Research On Current Control Performance Improvement For Three-phase Grid-connected Converter Under Weak Grid

Grid-connected converter is an important part of the renewable energy generation system,and plays an important role for the stable and high performance operation of the system.Since the renewable energy generation systems are usually located at the end of the grid and far from load centers,the public grid usually exhibits the weak grid characteristics,which puts forward higher requirements to the control of grid-connected converter.Under weak grid,the current control interacts with phase-locked loop(PLL)via the point of common coupling(PCC)voltage.And PLL dynamic has negative effect on the current control.Thus,the current control renders undesired performance,even result in system instability.Moreover,under weak grid,The PCC voltage feedforward,which is usually used to suppress start-up inrush current,also deteriorates the current control,and grid current will have harmonic distortion.Therefore,this thesis studys the PLL effect on the current control and the start-up characteristics for three-phase LCL-type grid-connected converter,from four aspects(compensation control of PLL small-signal disturbance on the grid current control,current controller parameters design,PLL controller parameters design,and start-up inrush current suppression),to improve the current control performance and its robustness to the grid impedance changes.The model of three-phase LCL-type grid-connected converter considering the PLL effect on the grid current control is established.The mechanism of PLL effect on the grid current control is analyzed.The influence of the PLL on the grid-connected converter output impedance is pointed out.These lays the foundation for the method proposed to reduce the negative effect of PLL on grid current control.For the small-signal disturbance of PLL on the grid current control of the grid-connected converter,this thesis proposes a small-signal disturbance compensation control to reduce the negative effect of PLL on the grid current control under weak grid,without changing the PLL parameters and structure.Moreover,this method is not affected by the circuit parameters variation.Then,converter output impedance is analyzed with different compensations.Finally,the proposed method is verified by experiment.In addition to the small signal disturbance introduced by the PLL,for the current reference change of grid-connected converter,the PLL also has negative effect on the grid current control under weak grid.However,the existing design method of current controller parameters neglects the impact of PLL,and therefore,it is hard for the current controller to mitigate the negative effect of PLL.Thus,in this thesis,an improved design of current controller is proposed to considering the PLL effect on the current control under weak grid.The margin constrains of current controller parameters are deduced based on amplitude margin and phase margin.The effect of current controller parameters on the converter output impedance is analyzed,and a design guideline to improve the current controller parameters is presented.With the improved parameters,in precondition of satisfying system stability margin under both stiff and weak grid,the negative effect of PLL on current control can be effectively mitigated.Furthermore,the current control has a strong robustness against wide-range variation of grid impedance.Finally,the experiment demonstrates the effectiveness of the proposed design method.In addition,the performance of PLL will also play a key role in the high performance running of the grid-connected converter.Under weak grid,the synchronous voltage of PLL usual appears varying degrees of voltage imbalance,harmonic distortion,and so on.As a result,the PLL performance will be greatly worsen,and has negative effect on the grid current control.However,the existing design methods of PLL controller parameter are for PLL independent systems and do not consider the interaction of the PLL and the current control.Regarding the issue above,this paper considers the PLL effect on the current control,and proposes a systematic PLL controller parameters design method under weak grid.For the PLL independent system,the region of PLL controller parameters is established based on the phase margin and harmonic attenuation.The effect of PLL controller parameters on the converter output impedance is analyzed,and a design guideline to improve the PLL controller parameters is presented.With the improved parameters,not only the PLL independent system has well dynamic and static response performance,but also the negative effect of PLL on current control can be reduced.Finally,the experiment demonstrates the effectiveness of the proposed design method.In addition to steady-state operation,the start-up characteristics of grid-connected converters are also crucial.Serious inrush current will be aroused when the grid-connected converter starts,especially in rectifier mode,if no effective control method is taken.Such inrush current may result in triggering overcurrent protection,and even damage the switching devices.The PCC voltage feedforward is usually used to suppress start-up inrush current.Unfortunately,it will introduce a positive feedback loop related to the grid current and grid impedance under weak grid,and therefore,large grid current harmonics will be aroused.To solve the above problems,this thesis analyzes the mechanism of start-up inrush current,the effectiveness of the voltage feedforward method for inrush current suppression and the influence of the voltage feedforward on the grid current control under weak grid.Then,the method of a fundamental positive-sequence d-axis component of filter capacitor voltage feedforward is proposed.The influences of different feedforward methods on grid-connected converter output impedance are analyzed under weak grid.With the proposed method,it is not noly possible to suppress the start-up inrush current but also maintain the grid current quality under weak grid.Finally,experimental results verify the validity of the proposed method.