Disturbance Observer-Based Control in the Synchronous Reference Frame for Grid-Connected Inverters in the Presence of Grid-Voltage Harmonics

Abstract

This paper proposes the design and testing of disturbance observer-based current control for an L-type grid-connected voltage source inverter (VSI) considering grid voltage distortion. In particular, the proposed current controller is designed to ensure the injection of high-quality currents into the grid despite the presence of grid voltage harmonics. In this work, the current controller is derived in the synchronously rotating (dq) frame to allow for one-to-two harmonic compensation. In such a frame, a disturbance observer is designed to estimate the effect of selected voltage harmonics, which is then compensated for by a feedback control law. Nevertheless, synthesizing a stable disturbance observer to cope with a large number of harmonics is still challenging. The main challenge in observer design lies in selecting suitable observer gains, whose dimensionality increases as the number of voltage harmonics increases. This challenge in selecting the observer gains is addressed in this work by using the concept of D-stability together with the linear matrix inequality (LMI) approach. This approach reformulates the stability problem of the observer into solving specific LMIs offline such that all observer eigenvalues are confined within a stable region D, which can be selected according to desired transient performance. An additional insight is that the resulting current controller structurally reduces to a proportional-integral (PI) controller augmented with a multi-resonant (MR) compensator, thus facilitating practical implementation. Laboratory-scale experimental results confirm the efficacy of the proposed controller in achieving good dynamic performance with accurate mitigation of harmonic voltage disturbances.