Research On Stability Of The Three-phase Half-bridge Voltage Converters In Parallel Operation

With energy and environmental issues becoming increasingly prominent, distributed power generation system has become a hot topic of research. Converter has been a very important part of distributed generation system to exchange energy. The stability of the system may be affected when the converters are operated in parallel.Therefore the study on the stability of the converters in parallel operation has great theoretical significance and applicable value.Firstly, the stability criterion of impedance is introduced as the theory foundation. Then the stability analysis and the output impedance model separately for the converter system with the filter function of LC and LCL are proposed in this paper. A method based on the passive damping is put forward, not only to inhibit the resonant peak of LCL filter but also to increase resistance in capacitor branch and keep the system stable.Secondly, to research the effects of output impedance interactions on system stability, take parallel system about two converters as example. Among them, LC filter is a voltage-source converter that can control the amplitude and frequency of the output voltage, and LCL filter is a current-source converter. On the basis of the stability criterion of impedance, the relationship between stability of the parallel converter and the output impedance ratio of the LC and LCL filters is presented. Theoretical analysis indicates that output impedance interactions have a great impact on the stability of the parallel system. And the system can reach a stable stage by changing the control parameters. The simulations of time domain are carried out, and the results demonstrate the validity of theory analysis.Finally, an experiment platform is built based on two 7.5 kW three-phase converters, and then the stability analysis of single converter and parallel converter are all validated by experiment. The harmonic analysis and the waveforms of output current and voltage are pointed out, and the experiment results prove the validity of theory analysis and simulations.