Resonance Damping And Passivity-Based Impedance Shaping For LCL-Type Grid-Connected Inverters

Promoting the proportion of the renewable energy in the power system is an important way to achieve carbon peak and neutrality targets.As the interface between the renewable energy generation system and the power grid,the grid-connected inverter is responsible for converting direct current into high-quality alternating current and feeding it into the grid.LCL-type grid-connected inverter has been widely used because of its strong switching harmonic rejection ability.However,LCL-type grid-connected inverter is prone to resonance instability,which is more likely to occur when the grid impedance fluctuates widely.This dissertation focuses on the resonance damping and impedance shaping methods of LCL-type grid-connected inverter,and its main contents are demonstrated as follows.The state-variable-feedback active damping and the filter-based active damping are two popular resonance damping strategies.The state-variable-feedback active damping aims to emulate a virtual resistor in the LCL filter so as to attenuate the LCL resonance peak.However,due to the digital control delay,the actual emulated resistor is a frequencydependent impedance,which weakens the damping effect or even leads to negative damping,reducing the system stability.Reducing or eliminating the negative-damping frequency range by optimizing the state-variable feedback function is an effective way to improve the system stability,and identifying the positive-damping frequency range of different feedback functions is critical to the optimization.In this dissertation,a general evaluation method for the damping characteristics of the state-variable-feedback active damping is proposed.The positive-damping frequency range of different feedback functions can be intuitively identified by the proposed method,and then their damping characteristics can be revealed according to the relationship between the LCL resonance frequency and the positivedamping frequency range.Taking the capacitor-current-feedback and capacitor-voltagefeedback active damping as examples,the corresponding positive-damping frequency ranges of several typical feedback functions are identified with the proposed method.The results are compared with those derived from the virtual impedance,and the effectiveness of the proposed evaluation method is verified by experiments on a 6-k W prototype.By cascading the digital filter into the forward path of the current loop,the filter-based active damping can change the frequency response of the loop gain so as to achieve LCL resonance damping.In this dissertation,based on the phase-frequency characteristics of the system loop gain,the phase constraint regions of the system stability on the digital filters are obtained,and then a general evaluation method for the damping characteristics of the digital filters is proposed.With the proposed method,the damping characteristics of several typical digital filters(including low-pass filter,all-pass filter,notch filter,etc.)are evaluated,and the corresponding LCL resonance frequencies are identified on the premise that the gridconnected inverter can work stably.Furthermore,three kinds of digital filters with strong robustness are found,and the guideline for the digital filter parameter design is summarized.The effectiveness of the proposed evaluation method is verified by a 6-k W prototype.Impedance-based stability analysis is a popular method for assessing the stability of the grid-connection system.The robustness of the grid-connected inverter against the grid impedance variation can be improved by shaping its output impedance according to the impedance-based analysis.To cope with the complex grid impedance which changes in a wide range,this dissertation studies the passivity-based impedance shaping method for the LCL-type grid-connected inverter with grid-side current control and inverter-side current control,respectively.As for the grid-side current control,the reason which results the nonpassive region of the inverter output impedance is revealed,and then a passivity-based impedance shaping method is proposed.This method is fulfilled by emulating a virtual series impedance and a virtual parallel impedance,which are realized by feeding back the grid current and feeding forward the point of common coupling voltage,respectively.With the proposd impedance shaping method,the passivity of the inverter output impedance is achieved up to the Nyquist frequency by appropriate design of the grid-current feedback function and the grid voltage feedforward coefficient.The effectiveness of the proposed passivity-based impedance shaping method for the grid-side current controlled gridconnected inverter is verified through experiments on a 6-k W prototype.As for the inverter-side current control,it is found that the passivity of the inverter output impedance can be achieved up to the Nyquist frequency by only virtual parallel impedance,which can be realized by capacitor voltage feedback.The effect of capacitor voltage proportional feedback on the inverter output impedance is analyzed,and it is pointed out that the non-passive region can only be narrowed by selecting appropriate weighted proportional feedback coefficient,but cannot be eliminated completely.In this dissertation,the passivity-based impedance shaping method based on capacitor voltage proportionalderivative feedback is proposed.By optimizing the proportional coefficient and the derivative coefficient,all frequencies’ passive inverter output impedance is achieved.It is also found that this method can improve the suppression ability of the grid-connected inverters to the low-order grid voltage background harmonics.The experimental results on a 6k W prototype validate the effectiveness of the proposed passivity-based impedance shaping method for the inverter-side current controlled grid-connected inverter.