ISSN 2319-8885 Vol.04,Issue.36, September-2015, Pages:7877-7882 www.ijsetr.com Simulation of a Synchronous Reference Frame Voltage Control for a Z-Source Single Phase Inverter with Renewable Energy Sources GUNDETI RAVALI1, K. VAMSEE KRISHNA2 1 2 PG Scholar, Dept of EEE, VBIT Engineering College, Ghatkesar, R.R. (Dt), TS, India, E-mail: ravali31@gmail.com. Asst Prof, Dept of EEE, VBIT Engineering College, Ghatkesar, R.R. (Dt), TS, India, E-mail: kvamsee11@hotmail.com. Abstract: This paper presents the control of single phase power converters with Z-source network for renewable energy sources. Deals with the design of an SRF multiloop control strategy for Z-source single-phase inverter-based islanded distributed generation networks. Proposed controller uses an SRF proportional–integral controller to regulate the instantaneous output voltage and a capacitor current shaping loop in the stationary reference frame to provide active damping, improve both transient and steady-state performances, a voltage decoupling feed forward to improve the system robustness, and Z-source is a multi resonant harmonic compensator to prevent load current harmonics to distort the inverter outputs and decreases the shoot through problem for a single phase inverter. Since the voltage loop works in the SRF, it is not straight forward to fine tune the control parameters and evaluate the stability of the whole closed-loop network. To overcome this problem the stationary reference frame equivalent of the voltage loop is derived. This method of approach can be used in high power applications to produce high voltage gain when compared to the conventional converter. Simulations using MATLAB/SIMULINK are carried out to verify the performance of the proposed system. Keywords: DC-AC Converter, Z-Source Network. I. INTRODUCTION Distributed generation (DG), mainly from renewable energy sources, has increased during recent years. Smallscale electricity generation units, such as microturbines, roofmounted photovoltaic and wind generation systems, and commercially available fuel cells, are being widely utilized at the distribution level. Almost all these systems utilize some kind of power electronic converters to provide a controlled and high-quality power exchange with the single-phase grid or local loads. A voltage source inverter (VSI) is the most common topology which can operate either in grid-connected or standalone mode. In stand-alone or island operation mode, i.e., when the grid is not present, the local loads should be supplied by the DG system, which now acts as a controlled voltage source. DC-link voltage ripple of Z-Source inverter is one of major concerns since it is directly related to inverter output power quality. The essential requirement is to control the system voltage parameters such as amplitude and frequency with fast dynamic response and zero steady-state error. Different control techniques for single-phase VSIs in standalone mode have been presented in the literature. Owing to availability and low cost of advanced digital signal processors and digital control strategies based on repetitive control, dead-beat control and discrete-time sliding-mode control have been proposed. Digital repetitive control is proposed to reduce harmonic distortions of the output voltage produced by nonlinear load, with its excellent ability in eliminating periodic disturbances. However, in practical applications, slow dynamics, poor tracking accuracy, a large memory requirement, and poor performance to non periodic disturbances are the main limitations of this technique. The deadbeat and sliding-mode controllers exhibit excellent dynamic performance in direct control of the instantaneous inverter output. Unique feature is that even with their fast response, they prevent overshoot. These techniques suffer from some drawbacks such as complexity, sensitivity to parameter variations and loading conditions and steady-state errors. Proportional–resonant (PR) control has shown superiority in eliminating the steadystate error associated to the tracking problem of ac generations. The proposed technique has also attracted increasing interests in instantaneous voltage control of singlephase VSIs. The PR controller has certain disadvantages, the mains being exponentially decaying response to step changes and great sensitivity. The possibility of instability to the phase shift of sensed signals and the synchronous reference frame (SRF) proportional–integral (SRFPI) controller is widely used for three-phase converter systems to obtain a zero steady-state error. In the SRFPI control, the electrical signals are all transformed to the synchronous reference frame, where quantities are dc and as a consequence the zero steady-state error is ensured by using conventional PI regulators. This transformation requires at least two orthogonal signals; thus, a fictitious second phase must be generated to allow emulation of two-phase systems. In this paper, SRFPI Copyright @ 2015 IJSETR. All rights reserved. GUNDETI RAVALI, K. VAMSEE KRISHNA controller is proposed to regulate the instantaneous output voltage. While the use of SRFPI controller in three-phase (3) systems is a mature topic, in single-phase systems, it has not Where, been yet properly investigated. Proposed multiloop structure I and V - cell output current and voltage; employs a simple inner capacitor current shaping loop to Io - cell reverse saturation current; provide active damping and improve both transient and T - Cell temperature in Celsius; steady-state performance. Also, voltage decoupling feed K - Boltzmann’s constant; forward is utilized to improve the system robustness and at q - Electronic charge; the same time simplify the system modeling and controller Ki- short circuit current/temperature coefficient; design. A multiresonant harmonic compensator (HC) is G - Solar radiation in W/m2; added to the suggested scheme which prevents low-order Gn- nominal solar radiation in W/m2; load current harmonics to distort the inverter output voltage, Eg - energy gap of silicon; particularly under distorted and nonlinear loads. Combining Io,n - nominal saturation current; the multiloop control, harmonic resonators, and the voltage Rs - Series resistance; feed forward with the SRFPI in single-phase systems has not Rsh - shunt resistance; been yet explored. The I-V characteristic of a PV module is highly nonlinear in nature. This characteristics drastically changes with respect to changes in the solar radiation and cell temperature..Whereas the solar radiation mainly affects the output current, the temperature affects the terminal voltage. Fig.2 shows the I-V characteristic of the PV module under varying solar radiations at constant cell temperature (T = 25 ºC). Fig.1. Power stage of a single-phase VSI. SRFPI control algorithm involves several reference frame transformations; therefore, the classical control techniques cannot be simply applied to evaluate the performance of the closed-loop system. Thus, single-phase equivalent of the SRFPI regulator is obtained and which significantly simplifies the controller design and stability analysis. Detailed design procedure with consideration of the practical implementation issues, such as the effect of loading conditions and the control delay is proposed. II. OVERVIEW OF A PHOTOVOLTAIC (PV) MODULE To understand the PV module characteristics it is necessary to study about PV cell at first. A PV cell is the basic structural unit of the PV module that generates current carriers when sunlight falls on it. The power generated by these PV cell is very small. To increase the output power the PV cells are connected in series or parallel to form PV module. The electrical equivalent circuit of the PV cell is shown in Fig.2 Fig.3. Current versus voltage at constant cell temperature T = 25 ºC. Fig.3 shows the I-V characteristics of the PV module under varying cell temperature at constant solar radiation (1000 W/m2). Fig.4 shows Current versus voltage at constant solar radiation G = 1000 W/m. Fig.2. Electrical equivalent circuit diagram of PV cell. The main characteristics equation of the PV module is given by (1) Fig.4. Current versus voltage at constant solar radiation G = 1000 W/m. International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.36, September-2015, Pages: 7877--7882 (2) Simulation of a Synchronous Reference Frame Voltage Control for a Z-Source Single Phase Inverter with Renewable Energy Sources current-source converter and provides a novel power III. PROPOSED SYSTEM The power stage of a single-phase VSI, consisting of an conversion concept. The Z-source concept can be applied to insulated-gate bipolar transistor (IGBT) full-bridge all AC-to-DC, DC-to-AC, AC-to-AC, DC-to-DC power configuration followed by an LC filter, is illustrated in Fig. 1. conversion. To describe the operating principle and control, Throughout this paper, the dc-link voltage is assumed to be this paper