High Power Wind Turbine HVDC Transmission Technology Based On Multi-phase Motor

The demand for new energy is growing fast under the pressing situation of energy shortage.As one of the most important part of new energy,the wind power generation is widely used.In high power field,the multi-phase motor has shown its superior performance and is attaching more and more attention.This paper focuses on the high voltage direct current grid-connected topology of multi-phase wind power generation system and the control method of the module multi-level converter on the high voltage direct current receiving side.Considering the special structure of multi-phase motor,a new topology of high voltage direct current grid-connected is proposed,which is characterized by one multi-phase motor with multiple symmetrical three phase windings,and one diode rectifier converter following each three phase winding.Each diode rectifier is followed by paralleled DC-DC converters and series sub-modules.The dynamic model of wind turbine is firstly introduced to explain the maximum power point tracking realization.Then the theoretical reason of the superior advantage of multi-phase motor on the elimination of electromagnetic torque ripple caused by low order harmonic stator current is analyzed.Next,two coordinate transformation methods used for modeling the multi-phase motor are introduced.The differences of the two electromagnetic models based on the two coordinate transformations are displayed.Then the working states of the DC-DC circuits are analyzed and its basic model and control theory are deducted.Finally the switching states of sub-module in series are researched and the principle of the high voltage direct voltage grid-connected is analyzed.To achieve the maximum power point tracking of the proposed topology,the traditional control scheme for three phase motor is extended.The control structure of outer speed loop and inner current loop for each three phase winding is adapted.The power transmission process is analyzed and by induced the relationship of the sub-module capacitance voltage and transmission current,the control scheme of outer capacitance voltage loop and inner transmission current loop is proposed.Computer models based on the MATLAB is established and the simulation results prove that the proposed topology shows good performances on the steady and transient process as well as the fault condition.The mathematical model of modular multilevel converter on the high voltage direct current transmission receiving side is induced.The result shows that the modular multilevel converter shows the same output characteristics with the two level converter,so that the traditional dual loop control of voltage loop and current loop for two level converter fits modular multilevel converter,too.The principles and characteristics of two modulation schemes for modular multilevel converter is introduced.The droop control for modular multilevel converter is explained and the shortage on balancing the frequency and voltage performances caused by converter power limitation under large load change is also explained.Based on the analysis,one new droop control method is proposed,which controls the frequency and voltage collapses in the same speed so that the frequency and voltage performances can be satisfied under large load change.Finally,simulation results prove that the proposed droop method shows the same performance with the traditional droop method under small load change,but much better performance under large load change.