Control Strategy Of Doubly-fed Induction Generators In Microgrid

With the intensification of energy shortages and environmental pollution problems,countries have begun to actively explore clean and efficient renewable energy sources.Wind power has become one of the effective methods to solve the problems.At present,the constantly increasing penetration rate of wind power in China and the increasing demand for electricity in areas with backward economy and inconvenient transportation have caused more and more wind power generation systems in microgrid to be applied.However,there are still many problems to be solved urgently,including stability threats caused by the lack of equivalent inertia in the power system and the power distribution problems during the operation of paralleled DFIGs.In this paper,VSG is used for solving the problem of the inertia loss.First,a mathematical model of the DFIG is established,combined with the mathematical model of virtual synchronous generator control,voltage and current double loops,and line impedance to establish a single unit model.The influence of the parameters in power loop are studied through the obtained mathematical model.Secondly,based on the obtained model and the conversion of the reference coordinate system,the state-space small-signal model of multiple doubly-fed induction generators is established.By mapping the root trajectory,calculating oscillation frequency,damping ratio and participating factors,the effects of reactive droop coefficients,damping coefficients and inertia on system stability are studied.It is obtained that the increase of inertia increases the system dynamic adjustment time,while the increase of damping coefficient is conducive to the system stability margin.the increase of reactive power droop coefficient will reduce system stability or even instability.For paralleled DFIGs operation,this article finds that the mismatch of line impedance is the main cause of power distribution problems.Two control strategies are adopted to improve the system,including virtual impedance and virtual capacitance.According to the analysis of the improved small signal model,the increase of virtual resistance will tend to be stable while the increase of the virtual inductance is opposite,but virtual resistance will reduce the output voltage value.The virtual capacitor has a small value range when the system of DFIGs keep stable.On the basis of using its own characteristics,it can improve reactive power sharing and the voltage deviation can be reduced.Finally,simulations verify the feasibility of applying VSG to DFIG in microgrid.It also shows that the control strategy proposed in this paper can improve the problem of reactive power distribution,which has certain research significance.