focuses on an example: a Z-source inverter for constant. This assumption can be simply realized by using a DC-AC power conversion needed in fuel cell applications. sufficiently large capacitance at the dc link. Fig 5.shows the proposed system of single phase inverter with Z-source network by using SRF voltage control. Fig.6. Z-source inverter circuit diagram. Fig.5. Single phase inverter with Z-source. IV. Z-SOURCE CONVERTER Z source network is a one type of DC-DC converter which is used to control the shoot through problem and also used to reduce the harmonics, electromagnetic interference and acts as a Buck-boost converter operation. Z source inverters are recent inverter topologies that can perform both buck and boost functions as a single unit as shown in Fig.6. A unique feature of Z source inverter is the shoot through state, by which two semiconductor switches of the same phase leg can be turned ON simultaneously. Therefore, no dead time is needed and output distortion is greatly reduced and thus reliability is greatly improved. Feature is not available in the traditional voltage source and current source inverters. The proposed Z source inverters are mainly applied for loads that demand a high voltage gain such as motor drives and as a power conditioning unit for renewable energy sources like fuel cells, solar, etc to match the input source voltage differences. The development in Z source inverter topologies provides a consecutive enhancement in voltage gain and output waveforms. A tradeoff between the boosting capability and component count is always a major concern to keep the cost stable. It is to be noted that increase in the passive components with suitable modifications can improve the performance of these types of inverters. The topological growth has been in terms of addition or reduction of passive component, inclusion of extra semiconductor switches, alteration or inclusion of dc sources and also changes of modulation schemes etc. Voltage buck inversion ability is also provided for those applications that need low ac voltages. The Z-source converter employs a unique impedance network to couple the converter main circuit to the power source, thus providing unique features that cannot be obtained in the traditional voltage-source and current-source converters where a capacitor and inductor are used, respectively. The Z-source converter overcomes the conceptual and theoretical barriers and limitations of the traditional voltage-source converter and V. PROPOSED CONTROL SCHEME The control of three-phase power converters in the DQ rotating reference frame is now a mature and well-developed research technology. However, for single-phase converters and is not as well established as three-phase applications. Main reason behind this lies partly in its more complex structure than the conventional stationary reference frame controller and also a secondary orthogonal signal that is needed to implement a single-phase controller in the DQ reference frame. Fig. 7 illustrates the suggested control scheme, which includes an SRFPI controller to regulate the instantaneous output voltage, an inner current shaping loop to provide active damping and improve both transient and steady-state performances, and a voltage-feed forward path to improve the system robustness. The capacitor current is selected as the feedback signal in the inner current loop, since it brings better disturbance rejection capability than the inductor current feedback. Indeed, because the capacitor current is directly proportional to the time rate of change of output voltage and gives some kind of prediction about output voltage distortions caused by nonlinear load currents and allows the inner control loop to compensate in advance. On the other hand, it is simpler and definitely more cost effective to sense the capacitor current instead of the higher ampere inductor current. It should be noted here that a practical difficulty in accurately measuring the filter capacitor current, particularly for high capacitances, is that the low-frequency current information is immersed by switching frequency currents. A low-pass filter in the current feedback loop may be required. In practice and to reduce the filtering requirements and the resultant phase delays, the LC filter capacitor should be chosen as small as possible. It is also noteworthy that the current ripple highly depends on the capacitor equivalent series resistance (ESR). In practice, to reduce