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PHOTO VOLTAIC MICROINVERTER CONTROL FOR GRIDCONNECTED APPLICATIONS 1 N.VASEEM RAJA 2 S.GURU PRASAD 1 M.Tech Student GCET,KADAPA,JNTU Ananthapur, AP-India,[email protected] 2 Assistant Professor, EEE Dept, GCET,KADAPA, JNTU Ananthapur, AP-India applied to both the dc–dc converter and the inverter Abstract-- This project work presents a gridconnected photovoltaic (PV) micro-inverter system and its control implementations. A half bridge dc-dc converter is used to interface the low-voltage PV module and grid under light and heavyloaded conditions . A full-bridge pulse width-modulated inverter is cascaded which injects synchronized sinusoidal current to the grid. A plug-in repetitive current controller accommodated with IIR filter is proposed to regulate the grid current and also to eliminate periodic harmonics,to achieve high power factor and low harmonic distortion under varying loads The model of the proposed scheme of PV microinverter control has been built using MATLAB/Simulink. ,here a constant voltage dc link decouples the power Index Terms—Micro inverter ,boost half-bridge converter, repetitive current control ,IIR filter distortions (THDs), power factor, and dynamic flow in the two stages. By contrast, the second configuration utilizes a quasi-sinusoidal PWM method to control the dc–dc converter in order to generate a rectified sinusoidal current (or voltage) at the inverter dc link. For best MPPT performance and output current quality, the first method of PV micro inverter is adopted in this paper. A full-bridge PWM inverter with an output LCL filter is incorporated to inject synchronized sinusoidal current to the grid. In general, its performance is evaluated by the output current total harmonic response. Repetitive control (RC) is known as an effective solution for elimination of periodic harmonic errors. An IIR filter is incorporated in RC I.INTRODUCTION In single phase grid connected photovoltaic power to obtain very high system open-loop gains at a large systems, the concept of Micro inverter has become a number of harmonic frequencies such that the future trend for its removal of energy yield harmonic rejection capability is greatly enhanced. In mismatches among PV modules, A boost-half-bridge this work, a plug-in repetitive current controller is DC-DC converter cascaded by an inverter is the most proposed. It is composed of a proportional part and popular topology, in which a HF step up an RC part, to which the IIR filter is accommodated. transformer is often implemented within the DC-DC conversion stage. Pulse width modulation (PWM) techniques employed in the PV microinverter system, where two major methods are mostly used. In the first, PWM control is The proposed current controller following superior features: 1) high power factor is obtained; exhibits the 2) current harmonic distortions (up to the 13th-order) III. DIGITAL DESCRIPTION OF PV MICRO caused by the grid voltage non ideality are INVERTER SYSTEM CONTROL minimized; A simple dual power processing PV inverter 3) better current regulation is guaranteed. employing in the paper is shown in Fig. 2. 4) fast dynamic response is achieved during the transient loads. II. MICROINVERTER EMPLOYING BOOST HALF-BRIDGE CONVERTER The boost is a popular non-isolated power stage Fig.2 Dual power processing inverter. topology, sometimes called a step-up power stage. Power supply designers choose the boost power stage A digital approach is adopted for the control of the because the required output is always higher than the PV micro-inverter system, as shown in Fig. 5. The input voltage. The input current for a boost power PV voltage VPV and current IPV are both sensed for stage is continuous, or non-pulsating, because the calculation of the instantaneous PV power PPV. At output diode conducts only during a portion of the the inverter side, the grid voltage Vg is sensed to switching cycle. The output capacitor supplies the extract the instantaneous sinusoidal angle θg, which entire load current for the rest of the switchingcycle. is commonly known as the phase lock loop. The inverter output current Iinv is prefiltered by a firstorder low-pass filter on the sensing circuitry to eliminate the HF noises. The filter output Iinv is then fed back to the plug-in repetitive controller for the inner loop regulation. Vdc