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 isefa@gazi.edu.tr, naltin@gazi.edu.tr, saban.ozdemir@gazi.edu.tr 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 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 978-1-4244-9843-7/11/$26.00 2011 IEEE
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 (<5%). II. THE PROPOSED THREE PHASE GRID INTERACTIVE INVERTER SYSTEM The proposed three phase grid interactive inverter circuit scheme is shown in Fig. 1. Renewable energy source is represented as V dc voltage source. This renewable energy source can be photovoltaic modules, fuel cells or a wind turbine. The wind turbine generator is usually an AC generator which output voltage is rectified. The system consists of a three phase voltage source inverter, a line frequency transformer and a LCL filter. A PI compensator is used for the current control. Proportional and integral gains of PI compensator are found by using Ziegler- Nichols method [11]. Inverter output voltage is boosted to the 230 V with a line frequency transformer. The line frequency transformer also prevents DC ripple injection in current waveform and provides galvanic isolation between the inverter and the utility line. In addition this transformer simplifies the grounding of the DC energy source. An LCL filter is employed to reduce the high frequency harmonic components in current waveform due to PWM switching and to reduce the output current THD. A. Implementation of Control System Hardware and Software System is controlled with TMS320F2812 ezdsp control board. Principle scheme of the control system is seen in Fig. 2. This controller board has a 32 bit processor with 150 MHZ clock frequency. This processor and control board is suitable for the control of the three phase grid interactive inverter with its 12 bit ADC and 16 bit PWM resolutions. And also an USB2000 controller JTAG emulator board has been used to communicate with the computer and online monitoring the system variables [12]. Required electronic equipments are designed and implemented for digital inputs and outputs, analog inputs, zero crossing detections and conditioning gate signals according to the ezdsp board specifications. Block diagram of the proposed system is depicted in Fig. 3. The flowchart of the control system is shown in Fig 4. As seen from the flowchart, grid voltage and frequency is measured and controlled if the connection requirements are obtained or not. Input CAP1 control the one of the line voltage and program call an interrupt every zero crossing. The zero crossings of the other two voltages calculated from line voltage by using 120 phase difference. Phase sequence is also important and same phase sequence must be obtained. Period of the line voltage is read with timer and reference sine wave is obtained by using a look up table. Fig. 2. The principle scheme of ezdsp TMS320F2812 board Fig. 3. Block diagram of the proposed system Fig. 1. Proposed three phase grid interactive inverter
Fig. 4. Flowchart of the control system Reference current of the inverter is defined according to renewable energy source to obtain maximum power. Inverter output current read via LA 55-P current transducer. PI current regulator is used to shape the inverter current. The switching signals are generated by using look up table, PI regulator, defined power factor and current value. The switching frequency of the three phase inverter is 9.2 khz. B. LCL Filter Design Different output filters such as L, LC and LCL obtained from different connection of inductors and capacitors are used at the output of the inverters. L filter is first order filter and has 20 db/decade attenuation at whole frequency range and needs very high switching frequencies. Although LC filter is usually used in uninterruptible power supply applications, the resonance frequency of this filter is varies with line impedance in a utility interactive inverter application [13]. A third-order LCL filter is very suitable for grid interactive inverter, which can achieve reduced levels of harmonics distortion at lower switching frequencies and with less total inductance [14-15].Therefore LCL filter is preferred in this study. The resonance frequency of the LCL filter is related with only filter components and is given in Eq. 1. 