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2016China International Conferenceon Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 Solid State Transformer application to Grid Connected Photovoltaic Inverters BAOLONG LIU, YABING ZHA, TAO ZHANG AND SHIMING CHEN College of Information System and Management National University of Defense Technology Changsha, Hunan Province, China The transformers have been widely used in different fields, the primary functions of which are voltage-transformation, isolation, voltage-stability and energy-transfer. Actually the conventional copper-and-iron based transformers are highly reliable, relatively inexpensive and rather efficient, but have some disadvantages: (1) heavy weight and large sizes for iron cores or copper windings; (2) voltage drop under load; (3) sensitivity to harmonics; (4) low performance under load unbalance or in presence of dc-offset; (5) environmental concerns regarding mineral oil[1-2]. The Solid State Transformer (SST) is a new power conversion equipment, which is based on power electronic converter and high frequency link. SST has the advantage of voltage regulation, reactive power compensation, bi-directional power flow control features etc[3-5]. SST has been regarded as one of the ten most emerging technologies in 2011 by MIT technology review[6]. In the grid connected PV system, two-stage PV converter with high gain dc-dc converter cascaded by inverter is most popular topology[7-8]. In this structure, the bulky electrolytic capacitors are likely to have reliability of thirty times less than other types under the same conditions[9-10]. Large electrolytic capacitor can be replaced by small film capacitors to extend the lifetime, but small dc-link capacitor may suffer from large voltage swing[11]. The large twice grid frequency ripple will not only affect the control system but also degrade the MPPT performance. which may not only affect the control system but also CICED2016 Session 3 Paper No 1009 II. Proposed Converter The scheme of the proposed converter is shown in Fig.1. It consists of two decoupled power processing stages. The DHB converter boosts the PV voltage and directly performs the MPPT. A single phase full bridge inverter will coordinate with the DHB converter to achieve the MPPT and delivery the maximum PV power to the grid. SST Inverter inverter stage isolated stage S1 C1 Lf C3 S3 C4 S4 iL f S2 C2 S5 S7 S6 S8 Grid I. NTRODUCTION result in low-frequency ripple energy propagating into the PV side and degrade the MPPT performance. Dual half bridge (DHB) could be used as the isolated stage in the low power SST application[12] . This paper proposes a novel single phase PV system based on dual half bridge (DHB) with small DC-link capacitor. In the proposed approach, the input PV voltage is directly controlled by the shift phase between the primary half bridge and secondary half bridge. Due to the use of DHB, the PV arrays are immune to large voltage ripple in the dc-link. The effectiveness of the proposed approach is verified by simulation results. VLVDC Abstract—In the paper, an architecture that includes a Solid State Transformer (SST) which is different from the conventional style is proposed. The photovoltaic system with SST consist of two power stages. First a dual half bridge (DHB) converter implement the MPPT functions. Second a grid connected inverter is used to stabilize the dc-link voltage and make the output current sinusoidal. The control technique is presented and validated by simulation implemented on a photovoltaic system with SST. The simulation results prove the effectiveness of the proposed photovoltaic system and the proposed control technique. Index Terms—Solid State Transformer (SST), Photovoltaic Inverter, dual half bridge (DHB) Fig. 1. Proposed inverter topology A) energy buffer Ideally, the voltage and current at the output of the grid connected inverter are line frequency, e.g.[13], 50Hz. Therefore the output power of the grid connected inverter consist mainly of average power (which is in dc) and ripple power (which is the second-order harmonic content at 100Hz). (1 ) Where V represent grid RMS voltage, I represents output RMS current, , and is the grid frequency. On the other hand, in the PV maximum power point (MPP), the PV current and Page/4 2016China International Conferenceon Electricity Distribution (CICED 2016) voltage are constant thus the power delivered by the PV is constant. This means that the dc link bus must absorb the 100Hz ripple power. Because 100Hz is a relatively low frequency, large bus capacitance is needed to reduce the bus voltage fluctuation and achieve sufficiently high ripple current ratings. B) PV Fig. 2 shows the equivalent circuit of the ideal PV cell. The basic equation that mathematically describes the I-V characteristic of ideal PV cell[14] is Xi’an, 10-13 Aug, 2016 (4) where is the phase shift ratio from the primary side H-bridge to secondary side H-bridge. Fig. 3 gives the average circuit model of the DHB converter. The average current flowing out of the secondary side H-bridge(see in Fig.3) during half of a switching cycle can be