Enhanced DC-Link Voltage Dynamics for Grid-Connected Converters

Abstract

Three-phase pulsewidth modulated converters are usually controlled to achieve sinusoidal currents in the grid side and tight voltage regulation in the dc-link capacitor. The implementation of linear controllers based on small-signal models is a well-established solution for these converters. However, the dynamic performance can be improved only to a limited extent and it deteriorates under different operating conditions which may result in the uncontrollability of the dc-bus voltage during transients. In this work, a state-plane model and control strategy are introduced to improve the dc-link dynamics for three-phase converters. The proposed method provides fast, reliable, and consistent transient responses under the entire operating range, including rectifier and inverter modes. First, a normalized model that describes the natural dynamic behavior of the converter is derived and represented in the state-plane. In this way, the dynamic evolution of the operating point is described in a graphical domain as circular trajectories with well-defined characteristics. The insight provided by these natural trajectories is used to develop a control strategy to improve the dc-link dynamics by selecting a unique combination of circular paths to solve the transients. As a result, the operating point follows well-defined trajectories to achieve fast and consistent dc-link voltage responses under a wide operating range. The effectiveness and feasibility of the proposed control strategy are validated by several simulation and experimental results.