Research On Harmonic Stability Analysis And Responsibility Identification For Multi-inverters Multi-PCC Grid-connected System

In the weak grid,a large number of grid-connected inverters may cause system stability problems,resulting in system resonance and endangering the system stable operation.Based on the impedance-based stability analysis,harmonic stability problems caused by the impedance coupling between the inner current loop of grid-connected inverters and the power grid have been analyzed and suppression strategies have been stuided.The main research contents are as follows:1.A stability analysis method is proposed for the multi-paralleled grid-connected inverter system.The Norton equivalent circuit model of the grid-connected inverter is first established.By dividing the global admittance into two parts,the relationship between the stability analysis of the classical control theory and the resonance analysis of the circuit is established.This method shows that the harmonic stability problem of the grid-connected inverter system can be equivalent to the resonance problem of RLC series-parallel circuits.The resonance point of the system in the negative damping frequency band is the necessary and sufficient condition for the harmonic instability.2.Considering the impact of both the resonance point and system damping,two methods are proposed to quantitatively analyze the responsibility of different inverters for system instability.Method 1 is called the result-oriented method,calculating system characteristics after assuming the inverters are removed,which can accurately reflect the influence of each inverter.Method 2,the cause-oriented method,taking the initial system state and the inverter state as the object,using four characteristics of the inverter output admittance as the evaluation criteria,and using the combination weighting method to determine the coefficients of each element,can quickly reflect the impact of each inverter on system stability.In a multi-node power system,a step-by-step method of determining the impact of subsystems on the entire system stabiltity is proposed,which effectively reduces the amount of calculation.3.A distributed active damping method is proposed for resonance suppression.Based on the grid voltage feed-forward strategy,an additional damping loop is applied to the inverter control system.A second-order lag corrector is used to reduce the impact of ignoring the prediction unit,and an improved adaptive tuning algorithm is proposed using the inverter output current as a reference.