Frank Breuer

Applications of superconducting fault current limiters in electric power transmission systems

The introduction of new generating facilities by independent power producers and increasing load demand can result in fault-current over-duty on existing transmission system protective equipment. Conventional solutions to fault current over-duty such as major substation upgrades, splitting existing substation busses or multiple circuit breaker upgrades could be very expensive and require undesirable extended outages and result in lower power system reliability. Less expensive solutions such as current limiting reactors may have unwanted side effects, such as increase in system losses, voltage regulation problems or possibly could compromise system stability. This paper discusses the benefits of superconducting Fault Current Limiters (FCLs) which can be economically competitive with expensive conventional solutions. Superconducting FCLs are invisible in normal operation and do not introduce unwanted side effects. The performance of a particular type of limiter, the Matrix Fault Current Limiter (MFCL) is presented and examples are provided on how it could relieve fault current over-duty problems. The use of this device in a particular application in the American Electric Power (AEP) 138 kV transmission grid is also discussed.

CURL 10: development and field-test of a 10 kV/10 MVA resistive current limiter based on bulk MCP-BSCCO 2212

Within the German project CURL 10 a full scale three-phase resistive current limiter was developed and successfully tested up to the nominal voltage and power (10 kV, 10 MVA). This is up today the largest HTS current limiter world wide. The device is based on bifilar coils of MCP-BSCCO 2212 bulk material and operates at T=66 K. Per phase 30 components are connected in series in order to obtain the required resistance. The electrical stabilization is based on a metallic shunt contacted continuously to the superconductor and allows electrical fields up to 0.6 V/cm. We report on the development of the superconducting components and their large-scale manufacturing processes including material, contacts, mechanical stabilization, high voltage insulation and quality control. In a series of preliminary small scale tests at different temperatures as well as in the final tests with the full scale prototype reliable current limitation could be demonstrated in the full range of prospective fault currents, also in the particular dangerous small load regime. Since April 2004 the demonstrator has been installed within a field test in the grid of RWE.

System technology and test of CURL 10, a 10 kV, 10 MVA resistive high-Tc superconducting fault current limiter

A full scale three-phase resistive high-Tc superconducting fault current limiter (SCFCL) designed for 10 kV, 10 MVA, has been developed, manufactured, and tested within a publicly funded German project called CURL 10. The device is based on 90 bifilar coils of MCP BSCCO-2212 bulk material. The operating temperature of 66 K is achieved by cooling of liquid nitrogen using two Stirling cryocoolers. Until today, this is the largest HTS fault current limiter world wide. We report on the design features, the composition, and the operation parameters of the SCFCL system. From April 2004 on CURL 10 is installed and tested within the network of the utility RWE at Netphen near the city of Siegen, Germany. The results of the laboratory test and the field test of CURL 10 are given.

Testing bulk HTS modules for resistive superconducting fault current limiters

Recent investigations show that high Tc bulk material is an attractive material option in resistive superconducting fault current limiters. Our investigation focuses on the short-circuit behavior of long samples of MCP-BSCCO2212 and polycrystalline melt-textured YBCO. The experiments have been performed for different operating temperatures, short-circuit duration and prospective short-circuit currents. The measurements demonstrate that a metallic bypass is needed to avoid destructive hot spots during quench. Quench and limitation tests of MCP-BSCCO2212 bifilar coils with integrated high resistive metallic shunt down to 65 K have successfully been performed. This type of module will be used for construction of a 10 MVA power system demonstrator.

Manufacturing and testing of MCP 2212 bifilar coils for a 10 MVA fault current limiter

A very promising option within the German 10 kV/10 MVA superconducting fault current limiter project CURL 10 is melt cast processed (MCP-) BSCCO 2212 bulk material. With tube shaped samples cut to bifilar coils and contacted in series, the required long lengths (160 m per phase) can easily be achieved. At the operating temperature of T=65 K the critical current density is near 4000 A/cm/sup 2/ and the voltage under limiting conditions reaches E/sub p/=0.5 V/cm. The contribution describes the design, manufacturing and testing of the superconducting components. In particular the development and characterization of a suitable electrical shunt is focused. Finally the finished components are characterized with respect to electrical homogeneity and under various limiting conditions. It could be confirmed that the single components each meet all the requirements defined within the project.

