Description du sujet de thèse

Methodological Tool for the Stability Analysis of Variable Structure MVDC Networks

Contexte de travail

MVDC (Medium Voltage Direct Current) targets power network operators aiming to expand their grid to integrate more sources (renewable) or loads (such as electric vehicles), offering a decentralized and renewable production solution (up to 200 km distance and 20–100kV).
Renewable energy and storage sources are connected to an MVDC microgrid through regulated power converters that are interconnected via filters and passive lines. In the case of a single production unit, the associated converter is high-efficiency and regulated with fast dynamics to ensure bus voltage quality while maintaining sufficient stability margins. Due to the intermittent nature of renewable energy sources and unpredictable load connections, ensuring stable operation becomes crucial—or at least determining the conditions under which the network remains stable.
In this kind of network, multiple generation units can interconnect to form a decentralized generator (DG) system that powers multiple loads. In such complex systems, due to low inertia and possible interactions between different bandwidths (controllers, passive filters, communication), instability may occur, particularly under demanding load conditions. This is especially critical when supplying Constant Power Loads (CPL), during reconfiguration of the network following DG/load connection or disconnection, or during potential system expansion to integrate new sources and/or loads. These constraints lead to stability issues across extended timescales, ranging from a few milliseconds to several minutes.
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Actions:
An initial phase consists of developing an MVDC network model with DGs and associated converters (analytical and/or simulation model). Scaling of a real network and compliance with standardization will be essential. A subsequent stability analysis will be conducted, leveraging converter control or topology adjustment via DG connection/disconnection.
A hierarchical control strategy will be explored to provide proof of global network stability in the presence of large disturbances while considering the potential for system expansion.
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Planned Steps:
• Literature review on tools for static and dynamic stability analysis of energy systems and DC/AC networks (impedance spectroscopy, Lyapunov methods, etc.).
• Study of large-signal synthesis methods to develop a generic and scalable formulation applicable in case of network reconfiguration or expansion. The concept of passivity, based on Hamiltonian formalism, appears suitable to tackle this issue.
• Design of hierarchical control strategies (droop, consensus…) for the MVDC network to ensure system resilience and maintain stability under large disturbances (load impacts, connection/disconnection of one or several DG units, and issues linked to communication systems such as delays and losses).
• Establish proof of global stability for the developed strategies under significant disturbances.
• Development of active stabilization techniques in line with the chosen formalism, to address resonance phenomena (high-frequency stability), notably caused by filter-controller interactions.
• Validation of the developed hierarchical strategies on a reference MVDC network within a HIL (Hardware-In-the-Loop) environment.

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Contraintes et risques

Exploration work in the framework of National PEPR TASE Architect project