Impedance Circuit Model of Voltage Source Converter With DC-Link Voltage Control Dynamics

Abstract

The impedance circuit model maps the control algorithms into the circuit topology of voltage source converters (VSCs). By analyzing discrete circuit elements, the model provides clear physical insight for the oscillation mechanisms triggered by the control dynamics. However, previous studies do not consider the outer-loop control and reactive power. To enhance the generality, this paper integrates the DC-link voltage control (DVC) loop into the impedance circuit model and thoroughly considers the non-unity power factor operating conditions. In this model, DVC dynamics is mapped as two equivalent impedances in the AC circuit, and interaction between the inner and outer loops is visualized by the interconnection of impedances. The analysis indicates that the perturbation of DVC dynamics on grid-connected current introduces the negative resistance effect at low-power levels. Moreover, the effect of reactive power on impedance circuit is mapped as a coupled current source linking the d-axis and q-axis circuits. As the inductive-reactive power increases, self-excited oscillations occur in the coupled source. According to the stability constraints of the coupled sources, a strict design method for the control parameters is proposed. Experimental results verify the effectiveness of the proposed model.