Effect of DG and induction motor load power rating on Microgrid transient behavior

This paper presents an analysis of the effect of power rating on the dynamic characteristics of a Microgrid. The purpose of such analysis is to permit the results of experimentation and simulation to be compared over Microgrids with varying rating. It is shown that nominal power and the parameters of the Distributed Generators (DG) and Induction Motor (IM) loads can be characterized by a nonlinear but tractable relationship. Then, a small signal analysis is used to observe the natural frequencies of the system for nominal power ranging from 1kVA to 1MVA. Results show that the natural frequencies of most DGs and IM loads decrease their damping coefficient as power increases. This has implications on the ability of the Microgrid to recover voltage after a fault triggered islanding and on the speed at which the control loops of the DGs have to be implemented. A non-linear model of the complete system is built in MATLAB/Simulink to observe the combined effect of varying all parameters with power rating. The results show that increase in per unit inertia of machines leads to a slower recovery after a fault.