Abstract
This paper presents seven nonlinear mathematical models of a dc/ac droop controlled inverter connected to an infinite bus. These models are produced using the singular perturbation method to decompose a previously proposed 13th order nonlinear model. The models orders are 13th, 11th, 9th, 7th, 5th, 3rd and 1st. Each model ignores certain control or structural component of the full order model and allows researchers to choose the model order that meets their required accuracy and complexity. Moreover, the effect of grid (line) impedance on the accuracy of droop control is explored using the 5th order model. Simulation results show that traditional droop control is valid up to R/X line impedance value of 2. Furthermore, the 3rd order nonlinear model improves the currently available inverter-infinite bus models by accounting for grid impedance, active powerfrequency droop and reactive power-voltage droop. Results show the 3rd order model's ability to account for voltage and reactive power changes during a transient event.