Voltage Stability Study Of Directly Dirve Wind Turbine With Permanent Magnet Generators Connected To Power Grid

Because permanent magnet synchronous generator has high wind energy utilization, andactive and reactive power can be independently controlled, the permanent magnet direct drivewind power system was attended in modern wind power generation system. More and morewind farms which base on the permanent magnet direct drive wind turbine were built all overthe world. The voltage stability of permanent magnet direct drive wind turbine whichconnected to the grid is an important issue for the experts.According to the voltage stability problems of permanent magnet direct drive wind farm,firstly, the mathematical models of permanent magnet synchronous generator and converterare analyzed. Then, the static and transient voltage stability of wind farm which connected tothe grid are studied. Specific research contents are as follows:Firstly, the mathematical model of permanent magnet synchronous generator andfull-power converter are derived, and then the machine-side converter and grid side convertercontrol models are analyzed. The machine side converter using rotor field oriented vectorcontrol technology, not only the maximum torque of the generator can be achieved, but alsothe active and reactive power of the generator output can be controlled independently. Thegrid voltage oriented vector control technology is used by network side converter, whichmade wind power unit output to the grid active power and reactive power that can becontrolled independently.Secondly, the voltage instability mechanism of grid-connected wind farm by singlemachine infinite bus system is analyzed. The impact factors of the wind farm voltage stabilityis obtained, then the wind farm is connected to WSCC9node system, the system bus voltagechange is analyzed by using P-V cure method. Then, according to the characteristics ofpermanent magnet direct drive wind turbine, the reactive power is issued to the grid in acertain range to adjust to the power factor. The static voltage stability of the grid is improvedby controlling the reactive power.Finally, transient voltage stability is studied by simulating the grid malfunction andanalyzing wind turbine output characteristics. A transient voltage control module is added tothe network side converter control system, and the direct-drive type units are proposed toreduce the input torque of the permanent magnet synchronous generator and grid malfunction.According to the power grid voltage drop, the transient voltage control module is regulated.The reactive power is issued by the network side converter to help restoring the grid voltage.