1 L1 + L2 = (1) 2 π L L C f r 1 2 Resonance frequency and the components values of the filter is selected according to switching frequency, line frequency, and inverter reference power, voltage, current and impedance values. The resonance frequency, capacitor C, inductors L1 and L2 values affect the performance of the LCL filter. Resonance frequency of the LCL filter can be calculated by Eq. 2. f 10 f sw g fr (2) 2 where the fg is the utility line frequency and fsw is switching frequency. Similar criterions are required for inductor and capacitor values. Normalized values will help to explanation of these criterions are given in Eq. 3-9. S = S n (3) U = U g (4) f = f g (5) S I = U (6) 2 U U Z = = (7) S I Z L = (8) 2πf C 1 = (9) 2πf Z where S n is the nominal output power of the inverter and U g is the utility line voltage. Filter capacitor C is decreased the power factor. So the capacitor value of the filter is usually limited with Eq. 10 because the unity power factor is desired in utility interactive inverter applications [16]. C = 0. 05C (10) Low capacitor values for any resonance frequency, increase inductor values and also higher inductor values increase the system cost and size. So the value of the capacitor is selected near the limits of Eq. 10. In addition total inductance value affects the requirement DC voltage level because of the voltage drop on the inductors. Higher DC voltage level will result in higher switching losses. Consequently, total value of the inductance must be lower than the 10% of L [15]. Different L1 and L2 values can be selected for a unique value of the total inductance. The relation between the L1 and L2 is related with the maximum allowed value of the current ripple at the inverter and the line sides. In the study this relation is selected as in Eq.11. L 1 = 2L 2 (11) The LCL line filter components values of the utility interactive inverter is calculated by using Eq. 2-11 and are given in Eq. 12-14. The resonance frequency of the filter is tuned 1950 Hz. L 10mH 1 = (12) L 5mH 2 = (13) C = 2µF (14) III. SIMULATION AND EXPERIMENTAL RESULTS The experimental set up of the proposed three phase current controlled grid interactive inverter is seen in Fig. 5. The proposed inverter is simulated with MATAB/Simulink. Inverter output current and line voltage is in Fig. 6. As seen from figure, inverter current is synchronous with the line voltage and the frequency. Also, inverter output current is in sinusoidal waveform as indicated in international standards like IEC61727, EN61000-3-2 and IEEE1547. Inverter output current total harmonic distortion level is achieved as 2.61% as seen in Fig 7.
Fig. 5. Experimental set up of the proposed inverter The proposed DSP based inverter is designed and implemented. Three phase inverter output currents are shown in Fig. 8. In addition, inverter output current and line voltage for one phase and the FFT analysis of the inverter output current is seen in and Fig. 9. It is seen from figures that, experimental results are similar with the simulation studies and inverter output current is in phase with the line voltage. Inverter output current total harmonic distortion level is measured as 3% and this value is meet with the international standards (<5%). The line frequency transformer connected at the output of the inverter is increase the output voltage level to the line voltage level, prevents the DC current injection and provide the galvanic isolation between the line and the inverter. The proposed inverter is designed to operate in three modes: unity power factor operation, lead power factor operation and lag power factor operation. Although, grid interactive inverters are operated in unity power factor, inverter based Fig. 7. Inverter output current harmonics distributed power generation systems can be used to inject active, reactive power and harmonics to the grid. Reactive power injection is important for smart grid and especially microgrid applications [17]. The unity power factor, lead power factor and lag power factor operation modes of the proposed three phase grid interactive inverter are seen in Fig. 11, Fig. 12 and Fig. 13 respectively. As seen from the figures, the power factor of the inverter can be controlled +0.95 to -0.95. In addition, the output current of inverter is in sinusoidal waveform and harmonic level of the current is in the limit of international standards in all three operation modes. Fig. 6. Inverter output current and line voltage Fig. 8. Three phase inverter output currents
