Research On Key Technique Of Multi-unit Parallel Converter In Wind Power Generation System

As a result of increasing energy requirement and more concern of environment protection, the use of wind energy for electric power generation becomes a vital alternative to replace the conventional energy. The technologies on wind power generation become a research hotspot. Offshore wind power generation has becoming a more and more important research direction due to its characteristics. The converter is one of the core technologies in wind power system. With the increasing of the capacity of single wind turbine, how to realize high-power converter in the low-voltage conditions is an important problem demanding prompt solution. It is well know that multiphase converters can be paralleled to increase the power rating, reliability, efficiency, as well as decrease the cost and current/voltage ripples. In addition, the parallel structure is very suitable for modular system design and system reconfiguration, providing better flexibility. So the converter parallel technology has a wide range of applications in the high-power occasions.However, the circulating current will occur when the device parameters, switch states, control parameters and so on between paralleled converters are different. The circulating current will increase switch stress and compel the converters to run under reducing the operating power level. The circulating current will cause the over-current faults in serious cases. Thus, the control targets of parallel-converter is realizing the consistency in the frequency, amplitude and phase of output voltages of the parallel units and finally realizing the load-sharing between the parallel units. At present, the study of paralleled single-phase UPS is more, but the study of high-power three-phase wind power converters is less. So, the issues of this paper have important practical significance.First, an overview of domestic and international wind power development and trend is introduced in the paper. Then, several popular patterns of parallel converter are discussed. The control strategy of three-phase grid-connected converter is introduced. The general definition of circulating current is given in the paper and the concept of circulating current percentage is proposed to evaluate the amount of circulating current. And then the causes producing circulating current are analyzed based on the mathematical model of circulating current in parallel-converter system. The characteristics difference of circulating current between centralized control and distributed control mode is pointed out. For the distributed control structure, theoretical analysis and simulation results of the proposed zero-sequence circulating current control strategies are given in the paper. According to feedforward control theory, the circulating current control strategy based on circulating current feedforward compensation is proposed. The hardware circuits and control software are designed. The results of hardware in-the-loop simulation experiment are given based on RT-LAB. The experimental results show that the proposed circulating current control strategies are able to reduce the DC and low-frequency circulating current effectively.