Load Flow Analysis And Reactive Power Optimization In Wind Power Systems With Doubly-fed Induction Generators

With more wind power integrated, the wind power units bring analysis andcontrol of the wind power systems a lot of new fairs. The steady states operationparameters and reactive output of wind farm play an important role in security andstability of the wind power systems. In this dissertation, the load flow model,reactive output and the optimal var distribution giving by active loss minimizationin wind power systems with doubly-fed induction generators is studied, andoutlined as following:At first, an load flow model for doubly fed induction generators is proposed.In existing research, the output characteristic of wind turbine is ignored. Theyregard the output as constant or proportional to the cube of the speed. The former isvery simple. The speed quantified under maximum power point tracking is incontradiction with the slip in load flow analysis on the basis of koptmodel ofturbine. In the latter, optimal coefficient koptis not quantified, when the speedrippled or they decrease the active output. Compared with the koptmodel, the Cpmodel has higher accuracy. The active power output and the minimum loss ofgrid-connected wind turbines is investigated by load flow analysis in wind powersystems with doubly fed induction generators.Secondly, an improved reactive power optimization algorithm for the DFIG isproposed. Reactive power optimization algorithm of the grid-connected windturbines is modeled, including the blade tip speed ratio, the pitch angle adjustmentmodel of wind turbine and the mathematical model of doubly fed inductiongenerator (DFIG). The effects of several parameters such as the current andcapacity of stator, rotor and converter, and so on are considered at the same time.Objective function is the reactive power limit. Cpmodel of wind turbineconsidering the blade tip speed ratio, the pitch angle adjustment model can decreasethe active output. The blade tip speed ratio is adjusted when speed is above therated wind speed. The pitch angle is adjusted when speed is below the rated windspeed. Numerical examples analyze and verify the reactive power output is becomelarger when they decrease the active output.Finally, the reactive power optimization considering the control characteristic of wind turbines in grid-connected wind farms is proposed. The reactive powerlimit of grid-connected wind turbines and the optimal reactive power distributionbased on the minimum loss of system active power are calculated. The numericalresults indicated that the reactive power limit of wind farm varies directly ascapacity of wind farm. The point-of-common coupling of wind farm has significanteffect on the reactive power limit of wind farm. The optimal var distribution insidethe wind power systems is given by active loss minimization. The voltage level ofwind power system is stability based on active loss minimization.