Stability Analysis Of Grid-connected Inverter Under Multi-control Mode And Research On Its Multi-model Control

Current-controlled grid-connected inverters have been widely used due to high power generation efficiency and good grid-connected power quality.However,large-scale new energy power generation is generally far away from the load center and usually requires long-distance lines and multi-stage transformers for power transmission.Therefore,the point of generating unit connection(PGUC)has a certain equivalent grid impedance.In addition,due to the randomness and volatility of new energy,its output power is constantly changing,resulting in the constant change of PGUC’s equivalent grid impedance,which makes the power grid present a weak grid state where the short circuit ratio(SCR)fluctuates greatly.In recent years,the complex dynamic interaction between the current-controlled grid-connected inverter and the weak grid impedance has caused a series of resonance and instability problems.The voltage-controlled grid-connected inverter can simulate the external characteristics of the synchronous generator,provide beneficial support to the grid,and effectively improve the stability margin of the grid-connected system under weak grids.However,its stability margin is low in strong grids.Voltage control presents instability risks in strong power grids.On the other hand,in order to compensate for the terminal voltage drop caused by the equivalent grid impedance and improve the long-distance transmission capacity of new energy,the transmission line may be connected to certain series compensation equipment.The series compensation equipment is mainly capacitive,which has a significant impact on the low frequency band of the equivalent grid impedance,increases the uncertainty of the grid impedance,and causes new unstable modes such as subsynchronous oscillation.The above-mentioned complex grid conditions have brought great challenges to the stable and efficient operation of new energy power generation systems.Neither voltage-controlled nor current-controlled grid-connected inverters can operate in a wide range when the grid impedance changes significantly.In response to the above problems,this paper establishes the sequence impedance models of current-controlled grid-connected inverters and voltage-controlled grid-connected inverters(droop control,virtual synchronous generator),and uses impedance analysis to analyze their grid-connected characteristics and oscillation mechanism in different grid models.And proposed a subsynchronous oscillation suppression strategy for voltage-controlled grid-connected inverters.On this basis,this paper proposes a multi-model control strategy that adaptively changes the control mode according to different grid status,so as to realize the stable operation of the grid-connected inverter when the grid impedance changes greatly.The main research work and innovations of this paper are as follows:1)Aiming at the stability problems of current-controlled and droop-controlled grid-connected inverters in resistive-inductive grid,the sequence impedance models of current-controlled grid-connected inverters,droop-controlled grid-connected inverters,and virtual synchronous generators(VSG)have been established respectively,and the impedance analysis method is used to analyze their grid-connected stability respectively.And the influence of control parameters,line impedance and other factors on its grid-connected stability is discussed.It is concluded that the grid-connected stability of current control and voltage control under the condition of resistive-inductive grid has good complementary characteristics.2)Aiming at the subsynchronous oscillation problem of droop-controlled grid-connected inverters in series-compensated power grids,the impedance analysis method is used to analyze the subsynchronous oscillation mechanism,and on this basis,a strategy for suppressing subsynchronous oscillation based on virtual resistance under the synchronous rotating coordinate system is proposed.And a sequence impedance model of droop control with virtual resistance strategy is established.Finally,through impedance stability analysis,it is proved that the virtual resistance strategy can effectively improve the grid-connected stability of droop-controlled grid-connected inverters in series-compensated power grid.3)Aiming at the problem of subsynchronous oscillation of VSG in series-compensated power grids,a sequence impedance model of VSG considering reactive power loop is established,and the impedance analysis method is used to analyze the mechanism of subsynchronous oscillation.Based on the analysis,a strategy for suppressing subsynchronous oscillation based on the virtual resistance under the three-phase stationary coordinate system is proposed,and the sequence impedance model of the VSG with virtual resistance is established.Finally,the effectiveness of virtual resistance strategy is proved by impedance stability analysis.It can effectively improve the grid-connected stability of VSG in the series-compensated grid.4)Aiming at the stability problem of the grid-connected inverter when the grid impedance changes greatly,a multi-model estimation method for identifying different grid status and a multi-model control based on the adaptive switching control strategy of different grid status are proposed,and on this basis,the multi-model switching method is studied.Finally,this paper built a 20 k W multi-inverter system platform,and verified the correctness of the theoretical analysis and the effectiveness of the proposed strategy through experiments.