The Study On The Black Start Of A DFIG-Based Wind Farm

Blackouts in electrical power systems are major accidents which have significant influence on electric power security,economy and society.With the growth of regional industrialization and economy,the power consumption has been increasing,the scale and complexity of power systems has been expanding,which can increase the odd of blackouts in power system.Hence,it is of great significance to do some researches in the fast restoration of power system during blackouts and reduce the loss because of this sort of emergencies by keeping electrical equipment safe.This paper has taken the doubly fed induction generator(DFIG)-based wind farm as research object and has done a detailed study on its black start plan.First of all,the DFIG-based wind turbine is considered as the black start source.A DC voltage source is connected to the DC bus between the PWM inverters in DFIG in order to provide the DFIG with an initial excitation voltage in the initial phase.Next,in order to enhance the voltage and frequency support ability of DFIG and make it adapt better to the needs of black start,a modified virtual synchronous control strategy is designed,based on the mathematical model of DFIG,by imitating the synchronous generator.This control strategy enables the DFIG to present the characteristic of inertia which is similar to the synchronous generator,then enhances the stability of islanding DFIG’s output voltage in order to have a better performance in supporting the voltage and frequency of microgrid.Moreover,considering the fluctuation of wind,a variable pitch control part is added.By using MATLAB/Simulink,the amelioration of frequency control ability by applying that the modified virtual synchronous control strategy and the feasibility of the black start of DFIG-based wind turbine are both verified.Then,the major steps to black start a DFIG-based wind farm is established.In addition,because of the weakness of system in the initial phase of black start procedure and the existence of motors and transformers,the impacts of loads cutting in on the wind power system in the initial stage of black start process is studied by comparing the volume,the type and the order of loads.Several rules are concluded for the application of loads in this situation.Finally,focusing on the stand-alone DFIG-based wind turbine with unbalanced and nonlinear loads,an improved VSynC strategy using second-order generalized integrator(SOGI)is proposed.The integration of a SOGI in the virtual synchronous reference frame enables the stand-alone DFIG to achieve the target of sinusoidal output voltage with unbalanced and nonlinear loads,reducing the amplitude of harmonic components in output voltage.The proposed method removes the sequence decomposition for stator voltage,which simplifies the control schema.