The impact of events in the Slovene high – voltage network on the power quality in the distribution networks

This work analyses the impact of different events in Slovene high-voltage network on power quality in the 20 kV and 0.4 kV distribution networks. In order to perform the analysis a simplified dynamic model of the Slovene power system was build in the program package Matlab/Simulink, using toolbox PowerSys. The obtained model is appropriate for simulation of electromagnetic transients in duration up to a few seconds. It contains simplified dynamic models of all main generators and power transformers installed in the Slovene power system, detailed models of 400 kV, 220 kV and 110 kV networks, simplified models of the neighboring power systems and models of important consumers. The 20 kV and 0.4 kV distribution networks are modeled in the details only in the area of interests. Everywhere else, they are represented as constant loads connected to the 110 kV network. The aforementioned dynamic model was applied to analyze the impact of different events in the Slovene power system on power quality in the 0.4 kV distribution network during transients and in steady states. The power quality in distribution network was analyzed on 0.4 kV busbars in five substations (Cerkno, Škofja Loka, Vevče, Šentjur, Rače) for the following set of events: a switch-off of a 300 MVA (400 kV/110 kV) power transformer in the substation Okroglo; a three-phase short circuit on 110 kV busbars in the substation Kleče; a three-phase short circuit on 20 kV busbars in the substation Rogaška Slatina and a switch-on in a pumping regime of pump-turbine plants Avče and Kozjak which are currently at the design stage. The results obtained show that aforementioned events in some points of Slovene distribution network can cause power quality distortion over that allowed by the power quality standards

Optimalno sledenje sončnih modulov Soncu ob upoštevanju izgub pogonskega sklopa

V delu je obravnavan dvo-osni sledilni sistem za sledenje sončnih modulov trajektoriji Sonca. Pri tem je predstavljena nova metoda za optimalno sledenje trajektoriji Sonca, ki upošteva tudi izgube pogonskega sklopa dvo-osnega sledilnega sistema. Cilj optimalnega sledenja je maksimalna pretvorba energije sončnega sevanja v električno energijo upoštevajoč na električne izgube pogonskega sklopa. Določitev trajektorije sledilnega sistema predstavlja v optimizaciji nelinearni in omejen problem, kjer ciljna funkcija ni na voljo v eksplicitni obliki. Za optimizacijski postopek se je uporabila stohastična metoda imenovana diferenčna evolucija. Pri tem je ciljna funkcija podana z napovedjo razpoložljivega sončnega sevanja, izgubami pogonskega sklopa in izkoristkom sončne elektrarne. Omejitve problema predstavljajo konstantna hitrost premikanja in minimalna sprememba kota premika pogonskega sklopa sledilnega sistema. Podani rezultati kažejo, da je optimalna trajektorija sledilnega sistema, odvisna predvsem od razpoložljivega sončnega sevanja, izkoristka sončne elektrarne, izgub pogonskega sklopa in omejitev, ki jih upoštevamo v sami optimizaciji.This work deals with the efficiency increase of the photovoltaic (PV) systems. The efficiency of PV systems depends mainly on the solar radiation reaching, the solar module, the efficiency and the types of solar panels, its temperature, DC/DC converter and DC/AC inverter. The efficiency of the solar radiation that reaches the module is the highest when the sunbeams fall aligned with the normal of the module. The influence of the technology and its usefulness is presented in [1], while the impact of the solar cell temperature on the energy conversion efficiency is discussed in [2]. The efficiency of the PV systems depends also on the impedance matching. The articles [3] - [5] deal with the determination of the electrical parameters to achieve maximum energy from the PV system through the Maximum Power Point Tracking (MPPT). The efficiency of the PV system can also be increased by the tracking systems. Primarily the one and two-axis tracking systems are used. The authors in [7] - [11] increased the efficiency of the solar energy conversion from 20% to 50% by using the tracking systems compared to the fixed PV systems. The tracking systems use closed- and open-loop control systems. The closed-loop control systems [12] use photo sensors to position the solar panels while the open-loop control systems [13] based on the mathematical algorithm provide predefined trajectories for the tracking systems. Both control systems have imperfections. When the weather is changing, such systems can spend more energy than they gain. This paper presents a new method for determining the optimal trajectories of the open-loop control system of the two-axis sun tracking system (Fig. 1) that assures the maximum efficiency of the solar energy conversion. For the two-axis sun tracking system the energy losses are determined as a function of angle (Figure 2). The aim is to determine the trajectory (tilt beta and aw) of the solar modules where in a specified period the energy production is maximum. To achieve the result a method for predicting the direct and diffuse solar radiation on the Earth\u27s surface in the form of the time dependent function is needed. The mentioned method was already presented in [17]. Using the predictions of direct and diffuse solar radiation on the Earth\u27s surface and (1) the solar radiation on the tilt surface is calculated.The angles described in (1) are also shown in Fig. 3. To solve the nonlinear and bounded optimization problem the method called Differential Evolution (DE) [18] is used. In the optimization process the target function is given by (5). Its value is evaluated by calculated prediction of the available solar radiation, the electric drive losses and the efficiency of the PV system. The problem bounds are given in the form of angular speed, lower and upper bounds for the both angles and angle quantization. The results are given for the two-axis tracking system of an active surface 7 m2 and the total efficiency of the PV system Ž = 0.085. The results presented in Figs 4 and 5 show that the trajectory determined by the proposed method gives the best results in terms of the increased energy production of the PV system. In this case the increased energy production compared to the step by step tracking system is not substantial. The ratio may change for the different types and numbers of the solar modules or the different energy consumption of the electric drive

Influence of small solar power plants on power quality

The solar power plants are usually connected to the distribution network in node with transformer. Small, mini and micro solar power plants are connected as real dispersed production in the middle or at the end of line as consumers.This paper describes measurements, according to standard SIST EN 50160, at 274 m long line, fed at one side from 20/0,4 kV transformer and at the other side from small solar power plant and in between are 15 users (houses)

Using a mathematical model to calculate the coefficient of high-harmonics increase in the traction current supplying a great number of electric- traction vehicles on electrified railways in Serbia

V članku so podani rezultati eksperimentov rasti vrednosti višjih harmonikov vlečnega toka na spojnih mestih kontaktne mreže na električno napajalno postajo v odnosu na vrednost istih han-nonikov toka, ki so prisotni na mestu napajanja elektrovlečnih vozil iz kontaktne mreže. Rezultati so dobljeni ob pogojih eksploatacije pri napajanju elektrovlečnih vozil na elektrificiranih progah Železnice Srbije