can be sensed which is Fig.1 basic schematic diagram of a boost converter. used as a reference voltage for tracking of the grid voltage. The grid current and the dc voltage The boost-half-bridge microinverter for grid connected PV systems is composed of two decoupled power processing stages. In the front-end dc–dc converter, a conventional boost converter is modified by splitting the output dc capacitor into two separate ones.. The center taps of the two MOSFETs (S1 and S2 ) and the two output capacitors (C1 andC2 ) are connected to the primary terminals of the transformer Tr , just similar to a half bridge. The boost-halfbridge converter is controlled by S1 and S2 with complementary duty cycles. The boost-half-bridge microinverter topology for gridconnected PV systems is shown in Fig. 3. references are represented by Iinv* and Vdc*, respectively. In order to achieve fast dynamic responses of the grid current as well as the dc voltage, a current reference feed forward,Iinvff is added in correspondence to the input PV power PPV. The magnitude of the current feed forward is expressed as follows: 𝐼𝑖𝑛𝑣𝑓𝑓=2𝑃𝑝𝑣\𝑉𝑔 (1) where|Vg | is the magnitude of the grid voltage and can be calculated by 𝑉𝑔 =1\2∫ 𝑉𝑔 𝑑𝜃𝑔 (2) Fig.3.Boost half bridge PV micro inverter. IV. DESIGN AND ANALYSIS OF PLUG-IN REPETITIVE CURRENT CONTROLLER Using an LCL filter in a grid-connected inverter system has been recognized as an attractive solution to reduce current harmonics around the switching frequency, improve the system dynamic response, and reduce the total size and cost. An undamped LCL filter exhibits a sharp LC resonance peak, which indicates a potential stability issue for the current regulator design. Hence, either passive damping or Fig.5 Proposed PV micro inverter control. active damping techniques can be adopted to attenuate the resonance peak below 0 dB. On the The plug-in digital repetitive controller is designed, other hand, a current regulator without introducing as shown in Fig. 4 .The conventional proportional any damping method can also be stabilized, as long controller with a gain of Kp2 is used to obtain fast as the LCL parameters and the current sensor dynamics. The RC is then plugged in and operates in location are properly selected. The current sensor is parallel with the proportional controller. e(z) and d(z) placed at the inverter side instead of the grid side. represent the tracking error and the repetitive disturbances, respectively. In an ideal RC, a unity gain is along the positive feedback path such that all the repetitive errors based on the fundamental period are completely eliminated when the system reaches equilibrium. However, in Fig.4 proposed plug-in repetitive controller. order to obtain a sufficient stability margin, a zerophase low-pass filter is often incorporated rather than the unity gain. This can be realized by cascading a linear-phase low pass filter Q (z) and a phase lead power converters. If the converter dynamics are compensator zk2.zk1 is another phase lead unit, disregarded in the MPPT control, undesirable which compensates the phase lag of inverter at HFs. transient responses such as LC oscillation, inrush current, and magnetic saturation may take place. The proportional controller modifies the transient Consequently,the conversion efficiency can be response and steady state error. It produces an output deteriorated or even malfunction of the converter signal proportional to error signal, and amplifies the may occur. error signal to increases the loop gain of the system. Table I summarizes the key parameters of the boost- In this work, Kp2 = 50,is chosen to increase the loop halfbridge dc–dc converter. gain. It is noticeable that larger Kp2 will result in a smaller tracking error during the transients. In a fourth-order linear-phase IIR filter has been synthesize for the repetitive voltage controller for UPS systems. Compared with the conventional linear-phase finite impulse response filters used for RC, the linear-phase IIR filter exhibits a flat gain in the pass band. Hence, it is a good candidate for the repetitive current controller . In practice, Q(z) is synthesized by cascading a second-order elliptic filter Table.1. boost half bridge converter parameters. . Table II summarizes the key parameters of the full bridge inverter. Qe(z) and a second-order all-pass phase equalizer