Research On Grid-connected Side Filter Of Wind Power Generation

Grid connected inverter is the key part of wind power generation system,inverter is the equivalent nonlinear load for wind turbine generator,which can bring a lot of harmonic,so the grid side filter is indispensable especially in wind power generation system.There are three types of the output filter of the grid connected inverter system: L,LC,LCL.Compared with the filtering performance of the three types,it can be found that the L,LC filter needs a large inductance to meet the high order harmonic filter in the grid connected inverter system with low switching frequency.Due to the high harmonic attenuation characteristics and the smaller inductance,the LCL filter is gradually replacing the traditional LC and L filters in the grid connected inverter system.But the introduction of LCL filter will bring resonance to the system,and the appropriate control strategy should be used to solve this problem.So this paper takes the LCL filter grid connected inverter system as the research object,and studies the current control strategy in two cases of the ideal grid voltage and unbalanced grid voltage.In this paper,the LCL filter is analyzed and designed firstly,and then the mathematical model of the grid connected inverter system with LCL filter is established under the condition of ideal voltage condition and unbalanced power system.Under ideal voltage conditions,the network side current feedback and the inverter side feedback control strategy are studied.Aiming at the resonance of LCL filter,the suppression effect of active damping method and passive damping method is discussed.Passive damping method is easy to implement,but increase the cost of the system;active damping can restrain the resonance peak,but the way of control is complicated,and the parameters deviation and the switching frequency of the inverter will restrict the active damping method.On the basis of this,the "LCCL" feedback control strategy is put forward based on the zero pole of the transfer function of the offset control system.The order of grid connected inverter control system will be reduced from three to one.Compared with the conventional current feedback control method,the method does not need complicated calculation,which can greatly reduce the pole gain of the control loop and improve the operation efficiency of the system.When the actual system is running in the unbalanced grid condition,the "LCCL" current feedback control strategy may not be applicable,so it is necessary to study the control strategy under unbalanced grid condition.Aiming at the problem of instantaneous power characteristic of traditional control strategy,this paper proposes a strategy of coordinated control of instantaneous power fluctuation and amplitude imbalance of network current under unbalanced grid condition.Then this paper analysis comparatively the effect of the suppression of resonance and the elimination of static difference in the grid connected inverter system with the PIR(proportional integral resonance)and PI(proportional integral)controller.It is found that,whether in low frequency or high frequency,the PIR controller is adopted under unbalanced grid voltage,and the filtering effect is better.According to introducing a resonant controller,control system's complexity increases,no damping control strategy is proposed and combined with the grid voltage feedforward inverter side current feedback,which can eliminate the grid current is controlled by the effect of the grid voltage,Through the verification of the strategy in the Matlab/Simulink simulation,it is found that,the waveform of grid current with grid voltage feed-forward in the control system is still symmetrical and not changed under unbalanced grid voltage conditions,which will not cause any harmonic pollution.