Magnetic and Dielectric Design of Auxiliary Power Supply for HVDC Applications : A high-frequency transformer with high power transfer capability and high voltage electrical insulation

Abstract: It is anticipated that massive amounts of energy will be transferred long distances via High-Voltage Direct Current (HVDC) links in the future and the prospect of having meshed HVDC grids is envisioned, for example the European super grid. Such a power system would benefit greatly if HVDC circuit breakers could reliably clear faults within the HVDC network. Different ways to break large direct currents have been proposed throughout the years and one distinguished concept is based on generating an artificial zero-crossing of the current and pass it through a mechanical interrupter as it opens. This concept is implemented in the Voltage-Source Converter Assisted Resonant Current (VARC) circuit breakers developed at Scibreak which require auxiliary power from an off-line supply unit to energize their electronic equipment. This thesis continuous and builds on research previously carried out at Scibreak on a special Auxiliary Power Supply (APS) concept for 525 kV HVDC applications. In essence, it is a unique modularized high-frequency transformer whose power transfer and voltage withstand capabilities are the cornerstones of its design. The APS must supply an adequate amount of power to drive the VARC circuit breakers with preferably high efficiency while also fulfilling the considerable insulation demands of HVDC grids. A feasibility study of this APS concept was carried out by building a parametric 3D model in the Finite Element Analysis (FEA) software Ansys Maxwell which includes all parts that affect both its magnetic and electrical properties. The initial model reproduced experimental results from a magnetic APS prototype and was then used to explore a plethora of different geometries and materials with regards to its magnetic and dielectric designs. Specific design candidates were selected for more in-depth analysis and experimental work. All obtained results together with knowledge of commercially available materials show that the APS holds great promise to meet the necessary design criteria for its HVDC applications. Its dielectric design is well suited to continuously handle an operating voltage of 525 kV DC, meet the required impulse voltage levels of the grid and properly shield the magnetic structure. It is expected to have a long life time where the design criterion was always 30 years in this work. Moreover, its magnetic design is anticipated to supply a few kW of active power with efficiencies between 80 to 95 percent and manage a continuous operating time of 5 min. Both design aspects are interchangeable to a decent extent in order to cope with one another and produce a compromised design.