Liang Zhen Lin

Study of Superconducting Fault Current Limiters for System Integration of Wind Farms

As electrical energy will be provided from renewable sources, the connection of a large number of wind farms to existing distribution networks may lead to the increasing fault levels beyond the capacity of existing switchgear, especially in urban areas. Fault current limiters (FCLs) are essentially expected to control the prospective short circuit currents. In this paper, investigations were carried out to assess the effectiveness of the resistive superconducting fault current limiters (SFCLs) for fault level management in wind power system. System studies confirmed that the superconducting fault current limiter (SFCL) could not only control the fault currents but also suppress the inrush currents, when wind farm has adopted in the case of the system interconnection. As a result, the highly efficient operation of the wind power system becomes more possible by introducing the superconducting fault current limiters.

Sequential coupling transport for the dark current of quantum dots-in-well infrared photodetectors

The dark current characteristics and temperature dependence for quantum dot infrared photodetectors have been investigated by comparing the dark current activation energies between two samples with identical structure of the dots-in-well in nanoscale but different microscale n-i-n environments. A sequential coupling transport mechanism for the dark current between the nanoscale and the microscale processes is proposed. The dark current is determined by the additive mode of two activation energies: Ea,micro from the built-in potential in the microscale and Ea,nano related to the thermally assisted tunneling in nanoscale. The activation energies Ea,micro and Ea,nano decrease exponentially and linearly with increasing applied electric field, respectively.

Performance of

The voltage and temperature of YBa2Cu3Ox (YBCO) tapes with different structures were measured at alternating-current overcurrent conditions in liquid nitrogen. A safety temperature limit of the tapes with different structure was obtained based on the measured critical-current degradation data of the YBCO tapes. According to heat conduction model, the electrical and thermal performances of YBCO tapes were analyzed for tapes with different stabilizers. The comparison was presented for simulation and test data. The results are helpful for the design of superconducting electrical equipment.

\hbox{YBa}_{2}\hbox{Cu}_{3} \hbox{O}_{x}

A theoretical model describing the angular dependence of critical current was proposed based on the critical state model and the effective mass theory. The critical currents of single-tape and a stack of three BSCCO/Ag tapes were measured under DC magnetic fields with different orientations and strength. The comparison of the theoretical and experimental results is presented.

Tapes With Different Structures Under AC Overcurrent Conditions

The measurement results of AC transport current losses in BSCCO/Ag tapes caused by AC transport currents and the simultaneously presented AC magnetic fields were reported. The measurements were carried out at 77 K and power frequency (50 Hz) for AC applied magnetic fields of 48, 80 and 120 mT, respectively. The angle between the direction of the magnetic field and the wide side of the tape varied from 0° to 90° with a step of 10°. Analyses on the angular dependency of the AC losses for BSCCO/Ag tapes were given. A theoretical model to describe the angular dependency of AC losses was proposed and compared with the experimental results.

A theoretical model for the angular dependence of the critical current of BSCCO/Ag tapes

The critical current and ac loss of a stainless steel reinforced Bi2Sr2Ca2Cu3O10 composite superconducting tape were measured under tensile stress/strain at 77K. By use of the definition of irreversible strain, a formula describing the variation of the critical current with strain was proposed. A relationship between ac loss and tensile strain was developed from Norris’ formula and the critical current–strain relation. It is shown that the experimental results agree well with the values calculated from our formulas.

The angular dependence of AC losses in BSCCO/Ag tapes under AC magnetic fields and AC transport currents

In the application of high temperature superconductors, the superconducting tapes are often exposed to magnetic fields with different orientations. Thus AC losses in such cases depend not only on the magnitude of the external magnetic field, but also on its orientation with respect to the tape surface. In this paper, AC transport losses of a BSCCO/Ag tape were measured at 77 K in 48 mT, 80 mT and 120 mT DC magnetic fields, respectively. The angle between the direction of the magnetic field and the wide side of the tape varied from 0 to 90 degrees with a step of 10 degrees. Using the theoretical model we had proposed for the angular dependence of critical current, we developed a theoretical model to describe the angular dependence of AC transport losses by modifying Norris formula. The comparison between the theoretical values and the measured data were presented and discussed.

Stress/strain dependence of ac loss and critical current of Bi2Sr2Ca2Cu3O10 reinforced tape

When operating in power applications, superconducting conductors often carry alternating currents (AC). Thus, understanding the quench behavior under AC conditions is essential for protecting high temperature superconductor-based devices in power applications. In this work, the quench behavior of Bi₂Sr₂Ca₂Cu₃O_x/Ag (Bi2223/Ag) tape with AC and DC transport currents is reported. The minimum quench energies and normal zone propagation velocities are measured for different DC and AC transport currents at 45 K. The frequency of the AC transport current is varied from 50 Hz to 400 Hz. AC losses are measured to analyse the background condition of AC quenches. The results are compared with those of a YBa₂Cu₃O_x (YBCO) coated conductor that was studied previously with identical conditions.

The angular dependence of AC transport losses for a BSCCO/Ag tapes in DC applied field

In most power applications, the superconducting tapes carry AC transport currents and are exposed to applied AC or/and DC fields with different orientations. Because of the anisotropy of high temperature superconductors, AC losses depend on both the strength and the orientation of the magnetic field. To achieve high current carrying capacity, several tapes are often stacked. Thus an estimation of AC losses in stacked tapes is necessary for the design of superconducting devices. Here, AC transport losses in single, double- and triple-stacked Bi2223/Ag tapes were measured under different magnetic fields. The measurements were carried out at 77 K with the field angle between the tape surface and the direction of magnetic field varied from 0/spl deg/ to 90/spl deg/. The experimental results on the angular dependence of the AC losses are reported and analyzed.

Quench Behavior of

An 1 MJ/0.5 MVA high-Tc SMES has been developed by the Institute of Electrical Engineering, Chinese Academy of Sciences. It has been operating in a real power grid since February 16, 2011 in Baiyin City, Gansu Province in northwest China. The SMES magnet is immersed in liquid helium and cooled by four G-M cryo-coolers to keep zero helium vaporization. To ensure safe operation of the system, protection of the HTS magnet is a necessity, which, however, cannot be complemented by only single protection. In this paper, coordinate protection of the system is introduced by an online measuring and control system and a power conversion unit, and the control principle is described in detail. By means of the coordinate protection, the temperature and pressure inside the SMES Dewar are controlled in the range of 4.3-4.5 K and 0.01~0.025 MPa, respectively. Long term demonstration of the SMES during a period of more than two years shows that the coordinate protection can ensure a stable and reliable operation of the HTS magnet.