Proceedings of the 2011 International Conference on Power Engineering, Energy and Electrical Drives
Torremolinos (Málaga), Spain. May 2011
An Implementation of Grid Interactive Inverter with Reactive Power Support Capability for Renewable Energy Sources Ibrahim SEFA, Necmi ALTIN, Saban OZDEMIR Gazi University, Faculty of Technical Education, Department of Electrical Education, Ankara TURKEY
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Abstract: In this study, a DSP based three phase current controlled grid interactive inverter with reactive power injection capability is proposed for renewable energy sources. The proposed voltage source inverter consists of a line frequency transformer, LCL filter at the output and PI current regulator. The proposed inverter system has been designed, simulated and implemented. Both experimental and simulation studies show that the power factor of the inverter can be controlled between 0.95 inductive and 0.95 capacitive. Also, inverter output current is in phase with line voltage in unity power factor operation. The inverter output current total harmonic distortion level measured as 3% and this value is in the limits of international standards for all declared operation range.
I.
INTRODUCTION
Various energy sources are investigated in parallel with the increasing world power demand. Solar, wind and the fuel cell electricity production systems are more popular among of these renewable energy sources. Initially, number of the studies about these technologies was limited because cost of the energy, produced from these sources, was higher than the conventional sources. But nowadays with decreasing costs of the fuel cells and photovoltaic systems, subvention of distributed energy resources, interest of these sources and number of the studies have been increased. The photovoltaic arrays and the fuel cells produce DC voltage. In general, induction generators, permanent magnet generators (PMG) and conventional excited synchronous generators are used in wind energy conversion systems. However, variable speed applications are preferred at low, medium to high power levels because variable speed wind turbines yield more energy capture than constant speed wind turbines, providing increased power generation at lower cost. PMG and synchronous generators produce AC power which, level of the voltage and frequency are continuously variable. Therefore, at first this energy is converted to DC and then it is converted to AC again via a grid interactive inverter in order to supply loads or utility grid [1]. Grid interactive inverters are used in grid connected renewable energy conversion systems to export the produced energy to the utility grid. In these applications, grid interactive inverters have been become one of the most important part of the system. With the increasing interest of the renewable energy systems, the grid interactive inverters
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are attractive point of interest of many power electronics researchers [2]. Grid interactive inverters can be designed as voltage controlled or current controlled. But a small synchronization error causes heavily overload the inverter in voltage controlled mode. A current controlled inverter is much less susceptible to this condition and recommended to control the export of power to the utility grid [2]. Different inverter structures are used in grid interactive inverter applications. Although, voltage source inverters are more popular, current source inverters attract attention with their ability of high resistant to short circuits and blocking reverse voltages [3]. In recent years, multi-level inverter structures are also investigated for especially high power levels. A line frequency transformer which used at the output of the inverter provides galvanic isolation between the DC energy source and the grid. Even though, line frequency transformers increase the system cost and size, they prevent the injection of the switching noise and DC current to the grid. Also high frequency transformer can be used embedded in DC-DC converter or DC-AC inverter. But, these ones cannot prevent the DC current and switching noise injection to the grid. In grid interactive operation, the current injected to the grid must be in phase with the grid voltage. So, phase and the frequency of the grid voltage had to be known to ensure unity power factor operation. Phase locked loop (PLL) circuits are used to obtain these data. These PLL circuits are used to generate reference current therefore, they are one of the most important part of the grid interactive inverters. Zero crossing detection of the grid voltage is one of the simple PLL method [4,5]. The grid interactive systems have a disadvantage that output capability of the renewable energy sources is related to natural effects. Week solar irradiation or wind speed levels force the whole system to be removed from the grid. To overcome this disadvantage of the grid-connected system, some multi-functional inverters have been presented. The systems include additional functions such harmonic current compensation generating and/or absorbing reactive power at a faster rate for the reactive-power compensation of rapidly changing industrial loads and for voltage regulation [6-8]. With the increasing application of renewable energy sources, more and more DG systems actively deliver
electricity into the grid and especially wind power generation is becoming an important electricity source in many countries. Consequently, grid codes now require wind energy systems to maintain active power delivery and reactive power support to the grid [9,10]. In this study a three phase grid interactive inverter with reactive power injection capability is proposed. A voltage source inverter structure is used in this proposed system. An LCL output filter is employed to reduce the high frequency harmonic components in current waveform due to PWM switching and to reduce the output current THD. The line frequency transformer is used to increase the inverter output voltage up to the grid voltage. The switching noise and DC current injections to the grid are prevented by this transformer. Inverter output current is shaped via the PI regulator. The proposed system is simulated and implemented. All processes, such as reading analog signals, generating PWM and control task are performed via TMS320F2812. Both Matlab/Simulink simulation and experimental results are show that power factor of the inverter can be controlled, and harmonics level of the inverter current is in the limits of international standards (
