Søren Krüger Olsen

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.

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.

AC losses in circular arrangements of parallel superconducting tapes

The DC and AC properties of superconducting tapes connected in parallel and arranged in a single closed layer on two tubes (corresponding to power cable conductor models with infinite pitch) with different diameters are compared. We find that the DC properties, i.e., the critical currents of the two arrangements, scale with the number of tapes and hence appear to be independent of the diameter. However, the AC loss per tape (for a given current per tape) appears to decrease with increasing diameter of the circular arrangement. Compared to a model for the AC loss in a continuous superconducting layer (Monoblock model) the measured values are about half an order of magnitude higher than expected for the small diameter arrangement. When compared to the AC loss calculated for N individual superconducting tapes using a well known model (Norris elliptical) the difference is slightly smaller.

Measurements of AC losses in different former materials

A high temperature superconducting cable may be based on a centrally located cylindrical support, a so-called former. If electrically conductive, the former can contribute to the AC losses through eddy current losses caused by unbalanced axial and tangential magnetic fields. With these measurements we aim at investigating the eddy current losses of commonly used former materials. A one layer cable conductor was wound on a glass fibre reinforced polymer (GFRP) former. By inserting a variety of materials into this, it was possible to measure the eddy current losses of each of the former candidates separately; for example copper tubes, stainless steel braid, copper braid, corrugated stainless steel tubes, etc. The measured data are compared with the predictions of a theoretical model. Our results show that in most cases, the losses induced by eddy currents in the former are negligible. However, for materials with a low resistivity the eddy current losses may become significant, e.g., for high purity Cu or Al.

Test results of full-scale high temperature superconductors cable models destined for a 36 kV, 2 kArms utility demonstration

Abstract Power cable systems using high temperature superconductors (HTS) are nearing technical feasibility. This presentation summarises the advancements and status of a project aimed at demonstrating a 36 kV, 2 kA rms AC cable system by installing a 30 m long full-scale functional model in a power utility. The expected benefits of such a system include reduced energy loss and increased power rating in a small cross-section. Electrical losses below 1 W/m/phase at 2 kA rms have been obtained in these conductors. The cable system consists of conventional electrical terminations in conjunction with thermal terminations, an HTS cable conductor including a flexible thermal insulation, a conventional room temperature dielectric, and a closed-loop circulating cooling system maintaining the temperature between 68 and 78 K. Critical issues before the commercialisation of this technology are the improvement of the thermal insulation, and the reduction of costs.

Measuring ac losses in superconducting cables using a resonant circuit: resonant current experiment (RESCUE)

A simple way to obtain true ac losses with a resonant circuit containing a superconductor, using the decay of the circuit current, is described. For the measurement a capacitor is short circuited with a superconducting cable. Energy in the circuit is provided by either charging up the capacitors with a certain voltage, or letting a dc flow in the superconductor. When the oscillations are started - either by opening a switch in case a dc is flowing or by closing a switch to connect the charged capacitors with the superconductor - the current (via a Rogowski coil) or the voltage on the capacitor can be recorded using, for example, a digital oscilloscope. The amplitude decay of the periodic voltage or current accurately reflects the power loss in the system. It consists of two components - an ohmic purely exponential one (from leads, contacts, etc.), and a nonexponential component originating from the superconductor. The method has been successfully applied for the measurement of the ac loss in a 1 m long superconducting cable model.

Nominal current test performance of 2 kA-class HIGH-TC superconducting cable conductors and its implications for cooling systems for utility cables

The current carrying performance of 3–10 m long superconducting cable conductor models has been evaluated. A reduced energy loss compared to conventional cables can be obtained using high-Tc superconducting materials due to the limited resistive and AC hysteresis losses in some conductor configurations. The conductors are characterised under DC and AC conditions. The current and voltage are recorded during the tests in order to determine the impedances and the losses of the cable models. Using a phase-sensitive measurement with two lock-in amplifiers, small losses can be accurately measured at high currents. The critical currents of these conductors are in the range of 1–3 kA, and AC losses smaller than 1 W/m are measured at 2 kArms. AC currents with peak values exceeding the DC critical currents are applied. Increased losses, in excess of the expected magnetisation losses are observed when individual layers in the cables saturate. The loss contributions from other components of the cable system are discussed, and the implications for the cooling apparatus for superconducting utility cables are determined.

The electrical aspects of the choice of former in a high T/sub c/ superconducting power cable

Centrally located in a superconducting power cable the former supplies a rigid means onto which to wind the superconducting tapes and enables a continuous supply of cooling power via a flow of liquid cryogen through it. Therefore, the choice of former has a broad impact on the construction and design of a cable. The diameter of the former determines the overall diameter of the total cable, influences the heat loss to the ambient and enters into the total AC-losses. Depending on whether the former is made of a good or poor electrical conductor, eddy currents in the former itself may also contribute significantly to the AC-loss of the cable; the choice between an open and a closed former determines how and where the pressure load (pressurized coolant) has to be accommodated. In this work the electrical impact of the choice of material and diameter of the former on the AC-loss of a cable conductor is addressed.