Design and Optimization of Solid-State Transformers for MV distribution and EV chargers: Challenges, Solutions, and Performance in 20kV AC to 1500V DC Conversion

1.    Introduction

Solid-state transformers (SSTs) represent a significant evolution in transformer technology, especially for high-frequency power conversion applications.

Traditional 50/60 Hz transformers are robust, widely deployed, and reliable, but they are large, bulky, and inefficient in some modern applications.

SSTs, operating at much higher frequencies, offer a promising alternative, enabling smaller, more efficient systems. However, the transition from low-frequency (50 Hz) to high-frequency designs involves several technical challenges.

This research aims to focus on SSTs with a specific application: converting 20kVAC to 1500VDC for fast EV chargers applications and DC distribution.

The thesis will address the design, modeling, and optimization of such a system, identifying the critical issues with traditional transformers, and proposing novel strategies for the successful implementation of solid state transformers.

2.    Problem Statement

While SSTs offer many advantages over conventional transformers (such as reduced size and weight, better efficiency, and enhanced functionality), they also present significant challenges, especially when converting high input voltages (20kVAC) to DC outputs of up to 1500VDC.

These challenges arise mainly from issues such as insulation, losses, thermal management, core material limitations, electromagnetic interference (EMI), and maintenance.

The aim of this PhD research is to investigate the specific challenges and limitations of high-frequency transformers in the context of SSTs and propose effective design strategies for their mitigation. It will explore how the change from a traditional 50Hz transformer to a high-frequency (up to several kHz) SST affects power conversion, losses, and system stability in medium-voltage to low-voltage DC conversion.

3.    Research Objectives:

1. Literature Review:

   • Analyze existing transformer designs (traditional and SSTs) with a focus on medium-voltage AC to DC conversion.

   • Study the state-of-the-art SST designs, materials, switching techniques, and control algorithms.

2. Identification of Key Challenges:

   • High-Frequency Design Limitations:

     • Evaluate the fundamental differences between 50 Hz and high-frequency transformers.

     • Investigate the material limitations, core losses, and insulation requirements at higher frequencies.

   • Losses and Efficiency:

     • Study the impact of high-frequency switching on system efficiency and propose methods for loss reduction (e.g., soft switching techniques).

   • Thermal Management:

     • Investigate the thermal challenges associated with high-frequency operation.

     • Propose cooling techniques and optimal layouts to improve thermal performance.

   • EMI and Insulation:

     • Assess EMI challenges in high-frequency transformers and propose shielding/grounding strategies.

     • Study insulation challenges in the context of higher switching frequencies and elevated voltages.

3. Design and Simulation:

   • Design an optimized SST capable of converting 20kV AC to 1500V DC.

   • Use advanced materials (e.g., wide bandgap semiconductors like SiC and GaN) to achieve high efficiency.

   • Develop simulation models to study the electromagnetic, thermal, and mechanical aspects of the design.

   • Propose and test different control algorithms to optimize voltage regulation, power factor correction, and efficiency.

4. Experimental Validation:

   • Build a prototype SST system based on the proposed design.

   • Validate simulation results and assess performance metrics such as efficiency, thermal stability, and EMI mitigation.

   • Test the prototype in a real-world scenario with a 20kV AC input and 1500V DC output.

4.    Expected Contributions:

• Development of a high-performance SST capable of converting 20kVAC to 1500V DC.

• Novel design strategies to overcome the challenges of core losses, switching losses, thermal management, and EMI in high-frequency transformers.

• A comprehensive framework for optimizing SST efficiency and performance in medium-voltage applications.

5.    The team you'll be part of

The PhD candidate will have the opportunity to be a part of the dynamic and innovative EV Chargers team within the eMobility Business Unit at Watt & Well. This team is at the forefront of developing cutting-edge solutions in the field of electric vehicle charging. Their expertise lies in designing and manufacturing high-quality power electronic components and systems that ensure efficient and reliable charging infrastructure for EVs.

The team is committed to sustainability and environmental consciousness and it aims to accelerate the transition to a greener and more sustainable transportation ecosystem.

 By joining Watt & Well, the PhD candidate will have the opportunity to work alongside industry leaders, contribute to impactful research projects, and make a tangible difference in the development and implementation of innovative EV charging solutions.

·       A Master's degree (BAC+5) in Electrical Engineering, Power Electronics, or a related field.

·       Strong background in power electronics, including experience with designing and modeling power converters.

·       Proficiency in simulation tools such as MATLAB/Simulink, LTspice or similar software.

·       Knowledge of control systems and experience in developing control algorithms.

·       Ability to work independently and collaboratively in a multidisciplinary research team

·       Good communication and technical writing skills.

·       Can meet up to deadlines

·       Languages English (Business fluent) and French (intermediate level)

·       Experience level: less than 2 years