Carsten Rasmussen

Test results of full-scale HTS cable models and plans for a 36 kV, 2 kA/sub rms/ utility demonstration

Cable systems using high-temperature superconducting (HTS) tapes are nearing technical feasibility. Several large-scale demonstrations are under way. This article summarizes the advancements and status of a development project aimed at demonstrating a 36 kV, 2 kA RMS AC cable system through installing a 30 m long full-scale functional model in a power utility substation. The HTS cable line is designed to link two medium-voltage transformer stations in an urban environment. The expected benefits of such a system include reduced energy loss, ease of installation, increased power rating in a small cross section, and insensitivity to the surrounding soil conditions. Results will be presented from tests on several 2 kA-class AC conductors. Electrical losses below 1 W/m at 2 kArms have been obtained in these cable conductors. The cable system consists of terminations, three HTS cables with conventional room-temperature dielectric and stress cones, and a closed-loop circulating cooling system maintaining the temperature between 74 and 84 K. Critical issues before the commercialization of this technology is the improvement of the thermal insulation, the reliability and maintainability of the cooling system, and the reduction of materials costs.

First operation experiences from a 30 kV, 104 MVA HTS power cable installed in a utility substation

An HTS cable with a voltage rating of 30 kV and a power rating of 104 MVA, has been installed and operated in the electric grid of Copenhagen Energy in the spring of 2001. This article describes the development phases, the system specifications, and the first experiences of operation under realistic conditions in the substation of Amager (AMK). Approximately 50 000 private and business customers are supplied from this cable. The load can be adjusted from 20% to 100% of the power supplied and the number of branches connected can be altered. This and other early HTS power installations are expected to act as ice-breakers for the HTS technology.

Alternating current losses of a 10 metre long low loss superconducting cable conductor determined from phase sensitive measurements

The ac loss of a superconducting cable conductor carrying an ac current is small. Therefore the ratio between the inductive (out-of-phase) and the resistive (in-phase) voltages over the conductor is correspondingly high. In vectorial representations this results in phase angles between the current and the voltage over the cable close to 90 degrees. This has the effect that the loss cannot be derived directly using most commercial lock-in amplifiers due to their limited absolute accuracy. However, by using two lock-in amplifiers and an appropriate correction scheme the high relative accuracy of such lock-in amplifiers can be exploited. In this paper we present the results from ac-loss measurements on a low loss 10 metre long high temperature superconducting cable conductor using such a correction scheme. Measurements were carried out with and without a compensation circuit that could reduce the inductive voltage. The 1 µV cm-1 critical current of the conductor was 3240 A at 77 K. At an rms current of 2 kA (50 Hz) the ac loss was derived to be 0.6±0.15 W m-1. This is, to the best of our knowledge, the lowest value of ac loss of a high temperature superconducting cable conductor reported so far at these high currents.

Optimization of termination for a high-temperature superconducting cable with a room temperature dielectric design

This paper describes the design considerations of a termination for a superconducting cable, based on tapes produced with the powder-in-tube method, with a room temperature dielectric design. Most important is the optimization of the current lead that leads the current from room temperature to cryogenic temperature. The current lead is optimized, using analytical as well as numerical methods. The paper proposes a current lead made of copper, with a constant cross-section area. With an optimized length-to-cross-section area ratio, the heat flow to the cold region is 43 W/kA for an uncooled current lead and 20 W/kA for a cooled current lead. The minimum loss in the entire termination is approximately 60 W/kA for a termination optimized for 2 kA. The paper describes why a gas-cooled current lead only reduces the total losses when used in connection with a multistep cooling machine.

Measuring AC-loss in high temperature superconducting cable-conductors using four probe methods

Measuring the AC-loss of superconducting cable conductors have many aspects in common with measuring the AC-loss of single superconducting tapes. In a cable conductor all tapes are connected to each other and to the test circuit through normal metal joints at each end. This makes such measurements considerably more complex, especially for samples of laboratory scale (1-5 meters). Here we discuss different measurement configurations using four probe methods and lock-in detection. We conclude that the voltage should be picked up at end of the connecting joints, and we show how the resistive contribution from these joints can be identified and subtracted from the measured data. We also show measurements which indicate that the size of the loop constituted by the voltage leads has no influence on the measurements.

Measuring the current distribution in a 10 m long high temperature superconducting cable conductor

Abstract To obtain realistic data on high temperature superconducting (HTS) conductors, a 10 m long cable conductor was built using 193 HTS tapes placed in eight concentric layers. To fully exploit the current carrying capability of all the HTS tapes and to minimise the AC losses the conductor was designed to have an almost even current distribution with respect to the HTS tapes. The outer diameter of the former was 35 mm and the outer diameter of the conductor was about 40 mm. A thin layer of electrically insulating Mylar TM foil was wound between each HTS layer. This way, the current in one particular layer may be treated as a tubular current sheet. The current in each layer was determined using custom made flat Rogowski coils placed between the superconducting layers. For this conductor, each layer was carrying

Calorimetric measurements of losses in HTS cables

A calorimetric test rig is used to investigate various loss components in a 10 m long superconducting cable model. A calorimetric technique, based on thermocouple measurements, is used to measure the losses of the 10 m long superconducting cable model. The current dependent losses are also measured electrically and compared with the losses obtained with the calorimetric method. The results obtained by the two methods are consistent. Based on an I/sup 2/ (current) fitting procedure, the loss, caused by the eddy current generated in the stainless steel cryostat housing, and the hysteresis loss generated in the conductor can be separated. From this result, it appears that the two contributions are roughly equal in size.

Design of a termination for a high temperature superconducting power cable

A cable conductor consisting of superconducting tapes wound onto a tight flexible tube (former) is placed inside a thermally insulating jacket (cryostat). This assembly is electrically insulated with an extruded polymer dielectric kept at room temperature. Cooling is provided by a flow of liquid nitrogen inside the former. The purpose of an end termination is to connect the superconducting cable conductor at cryogenic temperature to an electrical wire at room temperature and an external cooling machine at ground potential. Here we describe the design and construction of such an end termination. Aspects considered in the design include the thermal insulation of the termination, the transition from superconducting tapes to a normal conductor, the current lead carrying current between high and low temperatures, the transfer of liquid nitrogen over a high voltage drop and that of providing a well defined atmosphere inside the termination and around the cable conductor.

Short-circuit experiments on a high-Tc superconducting cable conductor

Abstract A high temperature superconductor (HTS) cable conductor (CC) with a critical current of 2.1 kA was tested over a range of short-circuit currents up to 20 kA. The duration of the short-circuit currents is 1 s. Between each short-circuit test the critical current of the HTS CC was measured in order to detect degradation due to the short-circuit current. During the over-current testing the current and voltage along the CC were measured as well as its temperature. Significant warming above the critical temperature occurs for short-circuit currents of 10 kA and above. No critical current degradation was found for currents up to 10 kA for 1 s, while some degradation (10%) was found for 20 kA.

Operating a Cryogenic Test RIG for a 10 Meter Long Liquid Nitrogen Cooled Superconducting Power Cable

One way to cool a high temperature superconducting cable is to circulate liquid nitrogen (LN2) by means of a mechanical pump through a sub-cooler and through the core of the cable.