Research On Control Strategy For Stand-alone DFIG-based Wind Power Systems

As the global energy shortage and environmental pollution caused by traditional energy generation become increasingly critical, wind power system has been rapidly developed due to its relatively mature technology and commercial potential, where AC-excited doubly-fed induction generator (DFIG) is the mainstream of wind turbine. Nowadays, the grid-connected wind power systems based on DFIG have been widely studied, but it cannot supply power for those areas where the extension of grid is not economically viable. However, the stand-alone wind power systems can meet the power demand in the remote areas nearby without the power grid support, which is of great significance to improve the environment, ease the power supply pressure and so on. This paper is dedicated to investigate a stand-alone DFIG-based wind power system and the related control schemes under the conditions of unbalance load, nonlinear load and energy storage, as well as the parallel operation control of multiple stand-alone systems. The related contents can be summarized as follows:(1) On the basis of the mathematical model of the DFIG, the power transmission characteristics in the different generation modes are analyzed in detail. Then, a control scheme of the stand-alone wind power system is proposed under the balanced load condition, which includes the vector-control strategies for the stator-side converter (SSC) and rotor-side converter (RSC) by means of the decoupling function of the intermediate DC capacitor. Simulation and experimental results verify the effectiveness of the proposed control scheme.(2) Due to the fact that unbalanced loads may cause the fluctuations of twice fundamental frequency of the active and reactive power, the electromagnetic torque on the stator side of the DFIG, a decoupled control scheme for the SSC and RSC is presented. To compensate the unbalanced stator currents, a three-phase four-leg converter is used as the SSC, which offers current path for the zero and negative sequence currents. As for the RSC, a direct voltage control method is presented to provide excellent voltage profile for the load, and then, an extended state observer is used to estimate the disturbance of the rotor current control loop, which can enhance the dynamic response and robustness of the stand-alone system. At last, simulation and experimental results verify the feasibility and effectiveness of the proposed control scheme.(3) To suppress the output voltage harmonics caused by the nonlinear loads, a control method for the RSC functioned as an APF is proposed in the stand-alone DFIG system, which results in an increase of the stator current harmonics. Then, an improved-control scheme is presented to reduce the harmonics of the output voltages and stator currents, at the same time providing the sinusoidal stator voltages. And the simulation results demonstrate the feasibility and effectiveness of the proposed methods.(4) Because of the intermittent and unpredictable nature of wind power, the output power fluctuation and the output voltage flicker would occur. For these reasons, a viable topology is proposed for the DFIG wind power system, where an energy storage device is installed on the intermediate DC-link of the converter. Based on this topology, two new control schemes are presented:one is that the SSC undertakes the stator voltage control, and the RSC is for the control of maximum power point tracking (MPPT); the other is that the SSC and RSC control the dc-link voltage and stator voltage, respectively, and the energy-storage converter is for the control of maximum power point tracking. The simulation results verify the correctness and effectiveness of the proposed topology and control schemes.(5) When the number of loads and power demand become larger, the stand-alone wind power system based on only-one DFIG may fail. To overcome this drawback, a droop-controlled scheme for power balance without communication is presented to control the parallel operation of multiple DFIGs. Based on the model of two DFIGs working in parallel under normal and load-switching situations, control schemes for the SSC and RSC are presented to obtain three-phase balanced stator voltage and steady dc-link voltage. At last, two DIFGs working in parallel are conducted on the simulation platform, and the results verify the effectiveness and viability of the droop-control based scheme.