Electric field in coated conductors carrying ac current under ac external magnetic field

Nov. 29 18:40-19:00

*Yusuke Sogabe1, Naoyuki Amemiya1
Kyoto University1

For use in ac equipment such as rotating machines, the resistance of coated conductors cannot be regarded as zero, but coated conductors must be regarded as conductors with finite resistance that follows their non-linear E–J characteristics. In particular, at relatively high frequencies, electric fields much higher than the defining electric field E0 of the critical current (generally 10−4 V/m) may be induced in the coated conductors. However, the formulation of the E–J characteristics of coated conductors used in numerical analyses is determined from measurements of the V-I characteristic of coated conductor in the electric field region of 10−5 V/m to 10−3 V/m generally. Namely, the electric field region governing electromagnetic phenomena in a coated conductor used for ac equipment may not match the electric field region targeted by the measurements used to determine the E-J characteristics in the analyses, which may affect the accuracy of the analyses. When evaluating the electromagnetic behavior of a coated conductor carrying an ac current under an ac magnetic field, it is not clear at what electric field region the E-J characteristics of the coated conductor are important.

We performed numerical electromagnetic field analyses of a single infinite-long coated conductor carrying an ac current under an ac magnetic field, and evaluated the electromagnetic field distribution inside the coated conductor to evaluate the electric field generated in the coated conductor under various ac environment. A single coated conductor with a width of 4 mm and a superconductor layer thickness of 2 mm was analyzed, and a power-law model was used for the E-J characteristics in the analyses. The critical current density Jc and defining electric field E0 were set at 2.5 ´ 1010 A/m2 and 1 ´ 10−4 V/m, respectively. Here, critical current of the coated conductor was 200 A, and Jc was assumed to be independent of the magnetic field for simplicity. The n-value was a parameter in the analyses and varied from 5 to 30. The amplitude of current was varied from 0 A to 150 A, and the amplitude of external magnetic field was varied from 0 T to 0.6 T. The frequency of current and magnetic field was from 20 Hz to 200 Hz, and the current and external magnetic field were in phase.

As a result, it was confirmed that the maximum electric field inside the coated conductor is almost independent of n-value while it is proportional to frequency. In the region where the external magnetic field can be considered sufficiently larger than the self-field due to the transport current, the maximum electric field is proportional to the amplitude of the external magnetic field. The magnitude of the electric field was found to be 0.1 V/m at an n-value of 30, a magnitude of current of 150 A, a magnitude of magnetic field of 0.6 T, and a frequency of 150 Hz, which is much higher than the defined electric field E0. This suggests that the E-J characteristics in the high electric field region must be taken into account when performing electromagnetic field analyses of HTS coils or magnets wound with coated conductors for ac equipment.

This work was supported in part by JST-Mirai Program Grant Number JPMJMI19E1, Japan, and in part by JSPS KAKENHI Grant Number JP22K14238.