Studies of high-temperature superconducting tapes and their application in power apparatus

University dissertation from Stockholm : KTH

Abstract: Intensive development in the field of high-temperature superconductors (HTS) has resulted in long length HTS with high current carrying capacity. The performance of HTS in the form of multifilamentary silver sheathed Bi-2223 tapes is close to the requirements for power application. Several power apparatus prototypes based on HTS have been and are being demonstrated. Recently the first commercial order for a HTS power apparatus was placed. Tennessee Valley Authority ordered two 12 MVA synchronous condensers with HTS rotor windings for reactive power compensation.In most power devices the conductor is carrying an AC transport current while it is exposed to an AC magnetic field transverse to the current path. In certain applications such as multi-layer power transmission cables or the control winding in a controllable reactor, the conductors are exposed to a magnetic field component longitudinal to the tape axis that is parallel with the current path. In this thesis, the losses in HTS tapes in longitudinally applied magnetic field are investigated and a single phase small scale controllable reactor with a loss optimized HTS control winding is designed, constructed and tested.An experimental setup to measure the losses in tapes exposed to longitudinal magnetic fields, transport currents, frequencies, and temperatures is presented. The system is based on a calorimetric loss measuring method where the losses are determined by the temperature response of the conductor when exposed to magnetic field and/or carrying transport current.Semi-empirical loss models are developed from the results of the loss measurements of non-twisted multifilamentary Bi-2223 tapes. The losses when the sample is carrying transport currents and is exposed to longitudinal magnetic fields are found to consist of hysteresis, flux flow, and eddy current losses. The hysteresis losses can be modeled with the critical state model. The flux flow losses can be described with a power law dependent current voltage characteristic.The loss models are well suited for the design of power devices in which HTS is exposed to longitudinal magnetic fields. Based on the knowledge of the losses a single phase small scale controllable reactor with a loss optimized HTS control winding is designed, constructed and tested. A simple model of the magnetic circuit of the controllable reactor is presented. Calculations from the model are compared with experimental measurements. The feasibility to design controllable shunt reactors with large linear dynamic range, low losses and limited harmonic distortion is demonstrated. Compared to a copper control winding the HTS control winding increases the dynamic range and reduces the size due to the high current density and reduces the losses of the control winding.