Qa(z). Q(z), Qe(z), and Qa(z) are expressed by the following equations Q(z) = Qe(z)Qa(z) (3) 0.1385+0.2564𝑍 −1 +0.1385𝑍−2 Qe(Z)= 1−0.7599𝑍−1 +0.2971𝑍 −2 0.1019−0.6151𝑍−1 +𝑍 −2 Qa(Z)=1−0.6151𝑍−1 +0.1019𝑍−2 (4) (5) Table.2. full bridge inverter parameters. V. BOOST CONVERTER CONTROL The PV voltage is regulated instantaneously to the command generated by the MPPT function block. Typically, the MPPT function block in pv converter/inverter system periodically modifies the tracking reference of the PV voltage, or the PV current, or the modulation index, or the converter duty cycles. In most cases, these periodic perturbations yield step change dynamic responses in VI. SIMULATION RESULTS In the proposed Boost –Half-Bridge PV micro inverter system diagram of PV system and MPPT and Boost-Half-Bridge converter and full- Bridge Inverter are designed in Matlab/Simulink. The control circuit diagrams for Boost-Half-Bridge converter and fullBridge inverter designed in Matlab/Simulink are shown in below figures. (A). SYSTEM OUTPUT WAVE FORMS The PV cell output current and output voltage and output power wave forms for both light load and heavy load condition are shown in below figures 8 and 9. Fig.6 Simulation circuit diagram for Boost-HalfBridge converter control. The subsystem for the MPPT algorithm for the Boost Half bridge converter control is shown in the following figure Fig .8 PV cell output voltage Fig.7 Simulation circuit diagram for MPPT algorithm. Fig .9 PV cell output power The simulation circuit diagrams for Full-Bridge – The proposed plug-in RC achieves a voltage THD as Inverter control is shown in below figure.7. low as 1.51% and current THD of 1.76% and 0.99 power factor. The proportional part in the plug-in RC enables the controller to respond to the abrupt reference change promptly. Meanwhile, RC part cancels the harmonic distortions in fundamental cycles. Fig.6 Simulation circuit diagram for full bridge inverter control. Fig 10. grid current under loaded condition several power factor (>0.99) and low THD (0.9%–2.87%) are obtained. VII. REFERENCES [1] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of single-phase grid-connected inverters for photovoltaic modules,” IEEE Trans. Ind.Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005. [2] Q. Li and P.Wolfs, “A review of the single phase photovoltaic module integrated converter topologies with three different DC link configurations,” IEEE Trans. Power Electron., vol. 23, no. 3, Fig.11 output voltage and current under loaded pp. 1320–1333, May 2008. condition [3] R. Wai and W. Wang, “Grid-connected photovoltaic (B)FFT Analysis of total harmonic distortion generation system,” IEEE Trans. Circuits Syst.-I, vol. 55, no. 3, pp. 953–963, Apr. 2008. The performance is evaluated by the output current [4] M. Andersen and B. Alvsten, “200W low cost module integrated utility interface formodular photovoltaic energy total harmonic distortions (THDs) by conducting the systems,” in Proc. IEEEIECON, 1995, pp. 572–577. FFT analysis for the output wave forms as shown in the following figure N. Vaseem Raja, Received B.Tech Degree from NBKR Institute of Science and Technology, SVU Under the Department of Electrical and Electronics in 2012. Currently he is working towards his Masters Degree in the Department of Power Systems from GCET ,KADAPA. His interest includes High voltage engineering, Power quality issues. Fig .12. FFT Analysis for THD VI. CONCLUSION A boost-half-bridge PV microinverter system with its advanced control implementations for grid-connected PV applications has been presented. A plug-in repetitive current controller was proposed and illustrated. Simulation of Repetitive current controller used as a control circuit for micro-inverter has been carried out. Simulation of proposed system is done by using MATLAB/Simulink. Performance metrics is taken as THD and power factor. The current injected to the grid is regulated precisely and stiffly. High S.Guru Prasad, Received B.Tech Degree from VAGDEVI institute of Technology and science, JNTU Hyderabad in 2005 and master degree from MAHALAKSHMI institute of Technology and science, JNTU Hyderabad Under the Department of Electrical and Electronics in 2008. Currently he is working as assistant professor in the Department of Electrical Engineering. His interest includes Electrical power systems and Electrical machines.