Research On Control Strategy For DFIG Connected To DC-Microgrids

With the construction and operation of large-capacity wind power base and trans-regional AC/DC power transmission passage, the influence of wind power generation on power system becomes more and more noticeable. Alongside DC-Microgrids technology and HVDC also have beening mature. In the topology with doubly-fed induction generator(DFIG) connected to DC-Microgrids, the energy conversion efficiency of the conventional way of connecting to the grid is reduced and its reliability gets worse. Therefore, the research on control strategy for DFIG connected to DC-Microgrids has great significance.Aiming to the application of the DFIG connected to DC-Microgrids, an improved topology for the DFIG connected to DC-Microgrids is taken into account in this thesis. The stator side loses the support of voltage and frequency of AC point of common coupling bus. A novel control method suitable to the stator side converter(SSC) and the rotor side converter(RSC) of the topology is proposed. First of all,SSC is controlled by air-gap electromotive-force orientation(AEO) with dynamic component of air gap excitation current. RSC is controlled by stator flux-linkage orientation(SFO) with dynamic component of stator excitation current. The independent control of stator voltage and frequency, the decoupled control of power and variable speed constant frequency of DFIG are achieved in the doubly-fed induction generator connected to DC-Microgrids. Besides, for the bus voltage variation of DC point of common coupling bus, a feedforward control of power is adopted, which can enhance the capacity of active power transmission of DFIG during the voltage variation. Eventually, the voltage of the stator side is generated by the modulation of the SSC in the improved topology, especially under the circumstance with the asymmetric fault of stator side, DFIG’s electromagnetic torque, active power and reactive power are analysed, it is found that torque and power contain ripples at twice fundamental frequency, which increases fatigue degree of drive shaft for the generator side and amplifies ripple of grid-connected power for the grid side. Therefore, a novel control method suitable to stator side converter and rotor side converter based on reduced-order resonant controller(RORC) is proposed in this thesis, DFIG’s torque and output power performance are improved. Under the RORC control conditions the transfer functions of stator current and torque control system are established, the amplitude characteristic andthe system stability of RORC control are analysed. The simulation results in Matlab/Simulink verify the correctness and validity of the proposed method.