Quasi-Z-Source Inverter for Photovoltaic Power Generation Systems Yuan Li 1,2 , Joel Anderson 2 , Fang Z. Peng 2 , and Dichen Liu 1 1. Dept. of Electrical Engineering, Wuhan University, Hubei 430072, China 2. Dept. of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USA Abstract-This paper presents a quasi-Z-source inverter (qZSI) that is a new topology derived from the traditional Z-source inverter (ZSI). The qZSI inherits all the advantages of the ZSI, which can realize buck/boost, inversion and power conditioning in a single stage with improved reliability. In addition, the proposed qZSI has the unique advantages of lower component ratings and constant dc current from the source. All of the boost control methods that have been developed for the ZSI can be used by the qZSI. The qZSI features a wide range of voltage gain which is suitable for applications in photovoltaic (PV) systems, due to the fact that the PV cell’s output varies widely with temperature and solar irradiation. Theoretical analysis of voltage boost, control methods and a system design guide for the qZSI in PV systems are investigated in this paper. A prototype has been built in the laboratory. Both simulations and experiments are presented to verify the proposed concept and theoretical analysis. I. INTRODUCTION Photovoltaic (PV) power generation is becoming more promising since the introduction of the thin film PV technology due to its lower cost, excellent high temperature performance, low weight, flexibility, and glass-free easy installation. However, there are still two primary factors limiting the widespread application of PV power systems. The first is the cost of the solar cell/module and the interface converter system; the second is the variability of the output (diurnal and seasonal) of the PV cells. A PV cell’s voltage varies widely with temperature and irradiation, but the traditional voltage source inverter (VSI) cannot deal with this wide range without over-rating of the inverter, because the VSI is a buck converter whose input dc voltage must be greater than the peak ac output voltage. Because of this. a transformer and/or a dc/dc converter is usually used in PV applications, in order to cope with the range of the PV voltage , reduce inverter ratings, and produce a desired voltage for the load or connection to the utility. This leads to a higher component count and low efficiency, which opposes the goal of cost reduction. The Z-source inverter (ZSI) has been reported suitable for residential PV system [1-2] because of the capability of voltage boost and inversion in a single stage. Recently, four new topologies, the quasi-Z-source inverters (qZSI), have been derived from the original ZSI [3]. This paper analyzes one voltage fed topology of these four in detail and applies it to PV power generation systems. By using the new quasi-Z- source topology, the inverter draws a constant current from the PV array and is capable of handling a wide input voltage range. It also features lower component ratings and reduced source stress compared to the traditional ZSI. A prototype which provides three phase 60-Hz, 208-V llrms ac has been built in laboratory. It is demonstrated from the theoretical analysis, simulation and experimental results that the proposed qZSI can realize voltage buck or boost and dc-ac inversion in a single stage with high reliability and efficiency, which makes it well suited for PV power systems. II. CIRCUIT ANALYSIS OF THE QUASI-Z-SOURCE INVERTER A. Quasi-Z-Source Inverter Circuit Figs. 1a and 1b show the traditional voltage fed ZSI [4] and the proposed voltage fed qZSI, respectively. In the same manner as the traditional ZSI, the qZSI has two types of operational states at the dc side: the nonshoot-through states (i.e. the six active states and two conventional zero states of the traditional VSI) and the shoot-through state (i.e. both switches in at least one phase conduct simultaneously). In the non-shoot-through states, the inverter bridge viewed from the dc side is equivalent to a current source. The equivalent circuits of the two states are as shown in Figs. 2a and 2b. The shoot-through state is forbidden in the traditional VSI, because it will cause a short circuit of the voltage source and damage the devices. With the qZSI and ZSI, the unique LC and diode network connected to the inverter bridge modify the operation of the circuit, allowing the shoot-through state. This network will effectively protect the circuit from damage when the shoot-through occurs and by using the shoot-though state, the (quasi-) Z-source network boosts the dc-link voltage. The major differences between the ZSI and qZSI are (1) the qZSI draws a continuous constant dc current from the source To AC Load or Motor C 1 C 2 L 1 L 2 D V in + - Figure 1a. Voltage fed Z-source inverter C 1 D L 1 L 2 C 2 V in + - To AC Load or Motor Figure 1b. Voltage fed quasi-Z-source inverter 978-1-422-2812-0/09/$25.00 ©2009 IEEE 918 Authorized licensed use limited to: Tsinghua University Library. Downloaded on July 05,2010 at 10:12:11 UTC from IEEE Xplore. Restrictions apply.