Ole Tønnesen

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.

Operation experiences with a 30 kV/100 MVA high temperature superconducting cable system

A superconducting cable based on Bi-2223 tape technology has been developed, installed and operated in the public network of Copenhagen Energy in a two-year period between May 2001 and May 2003. This paper gives a brief overview of the system and analyses some of the operation experiences. The aim of this demonstration project is to gain experience with HTS cables under realistic conditions in a live distribution network. Approximately 50 000 utility customers have their electric power supplied through the HTS cable. The cable system has delivered 226 GW h of energy and reached a maximum operating current of 1157 A. The operation experiences include over-currents of 6 kA due to faults on peripheral lines, commissioning, servicing and failure responses on the cooling system, continuous 24 h, 7 day per week monitoring and performance of the alarm system. The implications of these experiences for the future applications of HTS cable systems are analysed.

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.

Overcurrent experiments on HTS tape and cable conductor

Overcurrents in the power grid can have a magnitude of up to 20 times or higher than the rated current. This may cause problems and permanent damage to electrical equipment in the grid. High temperature superconducting (HTS) tapes are known to be sensitive to currents much larger than their critical current. In this light, it is important to investigate the response of HTS tapes and cable conductors to overcurrents several times the critical current. A number of experiments have been performed on HTS tapes and cable conductors, with currents up to 20 times the critical current. During overcurrent experiments, the voltage, and the temperature were measured as functions of time in order to investigate the dynamic behavior of the HTS tape and cable conductor. After each experiment, damage to the superconductors was assessed by measuring the critical current. Preliminary results show that within seconds an HTS tape (critical current=17 A) heats above room temperature with an overcurrent larger than 140 A. Similar overcurrent experiments showed that a HTS cable conductor could sustain damage with overcurrents exceeding 10 times the critical current of the cable conductor.

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.

Power applications for superconducting cables in Denmark

In Denmark a growing concern for environmental protection has lead to wishes that the number of overhead lines is reduced as much as possible and that the energy supply should be shifted to renewable energy sources, e.g. windmills. Superconducting cables represent an interesting alternative to conventional cables, as they have other characteristics than conventional cables and will be able to transmit two or more times the current. Superconducting cables are especially interesting as a target for replacing overhead lines. Superconducting cables in the overall network are of interest in cases such as transmission of energy into cities and through areas of special beauty. The planned large groups of off-shore windmills in Denmark generating up to 400 MVA or more will be an obvious case for the application of superconducting AC or DC cables. These opportunities can be combined with other new technologies such as high voltage DC (HVDC) based on isolated gate bipolar transistors (IGBTs). The network needed in a system with a substantial wind power generation has to be quite stiff in order to handle energy fluctuations. Such a network may be possible, e.g., using superconducting cables.

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.

Power applications for superconducting cables

High temperature superconducting (HTS) cables for use in electric ac power systems are under development around the world today. There are two main constructions under development: the room temperature dielectric design and the cryogenic dielectric design. However, theoretical studies have shown that the insertion of these cables in the network is not without problems. The network stability requirements may impose severe constraints on the actual obtainable length of superconducting cables. Load flow considerations show that it may be difficult to use these high current cables to their full extent. Short circuits in the network may require a special protection system.

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.