Michail Vasiladiotis

Analysis and Control of Modular Multilevel Converters With Integrated Battery Energy Storage

Multilevel converters and battery energy storage systems are key components in present and future medium voltage networks, where an important integration of renewable energy sources takes place. The modular multilevel converter offers the capability of embedding such energy storage elements in a split manner, given the existence of several submodules operating at significantly lower voltages. This paper analyzes such a converter structure under different operating modes. In order to eliminate the low-frequency components of the submodule output currents, the latter are interfaced to the batteries by means of nonisolated dc/dc converters. Control algorithms are developed for the balancing of the battery state of charges and the respective gain limitations are established. Unbalanced grid conditions are also taken into account through the theory of symmetrical components and solutions are proposed. Finally, the development of a down-scaled prototype is described and experimental results are presented.

Dynamic Analysis and State Feedback Voltage Control of Single-Phase Active Rectifiers With DC-Link Resonant Filters

Single-phase active rectifiers exhibit an inherent strong second-order current harmonic on their dc-link. The latter is often undesirable from a load point of view and is therefore chosen to be eliminated. A common method is the use of a resonant passive LC filter tuned at the frequency, which increases the order of the system, posing certain difficulties for the voltage control design. This paper presents a comprehensive analysis of the dc-link dynamics. The equations are linearized by means of small variations, and a respective closed-loop regulator is formed utilizing the partial state feedback control theory for pseudocontinuous systems. The control output is fed to an inner current regulator, following the well-known cascaded loop method. The stability of the proposed controller is validated throughout the whole operating point regime and its robustness against system parameter variations is evaluated. A comparison with a conventional controller proves the advantages of the utilized method. The theoretical analysis is verified by means of simulations as well as real-time implementation on an experimental laboratory setup.

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.

Model predictive-based control method for cascaded H-Bridge multilevel active rectifiers

The cascaded H-Bridge multilevel active rectifier is an emerging converter topology, which offers significant advantages, such as modularity and high flexibility for a wide range of applications, including traction systems, industrial automation plants, uninterruptable power supplies, and battery chargers. However, the need for stable operation of the H-Bridge cells at asymmetrical voltage potentials and unbalanced loads imposes demanding requirements, in terms of an advanced and accurate control strategy. This paper introduces a simple and powerful solution to the mentioned problems, based on constrained Model Predictive Control (MPC). The proposed nonlinear controller achieves low input current harmonic distortion with almost unity power factor, as well as independent regulation of the H-Bridge cells, both under steady state and transient conditions. The effectiveness of the novel control algorithm is demonstrated by means of simulations as well as preliminary experimentation on a single-phase laboratory setup.

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.