metrological performance improvement of a superconducting cable test station

The work presented in this PhD thesis concerns the metrological performance improvement of a superconducting cable test station based on superconducting transformers. The main cable’s parameter to be assessed –as a function of temperature and magnetic field– is the critical current, i.e. beyond this limit the phase transition to the normal state occurs. Ramping the current at levels in the order of the tens of kA can be achieved with superconducting transformers at moderate capital and operational cost. But, issues such as (i) accurate/precise measurements and (ii) monitoring of the secondary current during the device operation have to be addressed. In this regard, the goals of the thesis are the design, prototyping, and validation of a new cryogenic current transducer and effective monitoring system for test stations transformer-based. Among the available transducers for current sensing at room temperature, the DC current transformer (DCCT) provides measurement accuracy in the order of the hundreds of ppm (and below) with high repeatability. Thus, one of the aims of this work is to develop a DCCT capable of measuring currents at liquid helium temperature improving critical current tests capabilities. On the other hands, resistive losses in the secondary circuit of superconducting transformers require to be compensated; a suitable control of the secondary current is mandatory. Therefore, further enhancement follows the implementation of a fully digital system for monitoring the transformer operation. After illustrating the basic idea and the design steps for the cryogenic DCCT transducer, the detailed design of the DCCT sensing element is reported. To overcome state-of-the-art drawbacks a cryogenic permalloy is used for the DCCT cores and an optimized design is carried out for a rated current of 100 kA. Moreover, the concept of combining ferromagnetic and superconducting materials to increase the rejection of external disturbance field, up to 1.0 T, is exploited. In particular, the design of a cylindrical superconducting shield made of MgB2 composite is proposed. A fully digital monitoring system provides the basic tools for a proper operation of a superconducting transformer. The high-performance digital measurement and control systems are described. Two compensation mechanisms of the secondary current are proposed. The former is based on a standard Proportional-Integral algorithm and the latter implements a smart control strategy. The design of the cryogenic DCCT is validate via both numerical simulation and on field characterization of a lab-scale prototype. Finally, the monitoring system is validated carrying out critical current assessment of a superconducting cable.