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Spudic V.,University of Zagreb | Baotic M.,University of Zagreb | Jelavic M.,Koncar Electrical Engineering Institute
Automatika | Year: 2011

The new grid regulations require that a grid-connected wind farm acts as a single controllable power producer. To meet this requirement a traditional wind farm control structure, which allowed individual wind turbines to internally define their power production, has to be modified. This paper investigates the opportunity for wind turbine load reduction that arises from dynamic power control of wind turbines. The wind farm controller design is proposed that utilizes coordinated power control of all wind turbines to achieve the wind farm regulation requirements and to minimize the wind turbine loads. Source


Filipovic-Grcic B.,University of Zagreb | Filipovic-Grcic D.,Koncar Electrical Engineering Institute | Uglesic I.,University of Zagreb
International Review of Electrical Engineering | Year: 2011

The state of the oil-paper insulation is a key factor for assuring a reliable operation of power transformers. Recovery voltage measurement is one of the methods utilized to estimate the polarization spectrum. Analysis of the polarization spectrum is used to evaluate the condition of the insulation with respect to moisture content and aging. In this paper, the elements which represent a polarization process are determined from recovery voltage measurement test results by using a new method based on genetic algorithm. The method was successfully tested on a simple circuit and then applied to two power transformers located in hydropower plants. © 2011 Praise Worthy Prize S.r.l. - All rights reserved. Source


Filipovic-Grcic D.,Koncar Electrical Engineering Institute | Filipovic-Grcic B.,University of Zagreb
Electric Power Systems Research | Year: 2015

Testing of electrical equipment with AC currents is a common test in both high power and high voltage laboratories. It implies driving currents, sometimes several tens of kilo-amperes high, through a test object loop conductor. Test transformers are normally used as current sources, but the limiting factor is usually their relatively low rated voltage. Voltage on the test transformer is determined by product of loop impedance and test current. Proposed compensation method deals with reduction of loop impedance which consequently reduces voltage on test transformer and improves test capabilities. Experimental verification of the method was successfully carried out on two test objects: 110 kV cable and 600 kV transformer bushing. © 2014 Elsevier B.V. Source


Petrovic V.,University of Zagreb | Jelavic M.,Koncar Electrical Engineering Institute | Baotic M.,University of Zagreb
Renewable Energy | Year: 2015

To enable further growth of wind turbine dimensions and rated power, it is essential to decrease structural loads that wind turbines experience. Therefore a great portion of research is focused on control algorithms for reduction of wind turbine structural loads, but typically wind turbine rotor is considered to be perfectly symmetrical, and therefore such control algorithms cannot reduce structural loads caused by rotor asymmetries. Furthermore, typical approach in the literature is to use blade load measurements, especially when higher harmonics of structural loads are being reduced. In this paper, improvements to standard approach for reduction of structural loads are proposed. First, control algorithm capable of reducing structural loads caused by rotor asymmetries is developed, and then appropriate load transformations are introduced that enable presented control algorithms to use load measurements from various wind turbine components. Simulation results show that proposed control algorithm is capable of reducing structural loads caused by rotor asymmetries. © 2014 Elsevier Ltd. Source


Mikulecky A.,Koncar Electrical Engineering Institute | Stih Z.,University of Zagreb
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2013

Insulators or bushings are one of the most common cause of transformer failures especially those with fire and collateral damage. This can be prevented by application of off-line and on-line monitoring, especially by measuring of capacitance C and dielectric dissipation factor tanδ, but current physical understanding of these parameters should be improved. A complex research of temperature dependence of C and tanδ, which is based on measurements on a specially designed oil impregnated paper bushing models with different moisture contents in the insulation and during artificial ageing, is presented in the paper. Useful relationships, which enable calculation of tanδ as a function of observed influencing parameters, are also presented. © 1994-2012 IEEE. Source

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