Scalable Regenerative Power Converters for Accelerator Magnets

Electromagnets are used at The European Centre for Nuclear Research (CERN) in high-energy physics experiments to direct the charged particle beams. They store significant amounts of energy in the magnetic fields and are cycled on and off with a cycling period in the order of seconds. Powerful power converters with peak power capability 10 times larger than the average power usage are used to push the electromagnet's energy in and out of the magnet before and after the physics operations. The loads having a significant amount of energy which can be recovered, and the cycling nature of the load and the long operating times, resulting in millions of thermal cycles for these devices. This PhD thesis explores the possibilities of scalable converters for cycling electromagnetic loads with energy recovery. The thesis introduces the concept of a modular converter with separated storage and demonstrates how it can be used in a scalable way to achieve scalability in how the converter is configured and operated. It contains an investigation into the cost optimal design and a control structure for the converter's controller to handle the separated storage. The fundamental building modules, refereed to as bricks, can either be connected to the grid or to separate energy-storage components. Splitting the converter in to bricks with separated energy storage can enhance the flexibility of the system by scaling for storage requirements, power capabilities and grid connecting more independently. By doing a sweep of the different semiconductor power modules, storage units and output voltages the cost optimal combination of such a converter has been investigated. By expanding the calculations to include lifetime cost of operating the converters, SiC MOSFETs has a significant cost saving compared to the IGBTs currently used. The proposed modular converter enables independent power flow control among the bricks and five different strategies have been demonstrated. They manage the power flow and optimise the usage of the power converters in terms of cost, efficiency, reliability and precision. Energy recovery into the energy storage takes place by redistributing the current during inductive load ramp-down. The performance of the proposed modular converter is validated experimentally on a full-scale lab prototype rated at 800 kW. It is shown that up to 30% cost savings can be achieved by eliminating converter components in the storage and in the grid connection, while the converter performance on the load is maintained and the same amount of energy is recycled. A laboratory verification shows that the converter can operate with independent currents delivered from the bricks while respecting the total voltage and current reference and that the system can manage iv the losses of the converter without compromising the performance of the converter. The various strategies allows the converter to operate with different modes, for example, optimal utilisation of the energy storage systems, minimised current stress in the semiconductors or minimising the installed grid capacity