given by (5) (2) id avg vo iLVDC C Where is the current generated by the incident light (it is directly proportional to the Sun irradiation), is the reverse saturation or leakage current of the diode, is the electron charge (1.60217646 C), is the Boltzmann constant (1.3806503 J/K), (in Kelvin) is the temperature of the junction, and is the diode ideality constant. Load Fig. 3. average circuit model of the DHB converter D) grid connected inverter The dynamics of grid connected inverter for the inner ac current control is described by (6) Fig. 2. Single-diode model of the theoretical PV cell and and the dc link voltage operation equavalent circuit of a practical PV device Practical arrays are composed of several connected photovoltaic cells and the observation of the characteristics at the terminals of the photovoltaic array[15] is described as (3) Where and are the photovoltaic and saturation currents of the array and is the thermal voltage of the array with cells connected in series. C) DHB converter DHB converter can realize zero-voltageswitching for all the switching devices without auxiliary switch device, which enables the high switching frequency operation with low switching losses. DHB also enable seamless bidirectional power flow. As a result, high power density and high efficiency can be achieved. For DHB, the power transferred is given by[16] CICED2016 Session 3 Paper No 1009 (7) where and is the grid current and voltage, is the dc link voltage, and is the filter inductance and dc link capacitor. III. Control Scheme The control system is represented in Fig .4, in which there are three main control aims to be performed. First, to achieve the MPPT under the changing atmosphere, the proposed inverter system will controlled to track the maximum power by the MPPT unit which calculates the PV voltage reference. The DHB regulates the PV voltage by controlling the phase shift. Since the DHB converter is equivalent to a controlled current source[17] and is simplified as a first-order system, a PI controlled outer voltage is selected to control the phase shift and consequently the power drawn from PV module. Page/4 i pv v pv iac * vdc Predictive current control id* PI vac First order filter and DQ transformation FMA MPPT v*pv DC/AC Grid DC/DC vdc v pv i pv 2016China International Conferenceon Electricity Distribution (CICED 2016) PI idq Xi’an, 10-13 Aug, 2016 Max power 1526W Transformer turns ratio 11:16 Switching frequency 12.8KHz Leakage inductance (referred to primary) 0.372mH Dc link capacitor vdq Fig. 5 shows the voltage, current and power of the PV arrays. As it was expected the PV power reach the maximum power point. iq* Fig. 4. Control system block diagram Second, the dc link voltage should be regulated around its average value with desired bandwidth and phase margin. Since the ripple at frequency twice the grid is large, such a result will yield to grid current distortion which is not desirable. A Fast moving average (FMA) filter[18] that has a time window with the length corresponding to the period of second harmonic ripple is employed to filter the measured dc link voltage. A standard proportional integral (PI) controller is designed to control the filtered dc-link voltage, which give the current reference of the inner loop of the grid connected inverter. Third, the output current of the grid connected inverter is synchronized with the grid voltage for unit power factor. Due to the time-varying nature of single phase grid connected inverter, it is also difficult to achieve good performance. It is much easier to perform analysis and design controllers for single phase rectifier in DQ frame because all time-varying state variables of the single phase rectifier become DC time-invariant[19]. The second orthogonal imaginary circuit is created to satisfy the requirement of a minimum of two independent phases in the implementation of DQ controllers. a first-order all-pass filter is used to create the second orthogonal signal, which improves the response of the controller by removing the delay. The predictive current control is selected in the current control loop because of its faster dynamic response. Fig. 5. PV arrays simulation waveforms The DHB current is shown in Fig. 6. As can be seen the current reaches the same point after half switching period. Fig. 6. primary side current of the DHB converter The dc link voltage is shown in Fig. 7. As it was expected the dc link voltage is stabilized around 400V. The ripple twice the grid frequency is observed in the result. IV. Simulation Results The proposed inverter system was simulated in Matlab/Simulink. The simulated circuit parameters are shown in table I. Main simulation results are shown in Figs. 5-8. Fig. 7. dc link voltage simulation waveforms TABLE I. SIMULATION PARAMETERS Description Value PV Max Voltage 273.5V Dc bus average voltage 400V CICED2016 Session 3 Paper No 1009 The grid current and grid voltage is shown in Fig. 8. A unit power operation is achieved by the current control. Page/4 2016China International Conferenceon Electricity Distribution (CICED 2016) Fig. 8. grid current and grid voltage simulation