LVRT Technology Research Direct-drive Wind Power Systems

With the rapid development of the wind power industry, wind turbinetechnology has greatly improved. Compared with the double-fed wind powergeneration systems, full power converter is used to connect the stator and thegrid side of direct-drive permanent magnet wind power generation system. Andthere is smaller interaction between the grid and the generator. And it has a largerange of variable speed operation. There is not gearbox of the direct-drivepermanent magnet wind power generation system, by the way of reducing thenumber of driveline components and improving the efficiency of utilization ofwind energy, the reliability of the system increased. There’s a increasingproportion of direct-drive wind power system. When the grid failed, thelarge-scale wind turbine power removal will cause again an impact after thefailure on the power system stability, or even cause the collapse of the powergrid. It is necessary to study low voltage ride through capability in ensuring thestability of the whole power grid.Firstly, analyze and model the various parts of the direct-drive wind powersystem, including wind turbines, transmission, permanent magnet synchronousgenerator and grid side converter. Build the wind generator model of thethree-phase stationary coordinate system and two-phase synchronous rotatingcoordinate system, and on the machine side converter, we achieve decoupling tocontrol the active and reactive power by using a zero d-axis current control. Thevoltage oriented vector control technology is used to control the network sideconverter.Secondly, according to the regulations and national standards forlow-voltage operation of the power grid, research a variety of strategies of lowvoltage ride through of the direct drive wind power system. First, with theDC-side unloading circuit, in the case of single-phase short-circuit power,two-phase and three-phase short circuit to ground, the PMSG system issimulated and analyzed. In addition, two cases that increasing reactive power compensation circuit of the network side and increasing shunt resistor of thestator side were respectively simulated and analyzed.Finally, according to the technical requirements for the reactive powercontrol and low-voltage ride-through (LVRT), after a lot of references, animproved control method is used. The machine-side converter change thegenerator load torque to achieve maximum utilization of wind energy. At adepth of power failure, with the DC side circuit, the network-side converteroperating in the reactive priority output strategy of the grid side and combine thenew reactive power compensation circuit at the network-side, by controlling theturn-off device on and off, to achieve adjust the reactive power fast and smooth.Simulation results show that the used scheme effectively improves the reactivepower compensation of PMSG systems and low-voltage ride-throughcapability；And it ensure that the value of each battery is rapid and smoothtransition to protect the safety of wind power equipment during the time ofdepth failure.