A general discrete time model to evaluate active damping of grid converters with LCL filters

While LCL filters offer significant benefits for grid connected converters, they always require management of their inevitable filter resonance. Many active damping strategies have been proposed to resolve this issue, using various feedback combinations of the converter side, filter capacitor and grid side currents. However, these strategies are often presented as continuous time formulations that ignore the impact of converter PWM delays, and are also not usually evaluated on an exactly comparable basis. This makes it hard to select between them to find the most suitable alternative for a specific context. This paper presents a generalised discrete time model for a grid connected VSI with an LCL filter, fully accounting for all practical converter delays and second order effects. The model is arranged to allow any active damping strategy to be readily integrated and evaluated under identical operating conditions, so that a comprehensive comparative evaluation of its relative damping performance can be easily performed. Experimental and simulation results are presented to compare the relative performance of several well known active damping strategies, to illustrate the capability and flexibility of the generalised model.