Stefan Fink

ENSYSTROB—Resistive Fault Current Limiter Based on Coated Conductors for Medium Voltage Application

A new German government funded project for a resistive fault current limiter has started in September 2009 (ENSYSTROB) and is presented. The consortium includes partners from industry, research centers and utilities. It aims the construction of a 3-phase medium voltage current limiter (12 kV, 800 A) for the protection of the domestic supply in a power plant. A special feature of this application is the presence of large in-rush currents (4100 Ap for 50 ms, 1800 A for 15 s). The superconducting components are bifilar pancake coils consisting of pairs of YBCO-tapes in face to back configuration. Successful limitation experiments on prototype components with prospective currents in the whole range are shown. Also the stability with respect to high voltage could be demonstrated. The AC-losses under normal operation are shown, by simulation and experiment, to be smaller than the heat input of the current leads. The current limiting components are compared with similar elements based on BSCCO 2212 bulk material (Nexans SuperConductors).

Test Results of the High Temperature Superconductor Prototype Current Leads for Wendelstein 7-X

The Karlsruhe Institute of Technology (which is a merger of former Forschungszentrum Karlsruhe and Karlsruhe University) is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics. The current leads are of the binary type, the HTS part covering the temperature range between 4.5 K and 60 K while the heat exchanger covers the range between 60 K and room temperature being cooled by 50 K He. In total 2 prototypes and 14 series current leads are required with a nominal current of 14 kA and a maximum current of 18.2 kA. The paper describes the design and first test results of the prototype HTS current leads.

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.

Test Arrangement for the W7-X HTS-Current Lead Prototype Testing

The Karlsruhe Institute of Technology (KIT) is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics. Because the W7-X current leads are mounted with the warm end at the bottom a special test cryostat has been built and is attached to the main cryostat of the TOSKA facility of KIT. Two prototypes of these current leads are tested in this cryostat under W7-X relevant conditions. The test conditions with respect to cryogenic, control, current supply, data acquisition and quench detection of the test setup for the prototype test at TOSKA is described. The performance of the MC plug used to power the HTS current leads is described in detail.

Influence of the radial plates on the transient oscillations in the ITER toroidal field (TF) model coil using a finite element approach

The transient electrical behavior of the racetrack-shaped TF model coil for the International Thermonuclear Experimental Reactor (ITER) during an energy dump has been investigated using a 3D finite element model. Detailed modeling of the radial plates inherently takes into account the eddy currents in the steel structures. Hence, unlike recent investigations using ordinary lumped network models, the influence of the radial plates on the damping and attenuation of the transient oscillations could be considered. Transient analyses for rise times from 0.5 ms to 500 ms have been performed. Additional AC analyses yielded the transfer function of the coil. Alternatively, a lumped network model was established. Frequency dependent values of the network elements, i.e. effective resistances, self and mutual inductances, have been determined using 2D FE models in a frequency range up to 500 kHz. Thus, it was possible to implement frequency dependent effects into the network model as well.

High Voltage Testing of ITER Prototype Axial Breaks

Axial insulation breaks are needed in forced cooled cryogenic high voltage devices like fusion magnets for the separation of the high voltage area from the grounded pipe system. For the verification of the ITER requirement compatibility, a special type test series was defined for a set of cryogenic and room temperature prototype axial breaks. This paper gives an overview of high voltage type tests performed on a set of 5 cryogenic breaks and 3 room temperature breaks designed and manufactured by Babcock Noell. Direct voltage, alternating voltage, and impulse voltage tests were performed to reproduce in an economic way the different voltage loads during current operation, fast discharge, and fault events. In addition, for 2 cryogenic breaks, Paschen tightness was demonstrated at 35 kV DC.

