Alexandre Christe

State-space modeling of modular multilevel converters including line frequency transformer

This paper presents a detailed state-space modeling of a Modular Multilevel Converter (MMC) in combination with line frequency transformer (LFT). The classical MMC topology is compared with the recently proposed Open-End Winding MMC (OEWMMC) topology, which integrates the LFT in the converter arm. The line frequency transformer parameters are added in the model, some of which are often neglected in the available literature. Both models are verified and compared by means of numerical simulations in open-loop. Despite integration attractiveness offered by the OEWMMC topology, there are also some inherent drawbacks affecting the overall system sizing that are discussed in the paper.

Insulation Coordination for a Modular Multilevel Converter Prototype

This paper provides a detailed overview of the dielectric design and insulation coordination applied to a medium voltage modular multilevel converter prototype. The complete system has ratings of 0.5 MVA and is designed for connection to a 10kVdc supply with a system voltage of 6.6kVac. The choice of air as insulating and cooling medium requires careful considerations regarding the clearance and creepage distances with respect to the selected materials. The design considerations from the submodule to the cabinet level are presented in the paper, considering safety standards and their requirements for a selected over-voltage category and pollution degree. The final dielectric design is verified experimentally with dielectric ac withstand test and partial discharge measurements.

Virtual Submodule Concept for Fast Semi-Numerical Modular Multilevel Converter Loss Estimation

During the design phase of a modular multilevel converter (MMC), an accurate loss evaluation of the submodule (SM) plays an important role. In this paper, a method based on the analytical description of the MMC key waveforms that allows to directly obtain the average semiconductor and capacitor losses that each SM will experience is introduced, under different operating conditions or control schemes. To verify the proposed concept, the results are compared with the losses obtained from a switched model with closed-loop control, where the analytical MMC key waveforms are approached in steady state. The proposed method provides a great flexibility and a significant reduction of the simulation/computational time otherwise needed to evaluate SM losses under various operating conditions.

On the integration of low frequency transformer into modular multilevel converter

Various high power applications require power converters with large voltage step ratios, easily achieved by inclusion of a single or multiple transformers, which provide galvanic isolation at the same time. This paper presents, in a systematic manner, the necessary steps for the integration of a Low Frequency Transformer (LFT) into the Modular Multilevel Converter (MMC). Unlike the classical MMC that requires an external transformer for galvanic isolation, this work considers a transformer integration at the arm level resulting in a complete replacement of the arm inductors. Such galvanically isolated modular converters can be realized either in interleaved or stacked arrangements. The properties of each variant are discussed and compared with the classical MMC with external LFT, on the system design level but also from the control point of view.

Novel insight into the output current ripple for multilevel and multiphase converter topologies

When it comes to the computation / estimation of the switching losses in a multilevel converter, it is crucial to know the instantaneous value of the output current. If the average output current value is used instead, there is a non negligible error when the quality of the output current waveform is low. This paper addresses this issue with a generic and versatile fast numerical method for phase-disposition PWM that produces the exact switching pattern, hence the exact inductor flux, without having to run time-domain switched simulation(s), as the calculations are performed for the minimal set of points required to fully determine the inductor flux ripple at the switching instants.

Interactions between bandwidth limited CPLs and MMC based MVDC supply

With the improvement in the power electronic technologies, medium voltage dc (MVDC) electrical distribution systems are being considered for on-shore and off-shore applications. These future MVDC electrical distribution systems are expected to provide the possibility of easy interfacing of the renewable energy sources, improving the dynamics of the system and also help in reducing the carbon footprint of energy sources. Modular multilevel converters (MMCs) are used in high voltage dc (HVDC) applications and are being considered for MVDC applications as well. In this paper, we present an MVDC electrical distribution system where the source converter is an MMC and the loads exhibit bandwidth limited constant power load (CPL) behaviour. An analysis is carried out on the dynamic interactions between the MMC source converter and CPLs, considering varying distribution cable lengths between the source and the load, the filtering effort at the load end and different loading conditions.