Research On Parallel Control Technology For Three-phase Grid-tied Inverters

The parallel operation of grid-tied inverter is an effective way to improve the grid-tied power for new energy generating device. It can reduce design costs and realize modular design in parallel. The parallel control technology is a research focus in the field of power conversion, and the circulating current restraining is a difficulty for the parallel of grid-tied inverters. According to this problem, this thesis launched a research which was about the parallel control technology for three-phase grid-tied inverters. The main contents are as follows:Established a parallel topology, and analyzed the generating mechanism of circulating current. For a variety of parallel control structures were analyzed and compared, then according to their characteristics and actual demand, the parallel topology of grid-tied inverter was established; The generating mechanism of circulating current in parallel system was studied based on the pathway of circulating current. The impact of the circulating current on the parallel system was expatiated, and quantitative analysis for circulating current was carried out.Developed the averaged model of circulating current in parallel system, and researched the current control strategy of parallel system. The averaged model of circulating current and duty cycle in parallel system was developed, and the relationship between circulating current and the zero sequence duty cycle was studied. The current control strategy of parallel module was analyzed, and the difference of current control strategy for parallel system and single module was described.Proposed restrain schemes of circulating current in parallel system. A zero sequence voltage compensation scheme was put forward based on SVPWM, and according to the defect which was difficult to realize modular design, a variable zero-vector SVPWM was proposed. Two SVPWM modulations were studied, and a conclusion was obtained which was the distribution of zero vectors' action time can't effect system control targets, then, an improved SVPWM modulation for variable zero-vector was proposed. The zero sequence duty cycle was changed by regulating the distribution of zero vectors, and then the circulating current of parallel system was inhibited. The control structure of circulating current was given, and the implementation methods of variable zero-vector SVPWM which was applied to two and multiple grid-tied inverters were studied.Put up the simulation and experimental platforms for the 50 kW three-phase grid-tied inverter. The simulation circuit of two 50 kW parallel system was build based on the parameters and data of 50 kW three-phase grid-tied inverter, and two restrain schemes of circulating current and the main impact of circulating current were validated by simulations. The circulating current phenomenon and the inhibitory effect of circulating current by using variable zero-vector SVPWM under the differences of filter inductors, line impedance, trigger pulse, switching frequency and dead time were analyzed and validated in detail. In addition, the inhibitory effect of circulating current for multiple parallel systems and fault clearing of single module were validated. Finally, the method of software implementation for variable zero-vector SVPWM was described. The simulation results and data show the feasibility of restrain scheme for circulating current by using variable zero-vector SVPWM correctly.