Research On Current Control And Weak Grid Stability For LCL-Type Grid-Connected Converter

Grid-connected converter is the important interface which is applied in distributed power system based on renewable energy and the high-voltage direct-current transmission,etc.Compared with L filter,LCL filter is preferred for its superior high-frequency harmonic suppressing ability which is widely applied in grid-connected converters.However,high resonance peak of the LCL filter will have an adverse effect on the stability of the grid-connected converters.Passive and active damping methods are proposed to suppress the LCL filter resonance.The active damping strategy is favored by researchers which solves the resonance problem of the LCL-type grid-connected converters by means of control method.Nowadays digital control technology has been widely used in power electronic devices.It has the advantages of strong anti-interference capability,flexible control algorithm and consistency.The bandwidth of current loop and phase margin will be affected by the inherent digital control delay of digitally controlled grid-connected converter which can lead to system instability in some applications.Recently,weak grid has become a hot issue of concern.The high grid impedance makes the control and stability of the grid-connected converters severe.In the case of ignoring the grid impedance,the controller parameters design method can not guarantee the system stability when connect to the weak grid.This dissertation focuses on digital control delay,active damping strategy with single current feedback control and system stability under weak grid.Main contributions are summarized as follows:The stability analysis of digitally controlled LCL-type grid-tied converter with capacitor current feedback active damping is presented.The average switch models of the synchronous rotating dq frame controlled grid-tied converter are derived in this dissertation.The digital control delay includes computation and modulation delays,in which,the modulation delay is inevitable.The computation delay can arbitrarily vary by shifting the sampling instant.By using the generalized z transform,a discrete domain model is established,and the closed-loop transfer function of the active damping loop is obtained.Then,the influence of computation delay variation on system bandwidth,phase margin and stability is studied.The different sampling frequency.This dissertation points out that the high sampling frequency and small computation delay can improve the stability of digital control system.In order to reduce the effect of digital control delay,a delay compensation strategy based on volt second product balance is proposed,which is called area compensation scheme.This method is to compensate the computation delay in the next sampling period by redistributing the modulation wave reference.Finally,the validity of the theory and the effectiveness of the proposed area compensation scheme are validated by experimental results.In order to reduce the hardware cost,reducing sensors is an effective way.Recently,the single current feedback controlled LCL-type grid-connected converter has been paid more and more attention.In the digital control system,the single grid current feedback control can stabilize the system under the condition of high resonance frequency due to the existence of one step delay.However,because of the parameter dift and the grid impedance variation,the resonance frequency of the LCL filter will change.The system may be unstable.In this dissertation,a novel active damping scheme with single grid current feedback control is proposed.The second order resonant integrator filter is introduced into the current feedback channel to enhance the damping characteristics of the LCL filter near the resonance frequency.This active damping method can effectively improve the amplitude frequency and phase frequency characteristics near the resonance frequency.And it allows wide range variation for the sampling frequency and resonance frequency ratio.The physical essence of the scheme is analyzed from the view of physical equivalent circuit.Because of the influence of digital control delay,the system phase has a large lag.To ensure a fast dynamic response and a small range variation of phase,this dissertation gives out the condition of satisfying the minimum phase system.The condition of satisfying the minimum phase system is given.The tuning method of second order resonant integrator filter is presented.Under three different cases of sampling frequency and resonance frequency ratios,the experimental results verify the damping effect.The effectiveness of the active damping scheme is also validated.In this dissertation,the dq impedance matrix model of LCL-type three phase grid-connected converters is derived by using the small signal linearization method.The impedance model takes into account the influence of grid synchronization,i.e.,phase locked loop.The input of PLL is obtained by detecting the point of common coupling voltage.Considering the parameters of current loop,voltage loop and PLL,the variation of the system impedance is analyzed using the negative incremental resistor concept.The stability criterion based on GNC is utilized for determine the stability of grid-tied converters under weak grid condition.The impedance model can be used to assist the design of closed-loop parameters.It has a very good evaluation on the stability of the system.Moreover,the validity of the impedance matrix model is verified.