Control of a Modular Multilevel Matrix Converter for High Power Applications

Diego Soto
University of Magallanes
Avda. Bulnes 01855, Punta Arenas, 113-D, CHILE

Jaime Borquez
University of Magallanes
Avda. Bulnes 01855, Punta Arenas, 113-D, CHILE

Abstract:

A control scheme for a multilevel AC-AC converter is presented. The converter resembles a matrix converter but it uses the cascade converter in place of converter valve. This is a string of H-bridge modules, each equipped with a DC storage capacitor, as the building block of the converter. This yields a highly modular implementation approach which may be suitable for high voltage, high power applications. One of the main issues with these modular multilevel converter topologies is the regulation of the capacitors voltages. This is a prerequisite to successfully operate the topology. This work explains the basics of the multilevel matrix converter and develops its associated control schemes to operate it as AC-AC converter. Control objectives include: regulation of multiple capacitor voltages; control of the multiple valve currents; and control of the output voltages. The proposed scheme is verified experimentally and through simulations. Results show good system performance in rejecting load impacts; maintaining capacitor voltages and valve currents within normal values; drawing low distortion, and close to unitary power factor, currents from the line; and creating high quality output voltages which result in low distortion load currents. The matrix multilevel converter thus shows great potential for high performance, high power motor drives.

Keywords:

Modular multilevel converter, matrix converter, control of power converter.

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CITE THIS PAPER AS:
Diego SOTO, Jaime BORQUEZ, Control of a Modular Multilevel Matrix Converter for High Power Applications, Studies in Informatics and Control, ISSN 1220-1766, vol. 21 (1), pp. 85-92, 2012. ttps://doi.org/10.24846/v21i1y201210

1. Introduction

Prerequisites

Because of the limitations of the current generation of semiconductor devices (voltage and switching frequency), large power converters typically use multi modular topologies such as the multi-pulse and multi-level converters [1]. In this context, because of the highly modular structure and the simplicity of the modules, the cascade converter is one of the most suitable topologies for high power applications [1]-[3], specially for Static Var Compensators, such as StatComs, and Active Power Filter (APF), where floating capacitors can implement the multiple isolated DC voltages needed.

At first, implementation of applications which requires the cascade converter to process active power, such as in a frequency changing converter [4], may not seem practical if implemented as a back-to-back connection of cascade converters. The requirement of isolation amongst the multiple H-bridge modules needs either multiple standard isolation transformers or a complex multi-winding isolation transformer [2] and [4].

Recently, a novel modular implementation approach to power converters, which resembles the direct AC-AC topology, as that shown in Figure 1 (no back-to-back based), has led to a new family of cascade-based converters [5].

This new generation of converters are known as the Modular Multilevel Converter (MMC) topology [5]-[6]. In this approach the cascade converter replaces the high-voltage valve (typically implemented by series IGBTs) required to implement a standard high-voltage converter topology. This is, for example, the series string of H-bridge converters arranged in a single-phase bridge, three-phase bridge or even in a full matrix converter topology.

The main advantages of MMC topologies come from using the multilevel conversion approach, with reduced switching frequency, hence reduced power losses, high quality input and output voltages and currents; and its highly modular implementation applying one of the simplest converter modules (the H-bridge converter). This may significantly reduce the cost of a high-power installation (manufacture of the modules and assemblage on the installation site).

One of the main challenges of this new conversion approach is in the development of suitable control strategies which enable these complex converter topologies to be used in a practical form (e.g. as a frequency changing converter). Tight control of the multiple capacitor voltages is fundamental to the successful operation of such converters.

This work addresses the control of a full matrix MMC topology. This is a converter which, as shown in Figure 1, has three input phases (a, b and c) and three output phases (A, B and C), where each switching element (valve) of the matrix is a multilevel cascade converter. In particular, this manuscript develops a suitable control scheme to operate the converter of Figure 1 as a high performance AC-AC converter, capable of fast changing the amplitude, frequency and phase of the output voltages. This requires fast control of the valve currents, regulation of all of the capacitor voltages and fast control of the output voltages (to rapidly force the load currents, e.g. a motor). The remainder of this paper is organised as follows: Section 2 describes the basic operation of the cascade matrix converter; section 3 develops a dynamic model of the converter and develops the required control strategies, with emphasis on the regulation of capacitor voltages; and finally section 4 presents experimental and simulation results which demonstrate the performance of the converter to load impact.

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