Feedback Linearization Based Current Control Strategy for Modular Multilevel Converters

Abstract

Modular Multilevel Converters (MMCs) are multi input-multi-output (MIMO) nonlinear systems. The control systems for MMCs are required to simultaneously achieve multiple control objectives. Existing cascaded control strategies for MMCs are complex and the controller parameter design is not straightforward for the nonlinear systems with highly coupled states. In view of this, a feedback linearization based current control strategy is proposed for an MMC system in this paper. The nonlinear state function model of the MMC is presented and transformed to a linearized form with the help of the feedback linearization technique. Simple linear controllers are employed for the linearized system to regulate the output and inner differential currents of the MMC, which significantly reduces the difficulty in current controller design.

Existing System

Phase-Shifted Carrier Modulation.

Proposed System

In this paper, a nonlinear control strategy, where the differential geometry based feedback linearization technique [11] is employed, is proposed to regulate the output and inner differential currents of a single phase MMC system. A state function model of the MMC system is presented, which shows that the MMC is an MIMO nonlinear system. The feedback linearization technique is applied to transform the MMC nonlinear state-function model to a linear one. Linear controllers are designed for the linearized MMC system to control the output and differential currents respectively. The controller design process for the feedback linearization control strategy is significantly facilitated in comparison to that of the conventional cascaded control strategies, which employ multiple resonant controllers or repetitive controller for suppression of circulating current. To experimentally verify the effectiveness and the improved performance of the proposed current control strategy, both the feedback linearization control strategy and the conventional cascaded control strategy with plug-in repetitive controller proposed are implemented in a laboratory MMC prototype. The controller parameters of the feedback linearization current control strategy are designed according to linear control laws. The experimental results are presented to show the excellent steady state and dynamic performances of the proposed feedback linearization current control strategy.

Architecture

Block diagram of the overall MMC feedback linearization control system