the ESR effect, several low ESR capacitors are connected in parallel for the LC filter. Fig.7. Stationary (αβ) reference frame representation of the SRFPI controller. International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.36, September-2015, Pages: 7877-7882 GUNDETI RAVALI, K. VAMSEE KRISHNA Where Vll_pk is the peak value of the line-to-line voltage, Vg is the DC link voltage. The performance of a modulation scheme can be evaluated based on the following five aspects: (1) distortion of the output voltage or current; (2) power losses; (3) harmonic spectrum and EMI; (4) dynamic range; and (5) complexity. It is always desirable to minimize the distortion of the output voltage or current. It may change with the modulation index in a nonlinear curve. The power losses are related to the total number of switching actions in one switching cycle, and the current level at switching. Therefore, different modulation schemes may result in different efficiencies. A PWM scheme with minimized switching Fig.8. (a) Block diagram of the proposed control system losses is desirable especially for high power applications. and (b) its simplified representation. Harmonic spectrum of the output voltage or current is related to the EMI issue and acoustic noise. It is desirable to The choice of the proportional gain of the PI minimize the EMI and acoustic noise. Dynamic range refers compensator is a tradeoff between the attainable voltage to the maximum possible control level in steady state or regulation bandwidth and the control loop stability as shown during transient. It can also be interpreted as the ratio in Fig.8. In this paper, Kp is chosen to provide a desired between the maximum possible output and the input. It is bandwidth of ωbv for v/v loop. A robust performance of the desirable to have a higher ratio. For a voltage source inverter, control system and a minimum steady-state error will be then it means a better DC link voltage utilization, which is crucial ensured by means of proper selection of the integral gain of for high voltage applications. It is preferable to have a PWM the compensator. For the sake of simplicity, the following scheme that can be implemented easily, by either an analog analytical analysis to determine Kp is based on the means or digital means as shown in Fig.9. assumption that the integral gain Ki has almost no effect on the voltage regulation dynamics. (4) VI. PULSE WIDTH MODULATION TECHNIQUE The pulse width modulation (PWM) concept is borrowed from communication systems, wherever an indication is modulated before its transmission, and so demodulated at the receiving terminal to recover the initial signal. Constant idea may be applied to an influence convertor. in an exceedingly power convertor, the switch network has associate on/off nonlinear nature. The desired continuous wave form is modulated and reborn to digitized signals to management the switch network. Then the modulated signals at the switch network AC terminals area unit demodulated by the AC filter to urge the specified continuous voltage or current wave form. Normally a sinusoidal voltage or current is the control target for a power converter. The first PWM scheme was the sinusoidal PWM (SPWM) scheme and was proposed in 1964. Since the modulator has a great impact on voltage/current distortions, switching losses, and EMI, it is of great interest to the power electronics researcher. In the past there has been intensive research on this topic and there is much literature on it. All the proposed PWM schemes may be classified into four categories, namely, (1) SPWM and its derivations; (2) Optimal PWM (3) Space Vector Modulation (SVM); (4) Hysteresis and Bang-Bang type modulation; and (5) Random PWM. All the PWM schemes may be evaluated under a certain switching frequency and the reference signal frequency ratio, and the input and output voltage ratio, which is also named as the modulation index M. The definition of the modulation index M is given Fig.9. PWM technique. VII. SIMULATION RESULTS The below figs.10 to 13 shows the simulation circuit diagram of a proposed system and following shows the waveforms getting from the simulation diagram. (5) Fig.10. Proposed simulation circuit diagram. International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.36, September-2015, Pages: 7877--7882 Simulation of a Synchronous Reference Frame Voltage Control for a Z-Source Single Phase Inverter with Renewable Energy Sources alone