Impedance-Based Stability Analysis of Multi-Parallel Inverters Applying Total Source Admittance

The utility-scale wind and solar electricity production is typically connected to the power grid through multiple parallel three-phase inverters. One of the main issues in such grid-connected systems is the harmonic resonance caused by interactions between the grid and inverters. A common method for the analysis of these systems has been the impedance-based stability criterion. However, in systems that have multiple parallel inverters, the system complexity and challenges in obtaining the required impedance measurements may deteriorate the accuracy of the impedance-based approach. This paper discusses the aggregation of parallel inverters and the stability analysis of such grid-connected system. A simple method, based on impedance measurements, is shown for defining the allowable number of paralleled inverters so that the system remains stable. Experimental results are shown from power hardware-in-the-loop setup recently developed at DNV GL Flexible Power Grid Lab.