Research On Virtual Oscillator Control Strategy Of Microgrid Inverter Under Complex Working Conditions

With the rapid development of distributed Generation(DG)technology,micro-grid(MG)based on parallel inverters has become an effective way to solve energy shortages and alleviate the problem of environmental pollution from traditional fossil energy sources.An in-depth study of the control strategy of the power generation system is needed to improve the issues in MG operation.Virtual oscillator control(VOC),a new distributed control technology,shows great potential for application in off-grid and grid-connected inverters under its inherent interconnection-free self-synchronization mechanism.Compared with the traditional droop control,the MG system under VOC has higher redundancy and good anti-disturbance capability.This paper takes VOC as the primary research object,discusses the problems of power quality and power distribution in MG islanding operation systems under complex working conditions,and improves the stable operation performance of microgrids by improving the control strategy.When some complex operating conditions,i.e.,load variations and nonlinear unbalanced loads connected to the system,the VOC-based off-grid inverter system commonly suffers from harmonic distortion,voltage deviation,and frequency deviation.The harmonic suppression capability of the system is improved by introducing voltage and current positive and negative sequence separation techniques and improving the voltage and current dual closed-loop control structure to address the above harmonic distortion problems.An improved VOC scheme is proposed to improve the voltage and frequency deviation of the inverter system under different lines and conditions.The droop characteristics of the VOC are enhanced by designing a voltage compensator and frequency compensator and then alleviating the system voltage and frequency instability problem.The simulation model of the three-phase three-wire off-grid inverter system is built in Matlab/Simulink platform,and the effectiveness of the proposed control strategy in improving the harmonic distortion and voltage and frequency deviation problems is verified by comparing it with the conventional VOC.A VOC-based sequencing control strategy is designed to effectively suppress voltage distortion by combining the output voltage vector asymmetry of the three-phase four-leg voltage source inverter(TPFL-VSI)under unbalanced conditions to solve the problem of system voltage distortion under unbalanced conditions.The basic idea is to use the second-order generalized integrator(SOGI)to extract the positive and negative sequence components of the current in the first three bridge arms,use the VOC to regulate the positive sequence components,design the virtual impedance and multi-proportional resonance controller to suppress the negative sequence components,and develop the proportional resonance controller to control the zero sequence voltage in the fourth bridge arm.A simulation model of TPFL-VSI is established using Matlab/Simulink platform to verify the feasibility of the proposed sequencing control strategy.Considering factors such as virtual impedance and complex operating conditions introduced in the control system,the control accuracy remains poor by relying only on primary control to regulate the system frequency and voltage deviation.The line impedance varies from inverter to inverter,and the non-proportional distribution of active power is pronounced.To further improve the stability of system voltage and frequency and improve the active power distribution effect,a VOC-based hierarchical control strategy for islanded MGs is investigated to obtain frequency,voltage,and active power deviations by interacting electrical energy information between neighboring DGs and applying a consistency algorithm to get dynamic correction values,which in turn leads to improved control effects.An MG hierarchical control model with four TPFL-VSIs connected in parallel is developed to verify the strategy’s control effectiveness in improving the power system’s frequency and voltage stability and the capacity-based distribution of reactive power.