A wind turbine two level back-to-back converter power loss study Ivan Mrˇ cela University of Zagreb Faculty of Electrical Engineering and Computing, Croatia, Email: ivan.mrcela@fer.hr Damir Sumina University of Zagreb Faculty of Electrical Engineering and Computing, Croatia, Email: damir.sumina@fer.hr Filip Saˇ ci´ c University of Zagreb Faculty of Electrical Engineering and Computing, Croatia, Email: filip.sacic@fer.hr Tin Bariša University of Zagreb Faculty of Electrical Engineering and Computing, Croatia, Email: tin.barisa@fer.hr Abstract—Wind power conversion systems are a widespread renewable energy source and, as such, have been studied intensely over the past two decades. Wind energy conversion system is connected to the electrical grid through a power converter, an LC filter and a step-up transformer. It is necessary to know the efficiency of the whole energy transfer system and to know the efficiencies of each individual part of that system, i.e. converter, filter and transformer efficiencies, in order to achieve a high-efficiency energy conversion system. This paper presents a simulation model used to determine the grid- and generator- side inverter losses, LC filter and step-up losses, total converter losses and converter efficiency, as well as grid voltage THD and grid and converter current THDs. This paper also decribes the method used to determine the maximum torque per amp (MTPA) algorithm parameters, used to control the generator-side inverter. Simulations were done at nominal operating conditions, as well as for varied DC link and grid voltages, ranging from zero to nominal converter power. Index Terms—Back-to-back converter, internal permanent magnet syncronous generator, converter losses, converter effi- ciency, MTPA I. I NTRODUCTION Share of renewable energy sources in overall energy produc- tion has been continuously growing for the past two decades, [1]. In general, electric power is generated from wind using an electric generator and a power converter. A permanent magnet generator with a full-scale back-to-back converter is one of the more attractive solutions. The power converter’s efficiency affects the overall wind energy conversion systems (WECS) efficiency and depends on the power modules used, switching frequency, operating point, selected modulation and working temperature. The DC link is connected to the electric grid via an inverter, a grid-side output filter and a step-up transformer. The inverter is used for DC/AC voltage conversion with a predefined fundamental harmonic frequency and amplitude, via pulse-width modulation (PWM). Fig. 1 shows a basic electrical schematic of the analyzed back-to-back converter, together with its electrical source (wind turbine and PMG) and load (LC filter, step-up transformer and the electrical grid). The proposed paper’s objective is to investigate switching loss for a full scale two level back-to-back converter used in WECS, to determine the grid-side inverter loss, the generator- side inverter loss, the LC filter and the DC link loss, as well as the output voltage and current THD, all depending on generator true power variations, from generator idle to nominal generator active power, in order to determine the maximum energy conversion efficiency point. The goal is also to analyze the grid voltage and DC link voltage fluctuations influence on the converter power loss and on the grid-side voltages and currents. This paper addresses the aforementioned problems and focuses on analyzing and simulating the effects grid and DC link voltage fluctuations have on power converter losses. The presented analysis and simulation results represent this paper’s contribution. A laboratory model-based simulation model of a permanent magnet generator (PMG) and a 520 kW back-to-back converter was developed in PLECS in order to simulate the back-to- back converter power loss. Control structures of grid-side and generator-side inverters were developed and the controller’s parameters were adjusted using magnitude and symmetric optimums. An instantaneous switching and conduction losses simu- lation block was developed as part of a larger simulation model. Switching loss is approximated from semiconductor datasheets using a third-order polynomial of the instantaneous switching energy as a switching current variable. The power loss block contains additional switching detection and current direction blocks for each semiconductor switch in order to determine the instantaneous switching energy, used to de- termine the converter switching loss. Two modulations were selected: space vector PWM (SVPWM) with maximum torque per amp (MTPA) algorithm on the generator-side, and third harmonic injection PWM (THIPWM) on the grid-side. There are several ways to expand the sine modulation linear range, and THIPWM was chosen for it’s simple implementation and good overall results. Since the output filter and step-up transformer are an integral part of most converters connected to a wind turbine generator, their losses were simulated in an effort to determine overall efficiency of the system. 978-1-5090-1797-3/16/$31.00 ©2016 IEEE 308