Development of a Power Quality Conditioning System for Particle Accelerators

CERN is frequently experiencing power quality problems caused by transient voltage dips originating from the outside grid, causing issues with electrical equipment and, in the worst case, tripping the particle accelerators. This thesis will prove the feasibility and develop a power quality conditioning system to mitigate these voltage dips to ensure satisfactory power quality for CERN today, and in the future for even larger particle accelerators. The mitigation system is based on the back-to-back HVDC converter concept and, as the nature of the electrical network of CERN is entirely passive, the converter is operating as a remote-end converter. The purpose of this converter is to supply constant power during voltage dips occurring in the upstream network, and thus, completely decouple CERN from the grid. The topology of choice is concluded to be the HVDC modular multilevel converter, which has the added benefits of being able to connect to weak AC networks. In addition, it possesses reactive power support, low harmonic distortion as well as high modularity and reliability. This thesis has further categorised two mitigation strategies with the back-to-back converter: Increased line-current control and energy storage support, with the latter one being the primary focus. This is based on incorporating the energy storage in the converter topology by proper dimensioning of submodule capacitors. A statistical analysis of voltage dips is performed, optimising the energy storage for reduced investment costs. Furthermore, the system is designed for the electrical network of CERN, identifying optimal location and operational parameters. The converter is designed down to component-level and important aspects in the design are highlighted. The system is simulated with MATLAB & SIMULINK and the feasibility of the control system, dip mitigation and extra features is proven. The reliability of the designed submodule is reviewed, and the footprint of the converter is investigated. The total cost of the converter is estimated and cross-referenced with literature, and a net-present value analysis is performed based on the economic gain for CERN to employ such a system for the Large Hadron Collider. The thesis is finalised by reviewing the possible applications for the power quality conditioning system in the industry outside of CERN.