Analysis Of Resonance Characteristics And Research On Active Damping Suppression Strategy Of Photovoltaic Grid-connected System

Vigorously promoting the transformation of energy structure and rapidly building a novel power system with a higher percentage of renewable energy are both the evolution path of Chinese future energy system,which is also an significantly important measure to accomplish the target of “carbon peaking and carbon neutrality”.Photovoltaic（PV）power generation has gained more attention from the researchers and scholars because it riches in natural resources and is widely distributed in various regions.Therefore,making sure the safety of PV networking systems is increasingly important.The resonance characteristics of PV networking systems are closely related to the system network impedance,and the network impedance will change when the operating conditions and time of the system both change.Moreover,the system safety will be seriously threatened because of the low damping property of the PV networking system.Therefore,a correct quantitative analysis of resonance characteristics and designing the relevant suppression strategies of the PV networking system have the practical significance for improving the system safety.The paper makes an further depth study on the resonance characteristics and active damping methods of single-inverter-based grid-connected system which operates in variable grid impedance and parallel-inverter-based grid-connected system.The main work is as follows:（1）Analysis of resonance problem in a grid-connected system.This paper first develops a single inverter model using Thevenin’s equivalent circuit.The resonance problem of a single-inverter-based grid-connected system operating under variable grid impedance is quantitatively investigated with the developed model.Moreover,this paper develops a parallel-inverter-based grid-connected system model with single inverter Thevenin’s equivalent circuit model.The resonance problem of a parallel-inverter-based grid-connected system when the number of parallel inverters changes is quantitatively investigated with the developed model.Based on the developed model of grid-connected system,the system resonance frequency expression is derived,which develops a model and theoretical basis for resonance suppression strategies.（2）Active damping adaptive adjustment.This paper introduces the realization method of three-phase LCL-type inverter with active damping based on capacitive current feedback.An adaptive adjustment formula of active damping when the grid impedance changes in a wide range is further deduced.Moreover,this paper investigates the oscillation characteristics of inverter terminal voltage because of the sudden change of the grid-connected inverter output power.A grid inductance detection method based on the terminal voltage oscillation characteristics is proposed.An adaptive adjustment scheme of active damping of a grid-connected system based on the grid inductance detection method is further designed.According to the detected grid inductance,the active damping parameters of the inverter control system can be adaptively adjusted online,which can highly guarantee current waveforms to be smooth.Simulation results are provided to validate the effectiveness of the grid inductance detection method and the adaptive adjustment strategy of the active damping in a single-inverter-based grid-connected system.（3）Active damping optimization method.This paper focuses on the resonance suppression problem of a parallel-inverter-based grid-connected system.A PCC-voltage-feedforward-based and a virtual-conductance-based active damping methods are carried out,respectively.Based on the virtual-conductance-based active damping method,the virtual conductance in parallel with the filter capacitor is equivalent to a virtual resistance.Through the filter capacitor in parallel with the virtual resistance,this paper makes some analysis about the resonance suppression effect when the parallel-inverter-based grid-connected system employs different virtual resistances.Moreover,this paper proposes an approaches for selecting the optimal virtual damping.Based on the simulation results,the feasibility of PCC-voltage-feedforward-based and virtual-conductance-based active damping methods and the effectiveness of the proposed approaches for selecting the optimal virtual damping are validated,respectively.