Power supply for degaussing systems with high temperature superconductors

Abstract

Ships with a ferromagnetic hull distort Earth’s magnetic field in such a way that magnetic mines can detect them. On-board degaussing systems compensate for the magnetic signature, but the copper coils take up valuable space and cause losses. High-temperature superconductive (HTS) coils are a potential alternative because they don't have Ohmic losses. However, research is lacking on the application of HTS to degaussing. The objective of this thesis is to design an adequate HTS degaussing power supply.

A modelling framework is developed to quantify the magnetic signature and to estimate the degaussing parameters. Analytical and numerical modelling techniques are compared where the numerical provides the most accurate results but takes longer to calculate. Both are verified with an experimental test setup where the (reduced) magnetic signature was measured. A second test setup was built to demonstrate the performance and feasibility of HTS degaussing coils. The HTS coils are placed in a closed-loop vacuum-insulated cryostat filled with sub-cooled liquid nitrogen. The magnetic signature is measured in a static and dynamic case where the behaviour of the HTS coils is similar to that of the copper coils.

The high current-carrying capability of HTS has the advantage that fewer turns are required. However, a larger current density results in more power source and current lead losses. Both problems are solved using a cryo-cooled converter with low on-state resistance MOSFETs. A comparative study between four different topologies has been conducted, concluding that an H-bridge converter with cryo-cooled MOSFETs and a DC capacitor bank inside the cryostat can reduce the amount of losses by a factor 23 compared to a conventional converter. A cryogenic H-bridge converter with ten parallel MOSFETs per switching leg and an HTS load was built and tested. The converter produces a current of 50 A with an input voltage of 3.5 V and a duty cycle of only 0.025. The parasitic inductances cause unwanted effects regarding oscillations above a switching frequency of around 100 Hz because of the low PCB resistances.

A concern exists that a peak in the frequency spectrum can be detected in the magnetic signature around converter's switching frequency. Five switching frequency modulation schemes were tested to reduce this peak. A model of a full-sized frigate estimates the effect of the switching frequency on the magnetic signature. Of the tested schemes, random lead-lag shows the best results.

In this research, a framework is created where the magnetic signature and the degaussing parameters can be estimated. It was shown that HTS can be used to replace copper degaussing coils. Several converter topologies were compared, and it was shown that cryo-cooled MOSFETs can increase performance drastically. A cryogenic converter was built and tested successfully. A variety of switching frequency modulation schemes were tested, and it was found that random switching frequency modulation can be used to reduce the switching ripple in the magnetic signature.