Subject Ph. D.

Context

A key enabler in combating climate change is the transition toward renewable energy sources such as solar and wind power. This transition has driven a major paradigm shift over the last decade: energy production and storage are becoming increasingly distributed, and alternating current (AC) components are progressively being replaced by direct current (DC) devices. In this context, DC microgrids have emerged as a promising paradigm for integrating renewable sources and local storage. Their operation relies heavily on power electronic converters, which manage power flows among multiple sources, storage elements, and loads. Ensuring efficient, stable, and reliable operation of these converters is therefore essential. These challenges form the core of the ANR JCJC project CASIMIR.

The converters used in DC microgrids are commonly referred to as multiport converters because they interface several energy sources and loads through multiple ports. A key characteristic of these systems is that they exhibit degrees of freedom in their control inputs with respect to the primary control objective, namely maintaining the DC bus voltage.

The main challenge addressed in this thesis is to exploit these degrees of freedom to enhance performance while preserving stability and voltage regulation. The approach envisioned in CASIMIR relies on dynamic control allocation, which decomposes the actuation into three coordinated components: (i) a stabilizer guaranteeing voltage regulation, (ii) an optimizer distributing currents among the different ports to minimize losses or meet secondary criteria, and (iii) an annihilator ensuring that these secondary objectives do not interfere with the main regulation task.

While such techniques are well established for linear systems, extending them to nonlinear systems, such as many multiport converter architectures, remains largely unexplored. The strategy adopted here is to focus on a well-understood multiport topology as a first step toward generalizing dynamic control allocation to nonlinear systems.

Objectives

The thesis focuses on developing dedicated dynamic control allocation strategies for two multiport converter architectures: parallel Boost converters and the Three-Port Converter. Beyond ensuring an optimal distribution of currents among the different ports while maintaining voltage regulation, the thesis also aims to analyze the mechanisms underlying the degrees of freedom in these nonlinear systems. This understanding will be used to derive a general framework for dynamic control allocation, particularly for the systematic design of the annihilator component. The work is structured into the following tasks: - Build and validate a nonlinear dynamical model of the two multiport converter architectures. - Design a stabilizer ensuring voltage regulation at the DC bus, with an emphasis on Lyapunov-based methods. - Design an optimizer that distributes current among the converters to minimize conduction losses at steady state. - Design an annihilator guaranteeing that the voltage regulation objective is preserved. - Validate the complete dynamic control allocation schemes in both MATLAB/Simulink simulations and experimental setups, comparing their performance with standard control strategies (e.g., PI-based voltage control, droop control). The experimental setup will be partially developed by the candidate as part of the thesis. - Compare the performances of the dynamic control allocation strategies with respect to existing approaches from the power electronics community.

Depending on the candidate's interests and progress, an additional research direction involves exploring the passivity properties of multiport converters at its interconnection ports. The goal is to investigate how these properties can be leveraged to enhance the stability and performance of the whole DC microgrids through passivity-based control techniques.

This work lies at the interface between nonlinear control theory, power electronics, and energy systems.