waveforms V. CONCLUSION In this paper, a new topology for interconnection of PV modules to the grid is proposed. The topology consists of two stage converter: a DHB converter, and a grid connected inverter. The DHB converter implement the MPPT functions. The grid connected inverter injects sinusoidal current into the grid. A DQ frame predictive current control is employed to achieve the fastest dynamic response of the grid current. The simulation result shown the effectiveness of the proposed PV inverter system. REFERENCES [1] Dannier, A.; Rizzo, R., "An overview of Power Electronic Transformer: Control strategies and topologies," International Symposium on Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), pp.1552,1557, 20-22 June 2012. [2] E. Ronan, S. D. Sudhoff, S . F. Glover and D. L. Galloway, “Application of Power Electronics to the Distribution Transformer,” Conference Record of APEC 2000, New Orleans, February, 2000, pp. 861-867. [3] F. Wang, X. Lu, W. Wang, and A. Huang, "Development of distributed grid intelligence platform for solid state transformer," in Smart Grid Communications (Smart Grid Comm), 2012 IEEE Third International Conference on, Nov 2012, pp. 481-485. [4] T. Zhao, J. Zeng, S. Bhattacharya, M. E. Baran and A. Q. Huang, “An Average Model of Solid State Transformer for Dynamic System Simulation,” in Power & Energy Society General Meeting (PES), IEEE, 2009, pp. 1-8. [5] X.She, R.Burgos, G.Y.Wang, F.Wang, and A.Q.Huang, “Review of solid state transformer in the distribution system: from components to field application,” in Proc.IEEE.ECCE., 2012, pp.4077-4084. [6] She X, Huang A Q, Lukic S, et al. On Integration of Solid-State Transformer With Zonal DC Microgrid[J]. IEEE Transactions on Smart Grid, 2012, 3(2):975-985. [7] 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, pp.1320 -1333 2008. [8] Mirzahosseini R, Tahami F. A lifetime improved single phase grid connected photovoltaic inverter[C]. Power Electronics and Drive Systems Technology (PEDSTC), 2012 3rd. IEEE, 2012:234-238. [9] S. Poshtkouhi, M. Fard, H. Hussein, et al. A dual-active-bridge CICED2016 Session 3 Paper No 1009 Xi’an, 10-13 Aug, 2016 based bi-directional micro-inverter with integrated short-term Li-Ion ultra-capacitor storage and active power smoothing for modular PV systems[C]. Applied Power Electronics Conference and Exposition. IEEE, 2014:643 - 649. [10] 1. Knight, S. Shirsavar, and W. Holderbaum, "An Improved Reliability Cuk Based Solar Inverter With Sliding Mode Control" IEEE Trans. Power Electron., vol. 21, no. 4, pp. 1107-1115, Jul. 2006. [11] Zengin S, Boztepe M. Modified dual active bridge photovoltaic inverter for solid state transformer applications[C]. International Symposium on Fundamentals of Electrical Engineering. IEEE, 2014:1 - 4. [12] Zhao B, Song Q, Liu W, et al. Overview of Dual-Active-Bridge Isolated Bidirectional DC–DC Converter for High-Frequency-Link Power-Conversion System[J]. Power Electronics, IEEE Transactions on, 2014, 29(8):4091 -4106. [13] Hengsi Qin, Kimball, J.W. Closed-Loop Control of DC–DC Dual-Active-Bridge Converters Driving Single-Phase Inverters[J]. IEEE Transactions on Power Electronics, 2014, 29(2):1006-1017. [14] Villalva M G, Gazoli J R, Filho E R. Comprehensive Approach to Modeling and Simulation of Photovoltaic Arrays[J]. IEEE Transactions on Power Electronics, 2009, 24(5):1198-1208. [15] Villalva M G, Gazoli J R, Filho E R. Modeling and circuit-based simulation of photovoltaic arrays[J]. 2009, 14(1):1244-1254. [16] Haifeng Fan, Hui Li; "High Frequency High Efficiency Bidirectional DC-DC Converter Module Design for 10 kVA Solid State Transformer," IEEE Applied Power Electronics Conference, pp. 210-215, Feb, 2010 [17] Zhao T, Zeng J, Bhattacharya S, et al. An average model of solid state transformer for dynamic system simulation[C]// IEEE Power & Energy Society General Meeting. IEEE, 2009:1-8. [18] D. Dujic, A. Mester, T. Chaudhuri, A. Coccia, F. Canales, and J. K. Steinke, “Laboratory scale prototype of a power electronic transformer for traction applications,” in Proc. 14th European Conf. on Power. Electron. and Appl., 2011, pp. 1-10 Baolong Liu was born Changchun, China in 1988. He is currently working towards the Ph.D. degree in College of Information System and Management of National University of Defense Technology (NUDT). His main interests are power management and control in Energy Internet, control methods of power converters. E-mail: [email protected] Yabing ZHA was born in Jiangxi, China in 1968. He received his Ph.D. degree in system simulation from National University of Defense Technology (NUDT) in 1995. Now he is the dean of College of Information System and Management, NUDT. His research interests include conceptual modeling, system simulation and integration, simulation Verification, Validation and Accreditation (VV&A). Tao Zhang was born in Anhui, China in 1976. He received his Ph.D. degree in management science and engineering from NUDT in 2004. Now he is a professor of NUDT. His research interests include complex system modeling and optimization, battery management technology, energy internet technology. Page/4