Study On Direct Power Control Strategy For Open-winding Brushless Doubly-fed Wind Power Generator

Taking the project supported by National Natural Science Foundation of China(NSFC)and the Natural Science Foundation of Liaoning Province as background,aiming at the novel brushless doubly-fed generator(BDFG)applied in variable speed constant frequency wind energy conversion system(VSCF-WECS),a new direct power control(DPC)method with maximum power point tracking(MPPT)for BDFG and OW-BDFG with the open-circuited control winding fed by the dual two-level converters are presented.According to the relationship between the MPPT and the motor speed,the mechanisms of MPPT and VSCF are all implemented by selecting the voltage space vector switch combination strategy in control winding machine side converter(s).The main study contents in this thesis are as follows:(1)For the requirement of the reliability and the higher price performance ratio by generator in VSCF-WECS,a brushless type brushless doubly-fed wind power generator system is put forward.The space vector mathematical model of the BDFG in the three phase stationary and two phase arbitrary rotating coordinate systems are all derived in detail,the mechanism of “pole converter” under the rotor magnetic field modulation,the electromagnetic torque and power expressions are all given.The new hybrid rotor structure with higher magnetic field coupling ability and the mechanism of VSCF-WECS are all discussed,then the power flow relations of the BDFG under different operation modes are analyzed according to the three slip ratio definitions and the power expressions.(2)Considering the increasing objective development demand of the single machine capacity in VSCF-WECS,a novel kind of open-circuited control winding fed by the dual twolevel converters topology is presented for the first time.The voltage space vector and its sector distribution of the new three-level topology using the proposed dual two-level convereters fed with the isolated and common DC bus are given out.Compared with the traditional two-level converter and three-level neutral point clamped converter,the proposed OW-BDFG topologies only have half voltage and rated capacity in power devices and converter(s),but have the higher redundancy.Then,the realtionship of voltage and current in power and control winding are provided.At the same time,the common mode voltage elimination strategy for the OW-BDFG using a common DC bus is analyzed.In addition,according to the feature curves of wind trubine,the MPPT is implemented by using the selected dual two-level converters voltage space vector optimized by the minimum switching loss for OW-BDFG in VSCF-WECS.(3)Aiming at the stricter power control requirements,especially for the large capacity offshore WECS,the DPC method based on the direct torque control(DTC)mechanism and the instantaneous reactive power theory is adopted,which more suitable for the quick control of real and reactive power(hence the power factor).Combined with the proposed OW-BDFG fed by the dual two-level converters via the isolated DC bus,the DPC strategies with MPPT using the different optimized vectors(i.e.,short vector,middle vector and long vector)are designed respectively for OW-BDFG.Meanwhile,using the advantage of the higher redundancy in the OW-BDFG fed by the dual two-level converters,a type of hybrid vector scheme combinated with the middle and long vectors is adopted to optimize the DPC,the real and reactive power error comparison direct power control(PEC-DPC)and the sliding mode variable structure direct power control(SMVS-DPC)are proposed,and its controller is designed by Lyapunov stability theory.Based on the MATLAB/Simulink software,the detailed simulation results are carried out in depth for the BDFG and OW-BDFG of the above mentioned DPC strategies,which not only the correctness of the proposed DPC strategy is validated,but also the outstanding effectiveness of the optimized DPC strategy for the OW-BDFG is highlighted.(4)Selecting the SEED-DEC28335 board as hardware core of the control system,the laboratory test rig with DPC strategy for OW-BDFG in VSCF-WECS is built and implemented,where using the induction motor driven by its inverter as the prime mover to simulate the wind turbine operation,the main circuit topology is selected with the traditional two-level back-toback converter(s),then the DPC strategy with MPPT experimental test rig for BDFG and OWBDFG in VSCF-WECS are conducted and investigated,and the correctness,validity and feasibility of the proposed DPC for BDFG and OW-BDFG are all confirmed by the experimental results,which can provide a necessary basic data and technical reserves for the subsequent deep research about the related control method and strategy in OW-BDFG and for its population and application in VSCF-WECS.