Research On The Ripple Characteristic Of Z-source Inverters

The Z-source inverter(ZSI), as first proposed by Prof. F. Z. Peng, is a novel power converter topology with potentially attractive properties. The dc-bus voltage of the network can be easily adjusted. Shoot-through states are allowed in the ZSI which improves its reliability. In the process of ZSI design, the inductor current ripple and capacitor voltage ripple affect the parameter selection and control of ZSI, while the output ripple affects the output current. In this paper, we call them as ripples, which have been studied as follows.In order to reduce the volume of Z-Source inductor, there are generally two kinds of measures. One is adopting coupled inductor or other novel topologies and the other is improving modulation method to decrease the inductance current ripple. Different modulation methods result in different current ripples. In this paper has given the readers a basic calculation methods for ripples.The inductor current ripples under SVM6 and SVM4 modulation stragies were comprehensively analyzed, achieving the universally applicable calculation formula for current ripples. However, this paper proposed that the conclusion above is not precise. With simple SVPWM control, the current ripple of the Z-source inductor will be related to the phase angle θ of reference vector. In some angle intervals, the SVM6 results in the lower current ripples when compared to SVM4. While in other intervals, the conclusion will be opposite. The maximum current ripple of SVM4 in line period is lower than that of SVM6.With large inductance, the Z-source capacitor voltage ripple has been traditionally looked on as irrelevant to the inductance of the Z-source network. It is believed that the capacitor is discharged in the shoot-through state while is charged in the nonshootthrough state. However, the Z-source capacitor voltage ripple is dependent on inductance when a smaller inductor is used. In this case, the capacitor is both charged and discharged while in the nonshoot-through state. As such, using traditional calculation methods to ascertain the capacitor voltage ripple will result in big errors. In this work, an analysis of the critical inductance and capacitor voltage ripple for bi-directional ZSIs was presented. This work described the division of the operational modes of the bi-directional ZSI into Complete Inductor Energysupplying Mode(CIEM), Incomplete Inductor Energy-supplying Mode(IIEM), Zero-crossing Input Current Mode(ZPCM), Non-zero-crossing Input Current Mode(NPCM), Zero-crossing Inductor Current Mode(ZICM), and Non-zerocrossing Inductor Current Mode(NICM) based on varying Z-source inductances. The capacitor voltage ripple with small inductors was also derived in this paper. The accuracy of the proposed approach is higher than others.Output current ripple is an important issue for design and control of inverters. For example, it influences the torque ripple and noise amplitude for a motor drives. An analysis of the output current ripple of Z-source inverter(ZSI) was presented in this paper. The expression of current ripple under SPWM and SVPWM was derived. With SPWM control, VSI results in the sanme current ripples compared to ZSI.While with SVPWM control, In most angle intervals, VSI results in the lower current ripples compared to ZSI. Compared to the constant DC-link voltage stragy, the variable DC-link voltage stragy is more helpful to improve the ripples problem. By shifting between the shoot-through and non-shoot-through states, capacitors and inductors are charged and discharged, which boosts the dc-bus voltage.However, adding the shoot-through state provides a challenge to the design of the snubber circuit. It is found that traditional C, RC, RCD, and RCD clamp snubber circuits are not suitable for ZSI. When these circuits are used with ZSI, the shoot-through state causes larger energy losses, which greatly lowers the efficiency of ZSI. This paper presents an improved RCD snubber circuit for ZSI applications, which resolves the problems of voltage distortion and large energy losses. The result shows that the proposed RCD snubber circuit suppresses voltage spikes perfectly and the system efficiency has been improved.Finally, inductor,capacitor and RCD snubber circuit design was carried out by the author based on the analysis of the ripples.The power devices were selected also. An experiment platform of motor control supplied with variable DC-link voltage for the PMSM was set up. The result shows that the system’s steady state and dynamic performance was good.