Nicolas Cherix

Functional modeling and Energetic Macroscopic Representation of Modular Multilevel Converters

A key feature of Modular Multilevel Converters (MMC) is the continuous and independent nature of the branch currents. Therefore, a decoupled control of the line and partial bus currents is possible, unlike for classical topologies. To this end, several control strategies have been proposed in the literature. Unfortunately, some of these strategies appear to be rather complex and are prone to conflicting control loops. This paper uses the formalism of the Energetic Macroscopic Representation (EMR) in order to develop functional representations of the MMC phase legs, which reveals to be very helpful in the understanding of the intrinsic converter behavior as well as its control structures. Both coupled and non-coupled branch inductors are taken into account in the developments. In a second step, this paper shows how such representations can be used to deduce simple and effective control strategies through elementary inversion rules. As an example, a typical decoupled current control strategy is developed and its effectiveness is verified by simulations.

Accurate voltage ripple estimation and decoupled current control for Modular Multilevel Converters

The analytical solution for the sub-module voltage ripple equations of a Modular Multilevel Converter (MMC) is derived, based on the knowledge of the external voltage/current magnitudes and enhancing a concept previously presented in literature. In order to achieve high accuracy, all passive elements of the converter, common-mode voltage injection as well as intentionally imposed circulating current harmonics are taken into consideration. A three-phase grid-connected MMC is chosen as an application example. The line and circulating current dynamics are also derived, which show that they can be controlled independently by forming two respective feedback loops. Any additional voltage regulators are avoided. The proposed implementation utilizes proportional-resonant (PR) terms for the line current, making it thus possible to introduce explicitly any chosen harmonic component. Experimental results from a reduced-scale laboratory prototype verify the discussed concepts.

Fail-safe modular control platform for power electronic applications in R&D environments

This paper presents a modular control platform which is tailored for teaching and research applications. It aims at being entirely “student-proof” and features software-independent safety mechanisms, extensive signal conditioning and easy-to-use interfaces, dedicated to power electronics. A practical example with various HIL simulation results illustrates the attractiveness of the presented platform.

Single-to-three-phase direct AC/AC modular multilevel converters with integrated split battery energy storage for railway interties

This paper presents a global hierarchized control method for railway interties based on the direct single-to-three-phase AC/AC Modular Multilevel Converter with integrated split battery energy storage. The power and voltage decoupling between the converter submodules and the batteries is achieved through non-isolated dedicated DC/DC converters. The control objectives include submodule capacitor voltage and battery state of charge balancing. All possible converter operation modes are investigated by means of simulations. Moreover, potential three-phase grid asymmetries are considered and studied. In the latter, the integrated storage elements can be utilized to transfer the necessary active power to the railway network side irrespectively from three-phase grid faults.

Operation and control of single-to-three-phase direct AC/AC Modular Multilevel Converters under asymmetric grid conditions

Single-to-three-phase direct AC/AC Modular Multilevel Converters have gained a significant interest in the field of railway interties. The use of such an MMC-based structure implies more advanced control techniques as well as design requirements, especially in the case where the three-phase side experiences grid asymmetries. This paper proposes a global control scheme, which is capable of ensuring high performance in such operating conditions. In addition, the impact of grid unbalances on the MMC capacitive energy storage requirements is evaluated by using different line current control techniques with current amplitude limitation as well as taking into consideration a three-phase reactive power injection. The proposed concepts are validated by means of simulation results. Finally, preliminary experimental results from a down-scaled laboratory prototype are provided.

Sizing of branch inductors parameters in modular multilevel converters: An analytical approach

One of the key features of Modular Multilevel Converters is the existence of inductors inside the branches that constitute each of their phases. These inductors, which may be coupled or non-coupled, authorize a certain independency in the control of the branch currents and contribute to the protection of the converter as well as the buses the latter is connected to. Through the analysis of the various current loops that are involved in this converter technology, the present paper aims to present a general parameter design method and proposes several criteria in order to analytically size these branch inductors. The possible use of coupled inductors is considered and compared with the use of non-coupled inductors. Finally, a numerical example as well as simulation results illustrate and confirm the validity of the proposed approach.