Research On HF Z-source AC/AC Converter

Various kinds of AC/AC converters are commonly used for AC/ACpower conversion nowadays. The most popular topologies are indirect AC/ACconverters with DC link, direct pulse-width modulation (PWM) AC/ACconverters and matrix converters. For applications where only voltageregulation is need, direct PWM AC/AC converters such as the buck-boost andthe Cuk topology are preferred. These kinds of topologies have drawbacks:offering lower or higher output voltage, inverse polarity, complex topology,high harmonic current in line.To overcome the limits of traditional converters above, Z-source AC/ACconverters have been put forward. As a kind of direct PWM AC/ACconverters, the Z-source AC/AC converter has its uni-topology of LC network.The topology has merits such as simple circuit, easy adjustment and control,extensive applications for voltage regulation with modularity. The Z-sourceconverter can provide a larger range of output voltage, reversing ormaintaining phase angle, reducing in-rush and harmonic current. In this paper,solutions have been found for output voltage stabilization and harmonicsuppression of the Z-source AC/AC converter.The addition of Z-source network will have an effect on the dynamicproperty of the converter. The small signal model of Z-source network hasbeen created for the study of dynamic performance. The effects on the systemstability of LC values in Z-source network have been approached throughrelevant transfer functions. The close loop control has been designed to ensurethe output voltage stabilization. As the dynamic model and the close loopcontrol system were created based on the Z-source network, the results ofstudy are the same with other kinds of Z-source converters.Traditional researches on the output characteristic of Z-source AC/AC converter are commonly following the principles used in HF DC converters,which believe that the phase position and amplitude of output voltage is onlydetermined by the duty factor of the switch elements. Although this kind ofanalytical approach is simple, the impact of circuit parameters is ignoredbecause of the approximate treatment. In this paper, the state space averagemodel has been put forward, which takes the circuit parameters into account.The model takes all of the circuit parameters into consideration and offers amore precise mathematical model for the output characteristic analysis. Theimpacts of the Z-source network parameters on the amplitude and phase ofoutput voltage, and the stability of the system have been studied by timedomain simulation. The paper put forward several elements which can makeerrors of output voltage and the relevant solutions have been discussed as well.The output voltage can be stabilized by PI close loop control.The input voltage is generally considered as an ideal sine curve intraditional researches on PWM technology of converters. The possiblesituation of input harmonic distortion is always neglected. In many cases, theobjective reality of harmonic component in input voltage is far fromnegligible. Based on the traditional Z-source AC/AC converter, this paperproposed a novel harmonic elimination PWM technique which can eliminateall low order harmonic voltage contained in ac mains. By unsteady duty cyclecontrol, the new PWM technique can regulate the output pulse widthdynamically. It is achieved by the division signal between fundamentalcomponent and AC mains of input voltage. The controller only processesfundamental frequency but harmonics, consequently system complexity andcosts can be reduced effectively. Moreover, the modulation function is derivedwithout using any feedback harmonic content from output side. Thus it canoperate in simple open-loop mode, which means good stable and dynamicresponding performance. High power quality and voltage stabilization can beensured by the combination of the harmonic elimination PWM technique andPI close loop control.