Reactive Power Optimization In Distribution Network With Doubly Fed Induction Generators

Wind power industry has been given more and more attention, since it is the bestnew power in the sense of large-scale spread and commercial development. Duringthe future smart grid construction, grid-connected wind farm will bring reform intraditional power network. In the distribution grid within doubly-fed inductiongenerator (DFIG),this paper mainly studies how to reasonably utilize and distributereactive power to decrease network loss, improve the stability of system voltage andreduce the negative impact of the fluctuation of wind power on system.Based on mathematic model of DFIG,without considering stator flux change,this paper analyzes static VAR limit of DFIG with stator current limit, rotor-side,steady-state stability limit. The power characteristics of DFIG is studied bydeveloping the mathematical model of the DFIG, and the reactive power generationand absorption capacity for each generated active power of the DFIG wind turbine isanalyzed. The influence of wind velocity variation on DFIG reactive capacity is alsoconsidered. The reactive power limitation of DFIG can provide the reference forreactive power optimization of distribution network which is involved with DFIG.For utilizing the reactive power control of DFIG and considering restriction ofregulating times of traditional reactive-power regulation devices and reactive capacityof DFIG, the time-interval and hierarchical reactive optimization of strategy based oncoordinated control of Var sources is proposed. The strategy firstly divides load curveinto several periods by hierarchical clustering analysis algorithm. Then the stratifiedcontrol strategy is applied for reactive optimization in different periods. The sum ofnetwork loss and average node voltage offset are chosen as the objective in theproposed reactive power optimization algorithm.An improved particle swarmoptimization is used to calculate optimal running state，included DFIG all kinds ofreactive power regulation devices.The switching of transformer taps and capacitor arepre-performed according to the optimization result. Those tasks are supposed to be completed in the upper layer. The flexible control capacity of DFIG reactive power isused to to follow the reference voltage value which is provided by upper layer’optimization results in lower layer. Herefrom the combination of voltage control atgrid-connecting point during each time interval and global reactive optimization isimplemented. A simulation is constructed to verify the proposed control strategy. Theresults showed that proposed strategy can guarantee voltage stability of load nodesand system economic operation.