sadrži usklađeno upravljanje klimatizacijskim uređajima ovisno o proizvodnji fotonaponskih elektrana te o potrošnji u distribucijskoj mreži, a u cilju snižavanja vršne potražnje u distribucijskoj mreži.Ključne riječi: distribucijski sustav; napredne mreže; proizvodnja fotonaponske elektrane; upravljanje potražnjom; uravnoteženje potražnje i opskrbe u stvarnom vremenu