Research On The Droop Method For Power Converter Parallel Operation

The research on the parallel operation of power converters has become one of the hot issues in the field of power electronics. The parallel operation of power converters can increase the power rating of individual power converter and achieve redundant configuration, and then the reliability of the entire system can be improved. Consequently such configuration has been extensively utilized in the high performance power systems, such as computers, communication systems, and biomedical instrumentations. Control schemes of parallel operation can be mainly classified into two catalogues: master/slave control method and droop method. Droop control does not require any communication between converters, and therfore true redundancy can be achieved. Accordingly, droop method has drawn more and more attentions of researchers. This paper will mainly focus on the implementation approaches and the dynamic and steady performance analysis of droop method, and will systemically analyze and investigate the control schemes of improving the dynamic and steady performance. An experimental prototype comprising two three-phase AC/DC active power converters are constructed, and then theoretical analysis results are verified with it.There are mainly two approaches to provide output voltage droop characteristics in droop method: program voltage reference by load current and limiting the DC gain of the voltage loop compensator. In this paper, after reviewing some research results of literatures the design guidelines of these two approaches are provided, and the transient and steady performance of these two approaches are analyzed and compared in detail.In this paper, for the first time the method of using close-loop output impedance to analyze the transient performance of power converter system in parallel operation is proposed, and the relationship of close-loop output impedance and transient performance of parallel system is discussed, and then the requirements the close-loop output impedanceshould meet to guarantee good transient performance are derived. These requirements are defined as ideal close-loop output impedance in this paper. Based on the ideal close-loop output impedance, a kind of controller design method based on output impedance is proposed, and the maximum frequency bandwidth of output impedance is defined to facilitate the implementation of the output impedance based controller design method.Control schemes of improving the steady performance of droop method are analyzed in detail in this paper, and the disadvantages of nonlinear droop method and droop method with restoration are pointed out. Finally, a control method with adaptively close-loop output impedance adjustment is proposed to overcome the adverse effect of line impedance on the steady performance of droop method.