992 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 4, APRIL 2010 Design and Analysis of a Grid-Connected Photovoltaic Power System Bo Yang, Wuhua Li, Member, IEEE, Yi Zhao, and Xiangning He, Fellow, IEEE Abstract—A grid-connected photovoltaic (PV) power system with high voltage gain is proposed, and the steady-state model analysis and the control strategy of the system are presented in this paper. For a typical PV array, the output voltage is relatively low, and a high voltage gain is obligatory to realize the grid-connected function. The proposed PV system employs a ZVT-interleaved boost converter with winding-coupled inductors and active-clamp circuits as the first power-processing stage, which can boost a low voltage of the PV array up to a high dc-bus voltage. Accordingly, an accurate steady-state model is obtained and verified by the sim- ulation and experimental results, and a full-bridge inverter with bidirectional power flow is used as the second power-processing stage, which can stabilize the dc-bus voltage and shape the output current. Two compensation units are added to perform in the sys- tem control loops to achieve the low total harmonic distortion and fast dynamic response of the output current. Furthermore, a simple maximum-power-point-tracking method based on power balance is applied in the PV system to reduce the system complexity and cost with a high performance. At last, a 2-kW prototype has been built and tested to verify the theoretical analysis of the paper. Index Terms—Bidirectional power flow control, compensa- tion units, direct current control, maximum-power-point-tracking (MPPT) method, photovoltaic (PV) system, steady-state model. I. INTRODUCTION T ODAY photovoltaic (PV) power systems are becoming more and more popular, with the increase of energy de- mand and the concern of environmental pollution around the world. Four different system configurations are widely devel- oped in grid-connected PV power applications: the centralized inverter system, the string inverter system, the multistring in- verter system and the module-integrated inverter system [1]–[4]. Generally three types of inverter systems except the centralized inverter system can be employed as small-scale distributed gen- eration (DG) systems, such as residential power applications. The most important design constraint of the PV DG system is to obtain a high voltage gain. For a typical PV module, the open-circuit voltage is about 21 V and the maximum power point (MPP) voltage is about 16 V. And the utility grid voltage is 220 or 110 Vac. Therefore, the high voltage amplification is obligatory to realize the grid-connected function and achieve the low total harmonic distortion (THD). The conventional system Manuscript received February 28, 2009; revised July 27, 2009 and August 29, 2009. Current version published April 9, 2010. This work was supported by the National Nature Science Foundations of China (50737002 and 50777055) and by the China Postdoctoral Science Foundation (200902625). Recommended for publication by Associate Editor R. Burgos. The authors are with the College of Electrical Engineering, Zhejiang Uni- versity, Hangzhou 310027, China (e-mail: yangbo@zju.edu.cn; woohualee@ zju.edu.cn; diabloturen@zju.edu.cn; hxn@zju.edu.cn). Digital Object Identifier 10.1109/TPEL.2009.2036432 requires large numbers of PV modules in series, and the normal PV array voltage is between 150 and 450 V, and the system power is more than 500 W. This system is not applicable to the module-integrated inverters, because the typical power rating of the module-integrated inverter system is below 500 W [3], [4], and the modules with power ratings between 100 and 200 W are also quite common [5]. The other method is to use a line frequency step-up transformer, and the normal PV array volt- age is between 30 and 150 V [3], [4]. But the line frequency transformer has the disadvantages of larger size and weight. In the grid-connected PV system, power electronic inverters are needed to realize the power conversion, grid interconnec- tion, and control optimization [6], [7]. Generally, gird-connected pulsewidth modulation (PWM) voltage source inverters (VSIs) are widely applied in PV systems, which have two functions at least because of the unique features of PV modules. First, the dc-bus voltage of the inverter should be stabilized to a specific value because the output voltage of the PV modules varies with temperature, irradiance, and the effect of maximum power-point tracking (MPPT). Second, the energy should be fed from the PV modules into the utility grid by inverting the dc current into a sinusoidal waveform synchronized with utility grid. Therefore, it is clear that for the inverter-based PV system, the conversion power quality including the low THD, high power factor, and fast dynamic response, largely depends on the control strategy adopted by the grid-connected inverters. In this paper, a grid-connected PV power system with high voltage gain is proposed. The steady-state model analysis and the control strategy of the system are presented. The grid- connected PV system includes two power-processing stages: a high step-up ZVT-interleaved boost converter for boosting a low voltage of PV array up to the high dc-bus voltage, which is not less than grid voltage level; and a full-bridge inverter for in- verting the dc current into a sinusoidal waveform synchronized with the utility grid. Furthermore, the dc–dc converter is respon- sible for the MPPT and the dc–ac inverter has the capability of stabilizing the dc-bus voltage to a specific value. The grid-connected PV power system can offer a high voltage gain and guarantee the used PV array voltage is less than 50 V, while the power system interfaces the utility grid. On the one hand, the required quantity of PV modules in series is greatly reduced. And the system power can be controlled in a wide range from several hundred to thousand watts only by changing the quantity of PV module branches in parallel. Therefore, the proposed system can not only be applied to the string or multi- string inverter system, but also to the module-integrated inverter system in low power applications. On the other hand, the non- isolation PV systems employing neutral-point-clamped (NPC) 0885-8993/$26.00 © 2010 IEEE