Control System Design of a Three-Phase Active Front End Using a Sliding-Mode Observer

This article proposes a sliding-mode-observer (SMO)-based control strategy to regulate the dc-link voltage for a three-phase two-level active front end (AFE). The SMO is designed for the voltage control loop to estimate the external load which is abruptly connected to the AFE dc-link and consequently causes the dc-link voltage fluctuation. The estimated load value is used to compensate the voltage loop controller, therefore, the voltage loop gains more robustness against the load perturbation and its disturbing effect is greatly reduced. The effectiveness and advantage of the proposed control strategy has been verified through theoretical analysis, simulations, and the real-application experiments conducted on a 5 KVA laboratory AFE. The results show that the proposed control strategy provides obvious improvement of the dc-link voltage control performance comparing with the conventional PI controller and demonstrates stronger robustness against the operating point variations caused by the changes in external load and dc-link capacitance.

I. Introduction

Active front ends (AFEs) are grid-connected power converters based on controlled switches such as IGBTs. These converters are widely used in industry, where they work as interconnecting bridge between the renewable energies and the utility grid [1]–[8], also they can work as active power filters to provide the required harmonic currents to cancel out the harmonic contents on the utility grid [9]. These converters have numerous advantages. First, they can achieve unit power factor: on the one hand, it can provide a current with a very low total harmonic distortion (THD), which is reduced to 3% when normally is 25%–30% in the case of a conventional diode converter, thus it provides very clean energy; on the other hand, the controlled ac current is in phase with voltage, therefore, it appears as a pure resistive load. Second, they offer bidirectional power exchange between ac mains and dc-link, thus it is perfectly reversible and it can return energy back to the mains to reduce the power loss. Third, in the cost point of view, they allow to eliminate power factor correction systems and harmonic filters, thus the cost is reduced.