Research On Control Strategies Of Grid-connected Inverter In Direct-driven Permanent Magnet Wind Power System

Direct-driven permanent magnet wind power system is widely studied due to itssuperiorties of connecting grid with back-to-back dual-pulse width modulation (PWM)converter, no failure-prone gearbox and uninterrupted connecting ability during the grid fails.Grid-connected inverter is the interface between wind power system and grid. Its goal is toregulate the active power and reactive power respectively, meanwhile, to maintain stable ofDC-link capactitor voltage.Firstly, Direct-driven permanent magnet wind power system and double-fed asynchronouswind power system are compared. Then the main control strategies of grid-connected inverterare introduced, and the necessity of low voltage ride through (LVRT) capability is illustrated.After a brief introduction to the working principle and the topology of driect-drivenpermanent magnet wind power system, the mathematical model of permanent magnetsynchronous generator (PMSG) is built, and the models of voltage source rectifier (VSR) areset up in both abc static reference frame and dq synchronous reference frame.Then, the grid voltage oriented control (VOC) strategy is presented when the gridvoltage is normal. The voltage feed-forward outloop and current decouple inner loop areintroduced. PI controller is applied to both loops under dq synchronous reference frame. Thecontrol parameter design of the control loop is analyzed in detail. Then this control strategy isset up in Matlab/Simulink, the results verify that the grid-connected inverter can operateunder unity power factor or output reactive power as the requirement. The output current togrid can be with great characteristics of sine and total harmonic distortion is less than5%.Finally, the characteristics of system are analysed when grid voltage dips. The poweroutloop and unloading circuit are introduced to consume power for protecting the capacitor.The simulation results verify that this strategy limites the DC-link voltage within2.62%.Furthermore, grid-connected inverter operates in the reactive compensation mode and exportsreactive power according to how much degree the grid voltage dips. The results that gridvoltage dips20%and80%illustrate that reactive compensation mode can export reactiveeffectively and improve low voltage ride through capability of the direct-driven wind powersystem.