Evaluation of control and modulation methods for modular multilevel converters

Abstract

The modular multilevel converter is a promising converter technology for various high-voltage high-power applications. Despite the apparent simplicity of the circuit, the inherent dynamics of the converter and the balancing of the sub-module capacitor voltages impose high requirements on the control system, which can be implemented in quite different ways. To illustrate this, and to provide a guidance for future research on the subject, this paper presents an evaluation of four different control and modulation methods. The investigation is based on experiments on a down-scaled 10 kVA converter having 10 submodules per phase leg. The main items to be investigated are dynamics within the sub-modules, arm voltages and circulating currents. It is found that the suggested open-loop control method provides the fastest arm-voltage response and that the balancing approach based on a sorting algorithm is substantially faster and less complicated to implement than the method using a dedicated voltage controller for each sub-module.

Novel Control of a Modular Multilevel Converter for Photovoltaic Applications

“…In the other words, we can control the output and circulating current components by using a single controller in each leg, unlike the traditional closed loop controller in [33] and [34], where the modulation index was separately generated for each individual arm. Using a single procedure to generate the modulation index in each leg can simplify the experimental implementation of the MMC controller in practical applications.…”

Selected aspects of Modular Multilevel Converter operation

“…In the best case the difference current is constant and equals to 1/3 of the dc current i dc . Achieving this best case state is the aim of the MMC control method known as open-loop control [7,9] and other control methods [10,11]. Reducing the difference current to merely the dc component minimizes MMC power losses [12].…”

“…The ac component of the difference current, i diff,ac , is minimized (ideally down to zero) by the open loop control method [3,7] -as opposed to the MMC operating under the direct modulation method governed by (13), where the i diff,ac can be large.…”

“…For the purpose of this article, the direct modulation method [7,8] is assumed to be used in both MMC models. In this method transistors in submodules are switched according to sinusoidal modulating signals, without any feedback from capacitor voltages.…”

High-efficiency MOSFET-based MMC for LVDC distribution systems

“…Each submodule contains two MOSFETs and one capacitor which acts as an energy storage component that may be inserted in the series path or bypassed according to the switching state of the MOSFETs [8]. The use of multilevel modulation allows the generation of a low-distortion output voltage without the need for output harmonic filters or a high switching frequency [9]. With increased cell number, switching loss is virtually eliminated resulting in a converter design in which conduction loss dominates.…”