Zvonimir Klaić

Measurements and computation of electromagnetic field in transformer station 400/110 kV ernestinovo

This paper presents results of measurements and computations of the electric and magnetic fields of complex busbars, transformers and transmission line structures at transformer station 400/110 kV. The fundamental sources of low frequency electromagnetic fields in the transformer station are the primary elements of the 400 and 110 kV open substations and associated overhead lines. Due to the complexity of the geometry of the substation elements, it was necessary to apply a three-dimensional approach to the computation of the electromagnetic field. Measurements were performed by EMS 100 field meter and computations of electromagnetic fields were conducted for quasi-static states of electrical values at a power frequency of 50 Hz. The calculated electric and magnetic fields were compared with measured values and permissible levels according to Croatian regulations. Measurements were performed for real working conditions for twelve different profiles in the transformer station.

PROBABILITY DENSITY FUNCTIONS OF VOLTAGE SAGS MEASURED INDICES

Probability Density Functions of Voltage Sags Measured Indices Voltage sags can cause interruptions of industrial processes, which could result as a malfunction of equipment and considerable economic losses. Thus, it is very useful to see certain rules of voltage sags occurrence due to duration and depth. This paper presents statistical analyses of voltage sags in several domestic and industrial transformer stations. Voltage sag probability functions are calculated from actual measurement data, by means of a hill climbing algorithm. Lognormal and Weibull frequency distribution functions are used to describe distribution of measured voltage dips.

Propagation of the voltage sags through different winding connections of the transformers

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Upravljanje potražnjom u distribucijskom sustavu s fotonaponskim elektranama

In the last years, there was a great effort on inclusion electricity generation from the renewable energy sources in power system. Random renewable generation creates the imbalance between electricity production and consumption, which requires power plants with fast response and also storage systems. Generally accepted solution for load balancing is 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.

Review of Non-Traditional Optimization Methods for Allocation of Distributed Generation and Energy Storage in Distribution System

The integration of distributed energy sources transforms passive distributed grid, in which the energy flows only in one direction (from the source to the consumer), in an active one, in which energy flows in both directions. To maximize positive impacts, which distributed generation (DG) can provide to the distribution network, it is necessary to determine the optimal allocation of distributed generation. The optimal allocation can be determined by using the optimization method. There are two main categories: exact methods (traditional) and heuristic (non-traditional) methods. Exact methods search for global optimum while heuristic methods achieve satisfactory solutions with greater computation speed. This paper gives a brief review of non-traditional methods used for determining optimal location and optimal power of DG with the aim to reduce real power losses and to improve voltage characteristics. Also, there is a review of the application of those methods in determining the optimal power, optimal location and optimal cycle of charging/discharging of electrical energy storage systems.

Influence of load management in distribution network on voltages and active power losses: Case study

Production of photovoltaic (PV) power plants depends on weather conditions which can cause sudden and often unpredictable variation in their output power. Balancing the production from renewable sources with consumption requires utilization of power plants with fast response or energy storage systems what causes considerable costs. In recent years, in terms of balancing, the emphasis is on the higher role of the consumers. In this paper computer model of the part of distribution network is used to simulate and analyze demand side load management (using air conditioner installed in households) in the distribution feeder considering PV power plant expansion scenarios. Three case studies are analyzed: first with low penetration of PV plants in distribution system, second with moderate penetration of PV power plants and third with high penetration of PV power plants. Influence of demand side load management on network voltages and active power losses is investigated.

Microgrid energy management system in a public building

Penetration of renewable energy sources, as distributed sources, and market liberalization have introduced a new concept in the power system. This globally new concept is to some extent based on the stochastic production of renewable energy sources which creates imbalance between production and consumption. One possible solution for that imbalance is a generally accepted concept of microgrids. Microgrids as an integral part of the power system, combine distributed energy sources, energy storage and load. However, in order to develop and deploy microgrids it is necessary to ensure active management of distributed sources and distribution networks. The aim of this paper is to present implementation of a microgrid in a public building by describing the concept of microgrids and offering a variety of models. Apart from indicating the possibilities and advantages of microgrids, the models identify main technical drawbacks and limitations.

IMPROVING ENERGY EFFICIENCY IN BUILDINGS USING MICROGRIDS

Modern society has a growing need for the electricity. To protect the environment, future energy demand must be met with more environmentally friendly technologies, such as renewable energy sources. Because of its vast availability, solar radiation has been used for decades to generate electricity through photovoltaic systems (PV) for residential, educational, and commercial buildings. However, the growth of distributed generation (and renewable energy sources) across power systems in industrialized countries has created new challenges. Random renewable generation causes an imbalance between electricity production and consumption, so smart grids and microgrids may be solutions. In this article, we investigate improving the energy efficiency in the Faculty of Electrical Engineering building in Osijek by using a microgrid. To do so, we compared the total electricity consumption of the building and the production of a 10 kWp photovoltaic power plant on that building. The improvement in energy efficiency of the building produced a maximum savings of up to 10% of the building’s total electricity consumption.

Online power quality measurements and voltage sags analysis

The Faculty of Electrical Engineering in Osijek and HEP d.d. - Croatian Grid Company are partners in the EU project Leonardo Power Quality Initiative (LPQI). One part of activities carried out within that project includes measuring, analyzing and statistical data processing of power quality indices. This paper presents the result of online power quality indices monitoring at HEP Distribution System Operator in TS 110/35 kV Osijek 2 at the secondary side on one of two transformers which supply the urban cable network in the city of Osijek. Power quality indices were measured with a three phase power quality network analyzer QWave Power CO. Voltage events were classified according to the EN 50160 and IEEE 1159-1995, and RMS variation performance indices were calculated.

Power quality indices of distribution feeder in eastern Croatia under electricity market conditions

The reform of the Croatian energy sector began in 2000 in terms of harmonization with the European energy sector. Adoption of the energy acts has created necessary conditions for gradual opening of the national electricity market.