Control Of Circulating Current In The Parallel Direct-driven PMSG Wind Generation System

Recently the wind power generation system has been paid a lot of attention. Among all the techniques of wind power generation, the permanent magnet synchronous generator (PMSG) based wind power system is considered the most advanced one, due to its fine control performance and improved grid-connected power quality. As the unit capacity of PMSG wind generation system become larger, the capacity for converters should also be increased, so the high power conversion system becomes widely used.Among all the high power conversion techniques, the parallel converter system is the most advanced one. There are many topologies and control strategies for parallel converters system. According to the characteristic of wind power and its applicability, and to realize the modularization and redundancy design, the parallel technique is used in the converter design. This paper focuses on parallel converters system with common DC bus, which has established control techniques and is proper for modularization. Also, to reduce the harmonics of the grid current, carrier phase shifting (CPS) technique is used in the parallel system.However, there two key problems in the parallel converters, which are the current sharing of the units and the control of circulating current:current sharing of the units is very important for load sharing in the system, and the circulating current can increase the THD of each unit in the system. All these problems cause the increasing of current harmonics and have negative effects on the normal operation of the parallel system. On the meantime, although CPS improves the grid-connect power quality, it also brings the low-frequency circulating current, which increase the harmonics of ac-side converter current. To solve the problems mentioned above, a circulating current controller is designed based on the analysis of circulation mechanism and mathematic model. The controller uses the feedback of circulation as well as the control of driving signals to restrict the circulating current. The simulation takes different factors into consideration to find if the controller functions well. According to the simulation results, the circulating current controller can reduce the circulation obviously and decrease the harmonics of the current, which verifies the feasibility and excellent performance of the circulating current controller.