Makan Chen

Fault current limiter based on high temperature superconductors: different concepts, test results, simulations, applications

Abstract All electric equipment in a power system has to be designed to withstand the mechanical and thermal stresses of potential short-circuit currents. Any reduction of these currents can lead to significant cost savings. Among all current limiting devices, superconducting fault current limiters (SCFCL) offer ideal performance: in normal operation the SCFCL is in its superconducting state and has negligible impedance, in the event of a fault, the transition into the normal conducting state passively limits the current. Different high temperature superconductors (HTS) materials, like YBCO films, Bi2223 wires or Bi2212 bulk are under development for the use in SCFCL. Due to the brittle nature of HTS and the hot-spot problem, most HTS components for current limitation are composites comprising the HTS, a mechanical substrate or support, and an electrical bypass. The performance of the composites largely depend on the parameters: critical current density, I – V characteristics, thermal conductivity, thermal mass, and electrical bypass. Mainly two different concepts of SCFCL, namely, the “resistive” and the “shielded core” concept have been pursued in the past. In 1996 the first ever SCFCL was installed in a hydro-power plant. The device had a rated power of 1.2 MVA, it was of the “shielded core” type and was based on tubes of Bi2212-bulk material. The feasibility of the technology has been demonstrated in a one-year-endurance test. Recently more compact “resistive” SCFCLs based on the same Bi2212-bulk material have been developed. Theoretical models for the SCFCL show good agreement with experimental data. They are used to study the influence of SCFCLs in power systems in order to evaluate technical and economical advantages.

6.4 MVA resitive fault current limiter based on Bi-2212 superconductor

Abstract ABB has recently successfully developed and tested a single phase 6.4 MVA superconducting fault current limiter (SCFCL) demonstrator, which is based on a novel conductor design and innovative Bi-2212 ceramic fabrication technology. At present, it represents the highest rated power reported for HTS based SCFCL. The employed SCFCL component is a composite consisting of layers of bulk Bi-2212 ceramic, resistive metallic electrical bypass and fibre reinforced plastic (FRP). The Bi-2212 conductor is fabricated in sheets with an area of 30×40 cm2 by using a modified partial melt process and is subsequently structured into long length meanders. The as-processed Bi-2212 is non-textured and exhibits a uniform jc in the range of 3000–5000 A/cm2. The employment of a robust bypass facilitates a uniform quench in the SCFCL component during a fault event. Depending on the level of prospective fault current, a fault current is typically reduced to around 10 times nominal current in the first current peak and further to 2–5 times after 50 ms into the fault. Test and simulation results of the 6.4 MVA demonstrator, together with the application prospects of such Bi-2212 based SCFCL are presented and discussed.

Superconducting control for surge currents

Systems designed to use superconductors to limit fault currents in power grids are undergoing testing. The authors describe superconducting fault current limiters (SCFCL) which may be categorised into resistive or shielded core types. The features and operation of each type of device are outlined. Both the shielded-core and resistive types of SCFCL use the same amount of superconductor material to achieve a given limitation behavior. This is because the rated power per volume of conductor is determined by the product of fault-induced field and critical current, which is the same for both devices, assuming the same type of superconducting material is employed. The shielded-core limiter works only with AC currents and is much larger and heavier than the resistive SCFCL. While there is only one large program left in the low-temperature type of SCFCL, more than 10 major projects are under way worldwide on the high-temperature type of device. The main reason is the lower HTS cooling cost.

High temperature superconductors for power applications

Abstract High temperature superconductivity (HTS, discovered in 1986) remains an active area of research worldwide, because its higher T c and, thus, more economical cryogenic cooling have raised the prospects for electric power application. The discovery of MgB 2 has rekindled the search for new superconductors with higher T c . Recently, various acceleration programs have been launched in Europe, USA and Japan. The advance in HTS conductor has enabled the demonstration of various application prototypes, including, power cables, transformers, motors, and fault current limiters. However, full commercialisation of HTS application critically relies on the realisation of HTS conductors that are reliable, robust and low cost with low AC-losses. Worldwide activities are, therefore, focused on developing processing technologies to fabricate the so-called coated conductor based on YBCO to fulfil the stringent specifications. While a high critical current density of around 5 MA/cm 2 (77 K) has been achieved, the conductor cost is currently estimated to be 10–50 times higher than what would be accepted.

Fabrication and characterisation of superconducting rings for fault current limiter application

Abstract Bi-2212 superconducting rings (380 mm in diameter) were processed in a rotating furnace in flowing oxygen using Ag foil as the substrate. The powder was heated to the partial melt state, cooled to 855°C to develop superconducting phase, annealed in N 2 rich atmosphere, and finally cooled in N 2 from 700°C. This process has been optimised to achieve smooth surface finish and uniform thickness in the rings. Microstructure studies suggest that the as-processed Bi-2212 rings are highly densified and consist of random oriented Bi-2212 grain bundles. The Bi-2212 rings show a high T c onset of 96K and a typical J c of 1500 Acm −2 with I c of 2500 A.

Properties of YBCO films at high current densities: fault current limiter implications

We have studied the properties of superconducting strip lines, based on epitaxial YBCO thin films, at high current densities. Experiments performed with short constant current pulses show that a quasi spontaneous highly dissipative state (HDS) appears at high current densities. By carefully measuring the temperature of the YBCO line, we found that its temperature is still below T/sub c/ when the HDS occurs, indicating that this state does not originate from a thermal runaway. Once initiated, this HDS starts to propagate with an initial velocity larger than 100 m/s, which is one order of magnitude higher than thermal velocities. We also applied constant voltage pulses to the YBCO line, therefore simulating a real short circuit. We found that the spatial extent of the HDS along the line, a few microseconds after the short circuit, depends linearly on the applied voltage. These results allow explanation of the fast switching properties observed in superconducting fault current limiters (SFCL) and the fact that the peak current is limited at, typically, 3 times the critical current.