Transient modeling and analysis of pulse-width modulated Z-source inverter

The newly proposed Z-source inverter has been proven in the literature to exhibit both steady-state voltage buck and boost capabilities using a unique LC impedance network coupled between the power source and converter circuit. This paper now presents transient modeling and analysis of a voltage-type Z-source inverter. These aspects are found to be challenging and they need to be carefully investigated before attempting to design advanced control algorithms for controlling the Z-source inverter. Through detailed analysis, the paper identifies several phenomena on the DC and AC-sides of the inverter, which would result in the inverter having an inferior non-minimum-phase transient response. The DC-side phenomenon is associated with the Z-source impedance network, which is shown through small-signal and signal-flow-graph analyses to be having a right-half-plane zero in its control-to-output transfer function. Also, the AC-side phenomenon is shown through space vector analysis to depend on the time intervals of inverter states used for reconstructing the desired inverter output voltage. Based on the AC vectorial analysis, a method for improving the inverter transient response is also presented. Lastly, simulation results obtained using a switching-functional model and experimental results obtained using a laboratory prototype are presented for validating the described theoretical concepts.

I. INTRODUCTION

Z-source inverters are new single-stage electronic power converters with both voltage-buck and boost capabilities that have been proposed for use in fuel cell energy conversion systems [1] and motor drives with a front-end diode rectifier [2], [3]. Compared with traditional voltage-source (VS) and current-source (CS) inverters, the sole difference of a Z-source inverter is its X-shaped impedance network implemented using a splitinductor ( and ) and capacitors ( and ), and connected between the input power source and inverter circuit, as shown in Fig. 1 for a VS-type Z-source inverter. (VS-type inverter is chosen for study in this paper because it is generally more established and can conveniently be constructed using low-cost, high-performance insulated gate bipolar transistor (IGBT) modules or intelligent power modules). This unique impedance network allows both switches within the same phase-leg (e.g. S1 and S4 in Fig. 1) to be turned ON simultaneously to introduce a shoot-through (dc bus short-circuiting) state for boosting the inverter output voltage (similar in theory to that of a dc-dc boost converter). As reported in [4], multiple shoot-through states can conveniently be introduced to traditional continuous and discontinuous pulse-width modulation (PWM) strategies for controlling a Z-source inverter with all steady-state performance merits of these PWM strategies retained.