Virtual Inertia Control Strategy Of Doubly-fed Induction Generator

As a kind of most important renewable energy,wind power generation is developing rapidly along with strengthening protection of the environment,which causes new problems that are different from the traditional power grid.Wind turbines is operated with the active and reactive power decoupling strategy and synchronize with the power grid by phase-looked loop as now.Hence the rotor speed of wind turbines are totally decoupled with power grid which is opposite to synchronous generations.This is the reason why the inertia of the power grid reduces and the stability of the grid frequency declines.This thesis is focused on the problem of grid inertia reducing coursed by large-scale wind power connecting to power grid,Take the doubly-fed induction generator as an example,a current source control strategy applied to doubly fed induction generator set is established from the math deducing,and briefly analyzed the maximum power tracking control.By applying the traditional virtual inertia control strategy to the current loop of the doubly-fed induction generator,setting up its control block diagram containing the inertia and damping characteristics of the power grid,and its open loop transfer function is derived.By drawing the bode diagram of the open loop transfer function on condition of different virtual inertia coefficients and the cut-off frequency of low pass filter,the result of analyzing bode diagram shows that the larger virtual inertia coefficient can reduce the stability of the control system,while the lower cut-off frequency of low pass filter can improve the stability,but it will reduce the response speed of the virtual inertia control strategy.This thesis proposes adding a differentiation element to the conventional virtual inertia control strategy to retain its shortcomings and the effectiveness of the scheme is proved by the simulation.At the same time,in order to restrain the adverse effect of differentiation element,the equivalent damping of the control system is improved by damping the differentiation element.The stability and celerity of the control system is promoted at the same time.This thesis proved the effectiveness of virtual inertia control strategy by using the simulation experiment platform of induction generator.The experimental results show that this strategy can effectively restrain the frequency change rate of the power grid,which means that the inertia of the power grid is promoted.It is meaningful for promoting the stability of the power grid as the large-scale wind power connected increasing in the future.