mode, which guarantees zero steady-state error at the fundamental frequency and Z-source inverter is used to decrease the shoot problem and increases the voltage gain. Moreover, an inner capacitor current regulating loop brings active damping and improves both transient and steady-state performances. A voltage-feed forward path boosts the system robustness. A multiresonant HC actively prevents the loworder harmonic currents to distort the inverter output voltage. The single-phase equivalent of the SRFPI regulator was provided, which significantly simplifies controller design and stability analysis. Fig.11. Solar Voltage. Fig.12. Z-source output voltage. Fig.13. Waveforms of Ic,Io,Vdc,Vo. IX. REFERENCES [1] Fang Zheng Peng, “Z - Source Inverter”, IEEE Transaction on Industry Applications. 39: 2003,2. Wuhan,China. [2] G. Pandian and S. Rama Reddy, “Embedded Controlled Z Source Inverter Fed Induction Motor Drive” IEEE transaction on industrial application, vol.32, no.2, May/June 2010. [3] K. Srinivasan and Dr. S. S. Das, “Performance Analysis of a Reduced Switch Z -Source Inverter fed IM Drives”, Journal of Power Electronics, Vol. 12, No. 2, May/June 2010. [4] K.Niraimathy, S.Kr ithiga, “A New Adjustable - Speed Drives (ASD) System Based On High - Performance Z Source Inverter”, 978 – 1 – 61284 – 379 - 7/11 2011 IEEE, 2011 1st International Conference on Electrical Energy Systems [5] Y. Y. Tzou, R. S. Ou, S. L. Jung, and M. Y. Chnag, “High-performance programmable AC power source with low harmonic distortion using DSP based repetitive control technique,” IEEE Trans. Power Electron., vol. 12, no. 4, pp. 715–725, Jul. 1997. [6] K. Zhou, K. Low, D. Wang, F. Luo, B. Zhang, and Y. Wang, “Zero-phase odd-harmonic repetitive controller for a single-phase PWM inverter,” IEEE Trans. Power Electron., vol. 21, no. 1, pp. 193–201, Jan. 2006. [7] K. Zhang, Y. Kang, J. Xiong, and J. Chen, “Direct repetitive control of SPWM inverter for UPS purpose,” IEEE Trans. Power Electron., vol. 18, no. 3, pp. 784–792, May 2003. [8] R. Ortega, G. Garcera, E. Figueres, O. Carranza, and C. L. Trujillo, “Design and application of a two degrees of freedom control with a repetitive controller in a single phase inverter,” in Proc. IEEE ISIE, Jun. 2011, pp. 1441–1446. [9] R. J. Wai, C. Y. Lin, Y. C. Huang, and Y. R. Chang, “Design of high-performance stand-alone and grid-connected inverter for distributed generation applications,” IEEE Trans. Ind. Electron., vol. 60, no. 4, pp. 1542–1555, Apr. 2013. [10] C. G. C. Branco, R. P. Torrico-Bascope, C. M. T. Cruz, and F. K. de A Lima, “Proposal of three-phase highfrequency transformer isolation UPS topologies for distributed generation applications,” IEEE Trans. Ind. Electron., vol. 60, no. 4, pp. 1520–1531, Apr. 2013. [11] C. Trujillo, D. Velasco, G. Garcera, E. Figueres, and J. Guacaneme, “Reconfigurable control scheme for a PV microinverter working in both grid-connected and island modes,” IEEE Trans. Ind. Electron., vol. 60, no. 4, pp. 1582– 1595, Apr. 2013. VIII. CONCLUSION This paper has proposed an SRFPI controller to regulate the instantaneous output voltage of the single-phase inverter with Z-source network for renewable energy source in standInternational Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.36, September-2015, Pages: 7877-7882 GUNDETI RAVALI, K. VAMSEE KRISHNA Author’s Profile: Gundeti Ravali, received the B.Tech degree in electrical and electronics engineering from Vidya bharathi institute of technology,Warangal. She is pursuing M.Tech degree in power electronics & electrical drives from Vignana Bharathi institute of technology, Hyderabad, Telangana expected to receive in 2015.Her current research interests include simulation of a synchronus reference frame voltage control for a z-source single phase inverter with renewable energy sources. Email id: ravali31@gmail.com. K.Vamsee Krishna, Member of IEEE, received the B.Tech. degree in Electrical and Electronics Engineering from Kakatiya University and the M.E degree in Industrial Drives and Control, Electrical Engineering from Osmania University. He is currently an Assistant Professor with the Department of Electrical and Electronics Engineering, Vignana Bharathi Institute of Technology, Hyderabad, Telangana. His research interests are Control Systems, Embedded Controllers, and Power Electronics. Email id: kvamsee11@hotmail.com. International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.36, September-2015, Pages: 7877--7882