Observer Based Admittance Shaping for Resonance Damping in Voltage Source Converters with LCL Filter

Abstract

An observer based resonance damping controller is proposed for voltage source converters (VSCs) with LCL input filter connected to any arbitrary AC network. In prior art, design and analysis of observer based current controllers has been performed either in z-domain taking a discrete approximation of the LCL filter or in continuous-time but with the observer discretized for digital implementation; both methods introduce significant modelling error in the high frequency range. We use a continuous-time equivalent of the discrete observer and develop a comprehensive model of the VSC including the effects of discretization, controller delay, and PWM process. Leveraging the model, we propose a dual-loop current controller with an inner admittance shaping loop that achieves internal stability of the VSC by damping the LCL resonance without the need for additional sensors; moreover, passive converter admittance is achieved almost up to the Nyquist frequency which provides sufficient condition for stability for any uncertain network impedance that may lead to harmonic resonance within that frequency range. The resonance damping capability of the proposed controller is validated through hardware experiments.