Research On Control Strategy Of Dual Three Phase Permanent Magnet Wind Power System Based On Cascaded Vienna Rectifiers

With the increasing of single-machine capacity of wind turbines,direct-drive wind power systems place higher demands on the wind turbine converter topology.Two-level converter is no longer suitable for high-power wind power system due to its series of shortcomings,such as its inability to withstand high voltages and large output harmonics.Therefore,it is of great significance to study the topology of high-power wind power system,which can improve the efficiency of generation and increase the reliability of the system.In the multi-level converters,the power device bears a small voltage and the output harmonics are greatly reduced,while the multi-phase motors doubles the capacity of the system.Based on the advantages of multi-level converters and multi-phase motor,a scheme of dual three-phase PMSM wind power generation system based on Vienna rectifier cascade is proposed in this paper.The machine side consists of a dual three-phase PMSM and two series-connected Vienna rectifiers,and diode clamp type three-level converter is used on the grid side.This scheme can double the power of the system to meet the requirements of capacity and reliability of the wind power system.Firstly,the basic working principle of the system is analyzed,and two sets of mathematical models are established for the dual three-phase PMSG.One is a mathematical model based on vector space decoupling coordinate transformation,in which the dual three-phase motor is modeled in a unified manner.The model is similar to the single three-phase permanent magnet motor model,and the similar control strategy can be used.The other is a mathematical model based on double dq coordinate transformation,which regards the dual three-phase motors as two independent three-phase permanent magnet motors.Its advantage is high reliability,but the coupling between the two models can not achieve complete decoupling,which makes the control more complex.Based on the vector space decoupling model,the machine side adopt the double closed-loop vector control strategy of speed outer loop and current inner loop.A neutral point voltage balancing method based on carrier modulation is proposed for a single Vienna rectifier.By establishing the dynamic equation of capacitance voltage error,the zero sequence voltage can be constructed from the point of view of ensuringthe control stability,which can make the capacitance voltage error approach to zero and guarantee the good input current quality at the same time.The grid side adopt the double closed-loop vector control strategy of voltage outer loop and current inner loop to maintain the DC bus voltage stability.At the same time,the whole neutral point potential balance can be achieved by injecting zero sequence component from the modulation level.Based on the double dq coordinate transformation model,Because two sets of three-phase motor models are coupled with each other,the feedback linearization method is used to improve the current control loop,which realizes decoupling control and greatly simplifies the control structure.In order to solve the problem of global neutral point potential balance,a novel neutral point voltage balancing method for Equalizing power is proposed,which is based on characteristics of mathematical model of dual three phase motor.By balancing the power output of two Vienna rectifiers on the machine side,the whole midpoint potential balance is realized from the control plane,which simplifies the modulation of the grid-side converter to some extent.Finally,the correctness and validity of the two control strategies proposed in this paper are verified by the simulation platform of MATLAB/Simulink.