The Research On The Control Strategies For Grid-connected Inverter Under Unbalanced Network

With the rapid development of wind power industry, the important influence of power system operating scheduling、grid voltage and frequency stability and power quality after large-scale wind farms are connected with power grid is increasingly. Due to distinctive features of the position of wind site selection, most of the wind farm with weak grids connected away from the main power system and load centers, the flicker、unbalance and frequency fluctuation of grid voltage will affect the normal operation of the wind turbines, particularly when the grid voltage dip or short-circuit fault occur, they can lead to over-current protection, then the whole turbines will be disconnected from the grid. Large-scale wind turbines’ trip-off seriously cause the local inrush flow and frequency fluctuation of power system, thus the faulted power grids get worse, even cause international blackouts. Therefore, the study of the wind turbines’operation and control and the enhancement of wind turbines’low voltage ride through (LVRT) ability under the asymmetrical grid voltage conditions have theoretical significance and practical value. With a (directly-driven wind turbine with permanent magnet synchronous generator, D-PMSG) connected to power grid by back-to-back dual-PWM converter as the reach subject, the work mainly includes the following three aspects.Briefly introduced the structure and operating principle of a D-PMSG with back-to-back dual-PWM converter, and showed the mathematical model of wind turbine, permanent magnet synchronous generator and grid-connected inverter (GCI).The control system of D-PMSG is discussed, which include the control system of blade angle, generator-side converter and grid-connected inverter.According to the performance analysis of the mathematical model of the three-phase grid-connected inverter (GCI) and the fluctuation mechanism on DC voltage of D-PMSG under asymmetrical faults, the notch filter was introduced to conventional control strategy of phase-locked loop, the new dual current-loop control strategy (DCC) was proposed to eliminate the quadratic fluctuations of DC voltage and maintain the wind turbine stable, and the control strategy is adopted to implement LVRT of the grid-connected D-PMSG.Under the asymmetrical grid voltage conditions, by the introduction of proportional-integral-resonant (PIR) controller which are tuned at the synchronous angular frequency and can be applied in the stationary reference frame directly to the direct power control strategy (DPC) of the GCI, the novel DPC can stabilize the DC voltage and a energy consumption crowbar is used to reduce the net power and inhibit the rise of the gird current for the wind turbine un-off-line operation on grid faults.A1.5MW D-PMSG with back-to-back dual-PWM converter was simulated in the PSCAD/EMTDC environment, compared with the dual current-loop control strategy(DCC), the novel DPC strategy is simple,simulation results showed that it could implement unity power factor control with excellent dynamic performance, inhibit the rise of DC voltage and gird current effectively, and enhance the ride-through capability of D-PMSG wind generation system under unbalanced network fault conditions as well.