Renewable Energy Systems Instability Involving Grid-Parallel Inverters

This paper presents a study of renewable energy system stability based on impedance models of the sources and the grid. The focus is grid-parallel inverters controlled in the current injection mode. System stability analysis based on source output impedance and load input impedance is generalized to current-source systems. Output impedance modeling of current-controlled PWM inverters for grid interface of renewable sources is presented to provide the necessary impedance models for system stability analysis. The effects of current control loop design and output filter configurations on the inverter output impedance are considered. A practical method to measure grid impedance using a frequency analyzer is presented, and the associated difficulties due to the presence of harmonics in the grid voltage are discussed. An example system involving a single-phase grid-connected inverter is presented to demonstrate the application of the proposed system analysis method.