Claus Nygaard Rasmussen

L1 Adaptive Speed Control of a Small Wind Energy Conversion System for Maximum Power Point Tracking

This paper presents the design of an L1 adaptive controller for maximum power point tracking (MPPT) of a small variable speed wind energy conversion system (WECS). The proposed controller generates the optimal torque command for the vector-controlled generator-side converter based on the wind speed estimation. The proposed MPPT control algorithm has a generic structure and can be used for different generator types. In order to verify the efficacy of the proposed L1 adaptive controller for the MPPT of the WECS, a full converter wind turbine with a squirrel cage induction generator is used to carry out case studies using MATLAB/Simulink. The case study results show that the designed L1 adaptive controller has good tracking performance even with unmodeled dynamics and in the presence of parameter uncertainties and unknown disturbances.

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.

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.

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.

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.

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.