Stability Analysis And Control Strategy Research Of Grid-connected Inverter Under Weak Grid Connection

Renewable energy based distributed power generation system（RE-DPGS）is currently the main way to develop and utilize renewable energy on a large scale.The grid-connected inverter is the key interface device of RE-DPGS.It undertakes the important task of converting DC power into high-quality AC power and feeding it into the grid,and its performance has an important impact on the quality of grid-connected power.With the increasing penetration of RE-DPGS into large power grid systems,large power grids have shown weak grid characteristics with small short-circuit ratios.The stable and efficient operation of the inverter will have a negative impact,and in severe cases,it will cause problems such as harmonic resonance and even instability of the"machine-grid"interconnected system.For this reason,this paper takes the single-phase LCL type grid-connected inverter as the research object,deeply analyzes the dynamic interaction between the"machine-grid"interconnected systems in the weak grid,and carries out the grid-connected current control technology of the grid-connected inverter.Relevant research to improve the quality of grid-connected power and ensure the safe,reliable and high-quality operation of RE-DPGS has certain theoretical analysis value and practical application prospects.This paper mainly studies the related problems caused by grid-connected inverters operating under two common grid-connected current control technologies,current double-loop control technology and weighted average current control technology.Under the influence of the coupling of the control system,there are unfavorable hidden dangers that seriously threaten the safe operation of the system,such as low relative stability of the control system,easy to induce harmonic resonance and poor robustness.The main research content is divided into three parts:First,consider the current double-loop control technology of the grid-connected inverter using capacitive-current-feedback-active-damping（CCFAD）loop and direct current feedback loop,under the influence of digital control delay The CCFAD loop can be equivalent to a virtual impedance Z_eq（s）connected in parallel at both ends of the filter capacitor,and its negative resistance characteristic is the main reason for the poor robustness of the control system to weak grids or even instability.In view of this,this paper proposes a robust CCFAD method using a negative first-order inertia link for active damping of capacitive current feedback.By introducing a negative inertia link into the CCFAD loop,the phase lag caused by the digital control delay is compensated.The frequency stability analysis discusses the proposed method in detail,and gives the design process of the relevant parameters.Theoretical analysis shows that the proposed method can ensure that the equivalent virtual impedance of the CCFAD loop is always in the positive resistance characteristic range within the effective range of the resonant frequency of the LCL filter.In addition,the proposed method has good flexibility,which can save the use of a set of current sensors when capacitive-voltage-feedback-active-damping（CVFAD）is adopted and phase-locked.Secondly,the research is carried out on the problem that the resonant frequency of the control system is shifted when the impedance of the equivalent grid changes,and the relative stability of the control system is reduced.The analysis shows that the change of the equivalent grid impedance in the weak grid changes the frequency characteristics of the LCL filter,and the grid-connected inverter may cause the grid-connected current harmonic resonance due to the low stability margin near the open-loop cut-off frequency of the control system.Stability issues.The mechanism of harmonic resonance generation and the relationship between stability margin and filter resonant frequency offset are further revealed,and the minimum resonant frequency offset condition and the influence of the minimum resonant frequency offset on the grid-connected current controller are deduced..In view of this,this paper proposes a series of grid-connected inverter stability control methods for suppressing the resonant frequency offset in weak grids,and gives the detailed implementation scheme and parameter design steps of the method.After theoretical analysis,it can be seen that the proposed method can effectively suppress the influence of the filter resonant frequency offset on the steady-state and dynamic performance of the system in the mid-low frequency band under weak power grids,approximately achieve the zero offset of the resonant frequency of the control system,and the robustness of the control system.Guaranteed.Finally,the stability of weighted average current（WAC）control technology under the influence of weak grid and digital control delay coupling is studied.The WAC control technology achieves the purpose of reducing the third-order control system to a first-order control system by performing zero-pole cancellation on the loop gain of the grid-connected inverter control system.However,the traditional WAC control technology does not fully consider the impact of the digital control delay on the system.The existence of the digital control delay will make the WAC control technology unable to ideally reduce the order of the system,and the grid-connected inverter is robust to weak grids.Poor sex.In view of this,this paper proposes a strong robust WAC control method considering the digital control delay.By introducing a first-order high-pass filter into the voltage feedforward channel of the common grid connection point,the system reverse resonance peak generated by the digital control delay is detected.Complete compensation to improve the adaptability of grid-connected inverter to weak grid.