We are proud to support the research community through our engagement towards PhD students by providing contractual opportunities to promising talents and supporting them throughout their theses work. Our unique position as an institute for energy transition (ITE) allows us to bring together the worlds of academia and industry, combining their expertise to the benefit of all.

The 57 PhDs that have been undertaken since the beginning of the institute are a fine example of this cross-sector collaboration, providing valuable contributions to the energy sector, and developing the precise expertise needed to develop innovative technologies and solutions for the electricity grid of tomorrow.

The following PhD students are nearing the end of their research and will be presenting their final theses over the course of 2022. Read on below to find out what they have been working on!

Figure 1: Photograph of the square voltage test bench developed in the Ampère laboratory, Ecully, France

The inclusion of wide band gap (WBG) devices (such as SiC-MOSFETs) in the converters of future power networks will introduce electrical constraints in the form of repetitive fast rising square voltage waveforms. These waveforms can influence the discharge mechanism of insulators used in high voltage apparatus in a different way than in conventional AC sinusoidal waveforms. The aim of this thesis is to assess the insulation system under fast rising square voltages for power electronic applications using partial discharge (PD) measurements.

The test platform (see Figure 1) developed during this thesis can detect PD under repetitive fast rising square voltages by deploying a photomultiplier tube (PMT).

The existing platform rated at 30 kV and maximum switching frequency of up to 460 Hz, can be used for additional PD research in transparent mediums, as well as to develop tools for PD measurements in solids and online monitoring of equipment like medium frequency transformers (MFT). Future prospects include expansion to other diagnostic methods such as aging and breakdown tests and increasing the complexity of electrical constraints by superposing DC over square voltages; for the assessment of insulation for medium/high voltage equipment for future smart grid projects.

Keywords: Repetitive fast rising square voltage, partial discharge, optical detection, power electronic applications.

Thesis defended: January 2022

One of the main challenges in HVDC grids is protection after fault occurrence. The overall goal of protection is to ensure human safety and minimize the stresses on the system and components. This thesis studies the strategies for fault current interruption and isolation of the faulty zone in addition to maintaining the power flow in the healthy part of the HVDC grid. One of the contributions of this thesis is to define a strategy and design aspects of inductors and DC circuit breakers (DCCB) for different types of faults. Furthermore, a method is proposed for inductor sizing optimization, taking into account fault clearing strategies and operational constraints of components such as DCCBs and modular multilevel converters (MMC). Another contribution is a new proposal for MMC control during fault clearing which results in faster restoration of voltage in MMCs, better damping of MMC DC side voltage oscillations and a reduced drop of MMC DC side voltage.

Keywords: Faults, protection, MMC (modular multilevel converter), MTDC (multi-terminal DC) grid, inductor sizing

Thesis defense: February 2022

The benefits of HVDC technologies have been proven, with numerous projects already operational around the world. In Europe, the region with the highest density of HVDC projects is the North Sea. The projects in this region use underwater cables to interconnect countries or to collect the energy produced by offshore wind farms. However, all of these projects are point-to-point (P2P) links i.e., they only exchange power between two points. In the interest of creating an HVDC grid, this thesis studies the device needed to interconnect two or more P2P links, the dc-dc converter. As projects have different technical characteristics e.g., rated voltage, rated power, grounding; these interconnections are considered heterogeneous interconnections.

My work continues an open discussion on the development of an HVDC grid using projects that have already been installed. The future HVDC grid could interconnect multiple countries and renewable sources of energy thereby improving the quality of service while promoting the use of renewables. The North Sea offers a high potential for wind farms. The European Commission has estimated an increased wind capacity of about 25 times the current capacity of 12 GW by 2050. With the HVDC grid, clean energy can be distributed to northern countries and the rest of Europe with reduced power losses, which could reduce the carbon emissions linked to energy production and transmission. At the same time, the northern countries can sell hydropower when the wind is not blowing, and the European grid can export to the northern countries when the water levels are low or the wind is not available. In conclusion, it could be a development that can help to reduce carbon emissions while the whole region benefits from the power exchange between countries.

Keywords: DC-DC converter, HVDC, line topology, HVDC grid

Thesis defense: May 2022

In my thesis, I am seeking to automate the responses and actions needed for HVDC network operation and maintenance. I also consider the evolution of the network’s needs, such as the temporary loss or the addition of a new converter station. To do this, I use supervisory control theory in which the different elements of the network are represented by automata with defined behaviour, subject to discrete events. An algorithm allows me to generate supervisors that observe the network and make the automata react to the events, thereby ensuring the system functions effectively. This work enables the implementation of new and more efficient networks covering large geographical areas, which reinforces the integration of new renewable energy sources into existing AC networks.

Keywords : Supervisory control, HVDC networks, automation, formal methods, reconfiguration.

Thesis defense: Novembre 2022

The aim of this thesis is to simulate an electrical network with two distinct types of modeling: a small part requiring a precise simulation will be simulated using an ElectroMagnetic Transient (EMT) type model and the majority of the network which can be simulated in a coarser way will be simulated using a Transient Stability (TS) type model (which is more computationally efficient but less accurate). I hope this work will contribute to the efficiency of network simulation.

Keywords: Domain Decomposition Method, Co-simulation, EMT-TS.

Thesis defense: November 2022

The availability of HVDC links is crucial to secure the energy supply. In the context of DC links, the Modular Multilevel Converter (MMC) plays an essential role. Therefore, it is essential to be able to determine the state of health (SOH) of the converter elements (sub-modules). To estimate the SOH of the submodules, algorithms based on physical laws or artificial intelligence must be developed. The estimation of the SOH then allows us to calculate the remaining lifetime (RUL) of the converter and this makes it possible to select the optimal predictive maintenance strategy.

My work aims to understand the degradation process of the semiconductors present in MMCs and to determine their current health status. Understanding the current state of health makes it possible to better manage the converter’s operation, and in particular, to avoid untimely shutdowns which can have economic/social consequences. My work contributes to energy efficiency because it aims to avoid the interruption of HVDC links and to better manage planned outages (maintenance interventions).

Keywords: Modular Multilevel Converter, State of Health, Remaining Useful Life, Prognostics

Thesis defense: November 2022