Coordinated Reactive Power Voltage Control Between The Doubly-fed Wind Farm And Static Reactive Power Generator

Wind power grid connection technique has obtained the development by leaps andbounds as a new form of wind energy used in recent years. To the local power gridcontaining the grid wind farm, wind power grid capacity increasing makes wind powergrid node voltage more vulnerable to the network load fluctuations or wind speed blowinginto the wind farm fluctuations which leads to the phenomenon of voltage instability.At present, doubly-fed variable speed constant frequency wind turbine generator(Doubly-Fed Induction Generator, DFIG) has become the mainstream type for the windfarm choosing generators. DFIG can realize decoupling control of active and reactivepower by controlling the rotor excitation current, so that it has the flexible reactive voltageregulating ability. The reactive power output limit of DFIG will reduce gradually as theactive power approaches rating. So this article introduces Static reactive power generator(Static Var Generato, SVG) that has a fast response speed and a wide adjusting range tocompensate the reactive power, which makes the two kinds of reactive power source in thefactory/stand adjust the voltage of PCC together.In order to improve the safety, stability and economy of parallel operation of the windfarm, this paper puts forward a two level coordinated control strategy of reactive poweramong DFIG, the wind farm and SVG. Firstly, this strategy calculates the reactive powerdemand of the wind power system by the deviation of the measured value and thereference value of PCC node voltage, and then reasonably distributes the reactive powerbetween the wind farm and SVG to control the deviation of target node voltage minimum.At last, it makes the secondary reactive power setting among the wind turbine generatorsby the method of surplus reactive power limit proportion considering the sensitivity indexof active network loss/reactive power of the wind power system based on the reactivepower limit calculated by the constraints of the maximum currents of the stator and therotor of DFIG and the power factor. A multi-machine dynamic equivalent model of the doubly-fed wind farm based on thewake effect is constructed using electromagnetic transient analysis software PSCAD as theplatform. This method uses Jensen model partitioning the wind farm and does theindependent equivalent for each group of classes. The Study shows that this equivalentmodel can more effectively simulate the output characteristics of the wind farm comparingwith the commonly used single equivalent model. The reactive power control model ofSVG is constructed and the simulation results show that the model can track specifiedreactive power target. Finally, a complete set of reactive power coordinated control systemis established for the selected wind farm depending on the actual power grid of a region inthe northeast and a simulation analysis is carried out under the circumstances of loadfluctuation or wind speed fluctuation. Results show that the control strategy can not onlystabilize the target node voltage, but also makes the wind farm meets the requirement ofthe grid power factor at the same time and effectively prevents the doubly-fed machine thathas a large active power output happening jumping phenomenon due to the limit of theinverter current. On this basis, the control strategy can also optimize the active power lossof the wind power system, so that the wind farm can operate more safely, stably andeconomically.