(a) Fig. 9. a) Inverter output current and line voltage for one phase, b) FFT analysis of the inverter output current (a) Fig. 10. Unity power factor operation a) Inverter output current and line voltage for one phase, b) Active, reactive power and power factor values (a) Fig. 11. Lead power factor operation a) Inverter output current and line voltage for one phase, b) Active, reactive power and power factor values
(a) Fig. 12. Lag power factor operation a) Inverter output current and line voltage for one phase, b) Active, reactive power and power factor values IV. CONCLUSION In this study, a DSP based three phase grid interactive inverter with reactive power injection capability is designed and implemented. The proposed inverter can operate three modes: unity, lead and lag power factor conditions according to requirements of grid authorities. This capability is tailorable in accordance with future requirements. The experimental and simulation results show that, power factor can be controlled +0.95 to -0.95 and inverter output current quality meets the international standards all three operation modes. The proposed inverter is suitable for microgrid applications with its reactive power injection capability. ERENCES [1] M.M. Reis, B. Soares, L.H.S.C. Barreto, E. Freitas, C.E.A. Silva, R.T. Bascopé, D.S. Oliveira Jr, A variable speed wind energy conversion system connected to the grid for small wind generator, IEEE Twenty- Third Applied Power Electronics Conference and Exposition, pp. 751-755, 24-28 Feb., 2008. [2] S. B. Kjaer, J. K. Pederson, F. Blaabjerg, A review of single-phase grid-connected inverters for photovoltaic modules, IEEE Transactions on Industry Applications, Vol. 41, no.5, pp. 1292-1306, 2005. [3] K. Hirachi, Y. Tomokuni, Improved Control Strategy to Eliminate The Harmonic Current Components for Single Phase PWM Current Source Inverter, 19th International Telecommunications Energy Conference, pp.189-194, Oct. 1997. [4] S. K. Chung, Phase-Locked Loop For Grid-Connected Three-Phase Power Conversion Systems, Electric Power Applications, IEE Proceedings, Vol. 147, Issue 3, pp. 213-219, May, 2000. [5] J. Svensson, Synchronization Methods for Grid-Connected Voltage Source Converters, IEEE Proceedings Generation, Transmission and Distribution, Vol. 148, Issue 3, pp. 229-235, May 2001. [6] H.-R. Seo, G.-H. Kim, S.-J. Jang, S.-Y. Kim, S. Park, M. Park, I.-K. Yu, Harmonics and Reactive Power Compensation Method By Grid- Connected Photovoltaic Generation System, ICEMS 2009. International Conference on Electrical Machines and Systems, pp.1 5, Tokyo, 15-18 Nov. 2009. [7] H. Yu, J. Pan, A. Xiang, A multi-function grid connected PV system with reactive power compensation for the grid, Solar Energy, Vol. 79, pp. 101-106, July 2005. [8] W. Libo, Z. Zhengming, L. Jianzheng, A Single- Stage Three-Phase Grid-Connected Photovoltaic System with Modified MPPT Method and Reactive Power Compensation, IEEE Trans. on Energy Conversion, Vol. 22, No. 4, pp. 881-886, Dec. 2007. [9] Grid Code for high and extra high voltage, E.ON Netz Gmbh, Apr. 2006 [10] The Grid Code, National Grid Electricity Transmission Plc, U.K., May.2009. [11] J.G. Ziegler, N.B. Nichols, Optimum settings for automatic controllers, Transactions of ASME, Vol. 64, pp. 759-768, 1942. [12] Texas Instrument, Literature Number: SPRS174Q http://focus.ti.com/lit/ds/symlink/tms320f2812.pdf [13] M. Raoufi, M. T. Lamchich, Average current mode control of a voltage source inverter connected to the grid: application to different filter cells, Journal of Electrical Engineering, Vol. 55, pp. 77-82, 3-4 Nov., 2004. [14] L. Augusto Serpa, S. Ponnaluri, P. Mantovanelli Barbosa, J. Walter Kolar, A Modified Direct Power Control Strategy Allowing the Connection of Three-Phase Inverters to the Grid Through LCL Filters, IEEE Transactions On Industry Applications, Vol. 43, No. 5, Sept.-Oct. 2007. [15] Kui-Jun Lee, Nam-Ju Park and Dong-Seok Hyun, Optimal Current Controller in a Three-Phase Grid Connected Inverter with an LCL fitler The 7th International Conference on Power Electronics, EXCO, Daegu, Korea, October 22-26, 2007. [16] M. Liserre, F. Blaabjerg, S. Hansen, Design and control of an lcl-filterbased three-phase active rectifier, IEEE Transactions on Industry Applications, Vol. 41, No. 5, Sep.-Oct. 2005. [17] J. C. Vasquez, R.A. Mastromauro, J.M. Guerrero, M. Liserre, Voltage support Provided by a Droop-Controlled Multifunctional Inverter, IEEE Transaction on Industrial Electronics, Vol. 56, No 11, pp. 4510-4519, 2009.