Research On The Circulation Analysis And Suppression Strategy Of Three-Phase Grid-Connected Converter

Grid-connected converters are the core equipment in new energy generation,and the development level of converters plays a vital role in the wide application of new energy.With the rapid increasing of new energy generation,the demand for large-capacity converters is also growing.In the practical application of new energy power generation systems,multiple small-capacity modules are commonly used in parallel to improve the system power capacity.The parallel three-phase grid-connected converters sharing DC and AC bus can increase the system power level effectively.However,the circulating current will occur inevitably.The presence of circulation will increase the loss of power devices,reduce the current control effect,and may lead to equipment failure.Therefore,it is very important to study the mechanism of the parallel circulation of the converter and propose the suppression strategy.In this paper,the parallel three-phase grid-connected converter system is introduced firstly.And then the mathematical model of grid-connected converter is established,which includes the mathematical model of three stationary coordinate systems and two-phase synchronous rotation coordinate system.Aiming at the circulating current problem in the parallel system,the circulating current caused by asynchronous carriers and the modulation carriers are analyzed respectively,and the characteristic of circulating current in one cycle is obtained.The high frequency circulating current caused by asynchronous carriers is studied emphatically in this paper,and a novel method to suppress high frequency circulating current caused by asynchronous carriers of modules is proposed.In this method,the carrier period of slave module is adjustable and the carrier period of the master module is constant.When a large phase shift between modules occurs,an adjusting cycle is inserted to carrier of slave module to realize re-synchronizing with master module.Besides,the circulation problems caused by the injection of different zero-sequence voltages are studied.And a modulation scheme combining SPWM and SVPWM is proposed,which can increase the voltage utilization,also can avoid the low-frequency circulation caused by the injection of different zero-sequence voltages.Finally,based on the three typical cases causing zero-sequence circulating current(ZSCC),the corresponding suppression methods were proposed at the source.The proposed two methods were easy to be implemented without adding any hardware,and no communication was needed between modules except the dq current reference.With the proposed methods,ZSCC suppression was achieved satisfactorily,which was well proven by experimental results.The investigation presented in this paper can fully suppress ZSCC,which makes it more applicable in practice.In this paper,the circulation in the parallel system of three-phase grid-connected converter is taken as the research object.From the perspective of the converter modulation strategy,the different causes of the circulation are analyzed in depth.According to the generation mechanism of the circulation and its related characteristics,corresponding suppression strategies are proposed to meet the requirements of safe and efficient operation of the parallel system.