Paschen Problems in Large Coil Systems

When a breakdown of the thermally insulating vacuum occurs in an energized high power coil system (e.g. a leakage of the He-cooling system or to the surrounding air) the coils warm up and have to be discharged fast to avoid destruction. Due to the coil inductance and the necessary discharge time, a high voltage is produced in the coil and the feeder system. Therefore an unspecified pressure coincides with the occurrence of a high voltage at the coil and feeder system. It is well known that under these conditions arcing can occur at a magnet when it is not perfectly insulated even for voltages 1 kV (Paschen-effect). The gas dependent breakdown voltage minimum occurs typically in the range of some 100 V at a pressure value that depends from geometry and temperature. For lower and higher pressures the breakdown voltage is increasing. For a high power magnet system an insulation is necessary that can withstand high voltage in the 10 kV range although any possible pressure is applied to the system. Usually a pressure independent solid insulation is used to solve this requirement. In order to test the reliability of this electrical insulation under all pressure conditions, a so-called Paschen test has to be performed. This Paschen test is a high voltage test with pressures varying between good vacuum and ambient pressure. In this paper examples for insulation failures on large coils found with Paschen testing are given. In a series of experiments with samples that have been prepared with intended insulation faults the necessity of Paschen testing is demonstrated to identify insulation problems that are not visible under good vacuum or ambient pressure. Therefore, a Paschen-test is also a useful high voltage diagnostic tool to localize insulation faults in solid insulated magnets.

Investigation of the transient electrical behavior of the ITER central solenoid model coil (CSMC) during safety discharge

The electrical behavior of the ITER central solenoid model coil (CSMC) exposed to the voltage rise occurring during a safety discharge initiated by a counter acting current switch has been studied. A detailed network model has been set up to determine the transient overvoltages inside the windings of the CSMC when the coil is subjected to transient voltages. The analysis takes into account the frequency-dependent resistance of the conductor in case of high-frequency oscillations, and considers the influence of the extensive instrumentation cables of the coil. The models accuracy is demonstrated over a frequency range up to 30 KHz. The total inductance and capacitance of the coil model are in very good agreement with previously obtained measurements and design values. The discharge circuit has also been modeled in order to accurately simulate the discharge process. It was found that the terminal voltage generated during a safety discharge causes transient oscillations inside the windings. However, they do not cause overvoltages exceeding the maximum acceptable insulation stress. The influence of several parameters of the discharge circuit on the rise time and shape of the resulting terminal voltage was investigated. Controlling these values might be a measure to prevent excessive internal oscillations for larger coils with lower natural frequencies than the CSMC.

AC breakdown voltage of liquid nitrogen depending on gas bubbles and pressure

Liquid nitrogen is often used within superconducting high voltage apparatus as pressurized coolant and for electrical insulation purposes. A temperature increase of the super-conductor surface, e. g. up to about room temperature during a quench of a fault current limiter, may cause a decrease of the breakdown voltage within the apparatus by generation of nitrogen gas bubbles. A cryostat was equipped with a sphere to plane electrode arrangement for the examination of breakdown and withstand voltages of liquid nitrogen. The temperature of liquid nitrogen was kept at 78 K whereas the pressure values were adjusted to 0.1 MPa, 0.2 MPa and 0.3 MPa. Alternating voltage up to 230 kVrms was applied to the arrangement. Impact of bubble generation by heating impulses on the dielectric strength was not found for 0.3 MPa and it was found for 0.2 MPa only in case of low distances. This is different than the already known behavior at 0.1 MPa and boiling temperature.

Withstand alternating voltage of liquid nitrogen in the presence of gas bubbles

Liquid nitrogen is often used in superconducting high voltage apparatus for cooling and for electrical insulation purposes. A temperature increase of the superconductor surface, e. g. to room temperature during a quench of a fault current limiter, may cause a decrease of the breakdown voltage within the apparatus by generation of nitrogen gas bubbles. A cryostat was equipped with a sphere to plane electrode arrangement with adjustable gap for the examination of the breakdown and withstand voltages of liquid nitrogen. Bubble generation of the heatable plane was verified by camera observation. Alternating voltage up to 230 kVrms was applied to the arrangement under the pressure condition of 0.1 MPa (absolute). Degradation of the withstand voltage up to 50% was found by using a 500 W heater in comparison to the case with no heating.