Characterization of BSCCO 2212 bulk material for resistive current limiters

It is demonstrated that melt cast processed (MCP) BSCCO 2212 bulk material is an excellent candidate for resistive superconducting current limiters. For rod shaped samples it could be shown that a homogeneous voltage of more than 1 V/cm can be established over the whole length of the sample leading to a substantial limiting of short circuit current. The require long lengths of superconducting material can be easily obtained by machining the MCP-tubes into (bifilar) coils. The measured critical current density and its homogeneity over these large lengths were determined and proven to be sufficient for the designated application.

High voltage design, requirements and tests of a 10 MVA superconducting fault current limiter

In recent years the development of superconducting fault current limiters (SCFCLs) has made significant progress. Within the frame of the German project CURL10 a 10 kV, 10 MVA demonstrator of a resistive SCFCL, using MCP-BSCCO 2212 bifilar coils, has been developed and successfully tested. A world-wide first field test of a resistive SCFCL started in 2004. This paper reports about the high voltage design aspects and summarizes the high voltage tests performed within this project. The different tests confirm that the design requirements of 28 kV AC voltage and 75 kV lightning voltage are fulfilled. Even at worst case conditions of a quench and a lightning pulse at the same time no breakdown was observed.

Conceptual Design of a 110 kV Resistive Superconducting Fault Current Limiter Using MCP-BSCCO 2212 Bulk Material

Superconducting fault current limiters (SCFCLs) are new and attractive devices to limit short-circuit currents in power systems. In recent years, the technical feasibility of SCFCLs in medium voltage applications was successfully demonstrated in several field tests. In high voltage power systems the application of SCFCLs is very attractive too, because at this voltage level conventional devices to limit short-circuit currents are hardly applicable and system studies showed considerable economical benefits. Therefore, a German project started recently to develop a first 110 kV, 1.8 kA prototype of a resistive SCFCL. A magnetic triggered resistive concept using MCP-BSCCO 2212 bulk material will be used for the demonstrator. This paper reports about the conceptual design of this SCFCL and the project status. Focus is given on the main data of the 110 kV prototype, the SCFCL modules, the general design of the whole system and the most important high voltage design aspects. The calculations and estimations show that the conceptual design presented in this paper seems feasible and that a major technical challenge is to ensure a reliable electrical insulation system.

Fault current limiters - R&D status of two selected projects and emerging utility integration issues

Emerging technologies of nondisruptive fault current limiters (FCL) are of increasing interest to the electric power industry. In this work an update on two selected superconducting FCL projects is given. Successful testing of resistive type prototype three-phase devices at voltages up to 10 kV are reported. While such tests clearly indicate the technical feasibility of novel FCLs for medium voltage applications, several system integration issues are still not addressed properly. The paper discusses the need for standard FCL testing procedures and important impacts of FCL on existing and future power system protection systems. It is concluded that novel medium voltage FCLs may lead to substantial benefits on future all-electric navy ships from significant reductions in weight and size of equipment. Furthermore, it is proposed to study more thoroughly the impact of FCLs on protection systems by means of transient system simulations.

Analysis of Unsymmetrical Faults in High Voltage Power Systems With Superconducting Fault Current Limiters

An analysis of unsymmetrical faults for a 110 kV sub-grid coupling with a superconducting fault current limiter is conducted in this contribution. For the design of the super-conducting fault current limiters it is essential to identify the highest possible voltage during the limitation process. As reference the symmetric three phase fault which generally leads to the highest short-circuit currents is used. Unsymmetrical faults like single line-to-ground fault, line-to-line fault and double line-to-ground fault are investigated in dependence of the superconducting fault current limiter impedance. The analysis shows that the double line-to-ground fault leads to the highest voltage drop during the limitation in the relevant range of impedance. Consequently, this fault is a crucial factor for the design of the superconducting fault current limiter.