Small-Signal Stability Analysis And Novel Improvement Method For Grid-Connected Inverters In Weak Grids

The construction of a new power system with new energy as the main part is an important support path for China to achieve the energy structure adjustment and the carbon peaking and carbon neutrality goals.The whole network will continue to vigorously develop new energy and promote the orderly and rational integration of new energy into the grid.However,with the continuous increase of the installed capacity of new energy,the grid gradually shows the characteristics of weak grid s,which leads to the negative impact of grid impedance that cannot be ignored.Meanwhile,as the power interaction interface between new energy and grid,power electronic converter s are the key equipment and core technology to promote high-quality power generation,high-efficiency power transmission,and high-quality power consumption.However,in th e weak grid,the initially designed stable converter system often interacts with the grid impedance to induce broadband disturbances after grid connection.This not only deteriorates the stability of the grid-connected converter system but also seriously restricts the high proportion of new energy being connected to the grid.Therefore,to maintain the safe and stable operation of grid-connected converters in a high-permeability grid with new energy as the main part,it is urgent to develop small-disturbance stability analysis model s of the grid-connected converters and analyze the instability mechanism.On this basis,it is necessary to propose relevant control strategies to enhance the stability of the grid-connected converters system.This is crucial to p romoting the development and utilization of large-scale new energy in China and the acceptance of power electronics in the grid.Therefore,under the support of the National Natural Science Foundation of China(52077072)and the Science and Technology Project of China Southern Power Grid Corporation(GDKJXM20212013),this paper takes the grid-connected inverters under the weak grids as the research object and studies the model construction and equivalent transformation method of the grid-connected inverters in different control domains.Meanwhile,the instability mechanism of grid-connected inverters based on positive and negative feedback loop effects under typical control mode is analyzed.In addition,control methods to enhance the stability of grid-connected inverters are also proposed.The specific research work and innovation points of this paper are summarized as follows:(1)First,taking the LCL-type grid-connected inverter in the weak grid as an example,the impedance model of the grid-connected inverter controlled in theαβ domain and the dq domain is constructed.On this basis,combined with the frequency-shifting characteristics of the complex transfer matrix,the equivalent transformation from the single-input single-output(SISO)impedance model of the grid-connected inverter controlled in the αβ domain to the multiple-input multiple-output(MIMO)impedance model in the dq domain is realized.Meanwhile,combined with the structural characteristics of equivalent transformation from αβ domain imped ance model to dq domain impedance model,the equivalent transformation from MIMO impedance model of grid-connected inverter controlled in dq domain to SISO impedanc e model in αβ domain is deduced.The unified transformation of the impedance models of the grid-connected inverter in the αβ domain and the dq domain is realized.Moreover,in the weak grids,by analyzing the correctness of the equivalent transformation of the impedance model s of the grid-connected inverters in theαβ domain and the dq domain,it is pointed out that the stability of the LCL-type grid-connected inverter controlled in the αβ domain is stronger than that controlled in the dq domain.Finally,this conclusion is extended to L-filter and LC-filter grid-connected inverters systems.(2)From the perspective of positive and negative feedback loops,the positive and negative feedback loop effects formed by the phase-locked loop,the current inner loop,and the power outer loop in the small-signal control block diagram of the grid-connected i nverters are revealed,respectively.Meanwhile,the influence mechanism of positive and negative feedback loops on the stability of grid-connected inverters in weak grids is analyzed.Firstly,the difference of the influence of the positive and negative fe edback loops intro duced by the phase-locked loop at the current reference value s and the modulation signal s on the stability of the grid-connected inverter is analyzed.On this basis,an asymmetrical compensation control method to suppress the negative eff ects of the positive feedback loop is proposed,and its compensation characteristics are analyzed in detail.Secondly,in the grid-connected inverter considering the DC bus voltage dynamics,the positive and negative feedback loop effects formed by the current inner loop controller parameters in the voltage outer loop control loop are analyzed.The root cause of the obvious difference in the influence of the current inner loop and the voltage outer loop controller parameters on the stability of the grid-connected inverter is intuitively revealed.Finally,for the PQ power control grid-connected inverter,it is revealed that the PQ power control is a kind of asymmetric control in the weak grid.Moreover,by analyzing the characteristics of the positive and ne gative feedback loops introduced into the active/reactive loop,it is pointed out that the stability of the grid-connected inverter is more sensitive to the changes of the reactive loop parameters than the active loop parameters.On this basis,an asymmetr ic control method of active/reactive loop controller parameters is proposed,and its influence on the stability of grid-connected inverters is analyzed.(3)Based on the requirement of the magnitude-phase-frequency characteristic curve of the output impeda nce of the grid-connected inverter,a method for improving the stability of the grid-connected inverter by correcting the phase and magnitude of the impedance is proposed.Firstly,a low-frequency phase correction method of output impedance based on adapti ve resonant lag compensator is proposed,so that the frequency range where the output impedance of the grid-connected inverter is maintained above the-90° line is widened to the low-frequency band.The range of feasible phase angles for keeping the grid-connected inverter stable is increased.Subsequently,a current loop phase angle compensation control method matched with an adaptive resonant lag compe nsator is proposed,so that the phase-frequency characteristic curve of the output impedance in the middle frequency band is maintained in the range of the-45° line.This not only ensures that the grid-connected inverter has sufficient phase margin under weak grids but also increases the magnitude of the impedance significantly.In addition,to solve the problem that the output frequency of the phase-locked loop may fluctuate greatly when the operating point of the grid-connected inverter changes,a novel phase-locked loop structure with additional inertia and damping is proposed.The dynamic and steady-state performance of grid-connected inverters in weak grids is improved.(4)Aiming at the technical limitations of traditional impedance criterion in the stability analysis of adaptive control grid-connected inverters,a novel decoupling impedance criterion is proposed.Meanwhile,the novel decoupling impedance criterion is applied to the stability analysis of the proportional weight-based control loop param eter adaptive grid-connected inverter,which provides a theoretical basis for the application of the novel decoupling impedance criterion.On this basis,based on the novel decoupling impedance criterion,a grid-connected inverter adaptive impedance phase and magnitude correction control method with constant stability margin is proposed,which improves the stability of the grid-connected inverter under weak grids.In a word,given the shortcomings of the existing research on grid-connected inverters under weak grids,this paper has done related research work from the aspects of model construction and its equivalent transformation,instability mechanism analysis under positive and negative feedback loops,and impedance phase and magnitude reshaping.The purpose is to improve the stability of the inverter connected to the weak grid s and promote the safe,efficient and reliable grid connection of large-scale new energy power generation systems.