The electromagnetic properties of high temperature superconductors (HTS) are characterized with the explicit intent to improve their integration in electric power systems. A tape and a coil made of Bismuth Strontium Calcium Copper Oxide (BSCCO) are considered in the presence of electromagnetically active materials in order to mimic properly the electromagnetic environment typical of electrical machines. The characterization consists of the determining the critical current and the AC losses at different values of the frequency and the transport current. The effects induced by the proximity of the active materials are studied and some related experimental issues are analyzedc.

Superconductors have exceptional electromagnetic properties [

In this context, this work is a contribution to the characterization of tapes and coils made of Bismuth Strontium Calcium Copper Oxide (BSCCO) based HTS material, produced by Sumitomo Electric [

The characterization methods are presented in the next section, followed by the presentation of the main obtained results and their interpratation.

With the assumption that the electric current (I) is uniform in the section of the tape, the E(J) relation, i.e., the electric field (E) dependence on the electric current density (J), is deduced from the U(I) curves measurement, i.e., the voltage dependence on the electric current, in the superconductive element.

The four-point measurement method is used for the determination of the U(I) curves [_{c }=_{ }1 μV/cm. The critical voltage (U_{c}) of the sample is the product of the critical electric field “E_{c}” with the tape length “L_{t}” between the points of the potential measurement (_{c} = _{c} × _{t}), and the critical current (_{c} = _{c} × _{t}), where S_{t} is the tape section, is the current giving this critical voltage. The E(J) or U(I) dependence is generally expressed by the power law given by

The voltage measured across the element contains a resistive part which is in phase with the current and responsible for the losses, and an inductive part which is a few hundred times higher than the resistive part which is of interest. The measurement of the inductive part has been compensated to avoid to calibrate the apparatus its voltage level leading to considerable errors in the measurement of the resistive part which would be embedded in its error margin. We used a compensation coil connected in series with the measurement wires, and positioned near the power supplying cable (

The distance separating the compensation coil and the supplying cable is varied such as the mutual inductance (M) becomes in the same order as the self-inductance (L) of the characterized sample. There is no need to eliminate completely the inductive part; it is sufficient to bring it to a value close to the resistive part (Ulosses). The reference in the synchronous detection is the supply current signal. The signal form of the measured voltage is non-sinusoidal due to the non-linear behavior of the superconductor, related to the nonlinear dependence of the electric field on the electric current (_{rms}, U_{1rms}, and φ1 denote respectively the RMS of the current, the RMS of fundamental of the voltage signal, and the phase shift between them.

The main disadvantage of this method is that it only works if the reference signal is sinusoidal. For other forms of current, one can use a numerical integration on an oscilloscope, as follows, where T_{e} is the sampling period and T = N × T_{e}:

The DI-BSCCO (dynamically innovative BSCCO) type tape, manufactured by Sumitomo Electric [

Tape | Coil | ||
---|---|---|---|

Thickness | 0.33 mm | Inner radius | 35 mm |

Width | 2.8 mm | Outer radius | 50 mm |

Length | 20 cm | Number of turns | 38 |

Critical current | 70 A@77 K | Tape length | 10.15 m |

The U(I) curves measured for the tape in self-field is given in _{c }=_{ }69.5 A, while the data provided by the constructor announce a value of 70 A. This difference may be due to an aging of the material over time.

The U(I) curves measured for the coil for the configurations described in

The measured losses with respect to the RMS value of the current, in self-field condition, are given in

This work constitutes a contribution to the experimental characterization of HTS for the improvement of their integration in electric power systems, in particular in electrical machines. Configurations reproducing the electromagnetic environment of superconducting coils in an electric machine have been studied, highlighting the influence of the proximity of magnetically active materials on the critical current and the AC losses. Some experimental issues and their solutions have also been addressed. Moreover, the provided experimental results can be used for the validation of future calculation models.

As future work, the characterization will be extended to a rotating magnetic field, in multiphase configuration, to better approach the electromagnetic environment of an electric machine.

Voltage [V]

Electric current [A]

Electric field [V/m]

Electric current density [A/m^{2}]

_{c}

Critical voltage [V]

_{c}

Critical electric field [V/m]

_{c}

Critical current [A]

_{c}

Critical current density [A/m^{2}]

Creep exponent characterizing the steepness of the transition from the superconductive to the normal state

_{t}

Tape length [m]

_{t}

Tape thickness [m]

_{losses}

Voltage in phase with the current [V]

Self-inductance of the superconductor [H]

Mutual inductance of the superconductor [H]

Average AC losses [W]

_{rms}

RMS value of the current [A]

_{1rms}

RMS value of the fundamental of the voltage signal [V]

_{1}

Phase shift between the fundamental of the voltage and the current [rad]

_{e}

Sampling period [s]

Period [s]