K. Yamagishi

Influence of AC external magnetic field perturbation on trapped magnetic field in HTS bulk

Abstract Electric motors and actuators are most promising future applications of HTS bulks. The HTS bulks in these electric machines are exposed to AC magnetic field perturbations. The perturbations cause AC losses in the bulks and also affect the magnetic field trapped in the bulks. In a previous work we measured AC losses in a YBCO bulk subject to the AC magnetic field and found the loss characteristics well followed the Bean model. In this work we experimentally studied the influence of the AC perturbations of the external magnetic field on the trapped field in a YBCO bulk. The experimental results showed that the application of the AC magnetic field decreased the trapped magnetic field even to zero. In this paper, the experimental results are presented and a reason for the decay of the trapped magnetic field is discussed based on the Bean model.

Decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work it was observed that trapped magnetic field in a YBCO bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. To study a reason for decay we investigated temperature rise of a YBCO bulk subject to the AC external magnetic field by use of thermocouples. A case was observed that the temperature rise of the bulk reached almost the critical temperature. This result shows that the decay of the trapped magnetic field was related to the temperature rise due to AC losses in the bulk.

Study on islanding of dispersed photovoltaic power systems connected to a utility power grid

Photovoltaic (PV) systems and other dispersed power systems need to be connected to a utility power grid for the systems to work effectively as energy sources. Protection against islanding is one of the most important techniques to develop for the dispersed power systems supplying surplus power back to the utility power grid. It is considered that active methods where the outputs of the power systems are actively perturbed are necessary to detect islanding. Various active methods have been developed. It has been shown that these methods work when a single power system is connected to one power distribution line. However, most of these methods have a limit in detecting islanding when multiple power systems are connected to one distribution line. In this paper, we compare the detection characteristics of typical active methods, frequency shift methods, a pulse perturbation method and a method using a correlation technique by simulation analysis for the case of multiple PV power systems. The study shows that the method using the correlation technique is the most promising when multiple power systems are operated on one distribution line.

Detection of islanding of multiple dispersed photovoltaic power systems

Protection from islanding is one of the most important R&D items for photovoltaic (PV) and other dispersed power systems connected to a utility power grid and supplying back surplus power. To protect against islanding, dispersed power systems should be able to detect loss of power in the power grid and be disconnected from the grid immediately. However, a reliable method for detecting the islanding, especially when multiple dispersed systems are connected to one power distribution line, has not yet been developed. We have developed a new method to detect the islanding using correlation techniques. In this article the detecting ability of the correlation method is analyzed when multiple power systems are connected to a power distribution line and compared with that of a usual voltage perturbation method. It is shown that the correlation technique is effective for the case of multiple connection of the PV systems to line, while a usual voltage perturbation method is not effective.

AC Losses in YBCO Coated Conductors Subjected to Tensile Stress

Stress dependencies of AC transport current losses, magnetization losses and total losses were investigated applying tensile stresses to an YBCO (IBAD/PLD) conductor. Measured transport current losses showed slight stress dependence in the range of 0~1 GPa. That can be explained by stress dependence of the critical current of the conductor. The magnetization and total losses are less sensitive to the stress than the transport current losses

Development and electrical test of a 30 kVA class fully superconducting generator

We are developing a 4 pole 50 Hz 30 kVA class fully superconducting generator to investigate the characteristics of superconducting armature windings subject to a rotating magnetic field. Compared with other fully superconducting generators that have been developed and tested, our machine is designed to obtain a higher armature current. It is generally observed in coils wound from AC superconducting cable that the AC quench current of the coil is much lower than its DC quench current. Countermeasures against these AC current degradation phenomena were developed and applied to the AC cable and armature winding of our generator. Results of the electrical tests of the generator are presented and the validity of the countermeasures is discussed. >

Development of prototype superconducting linear induction motor for steel making processes

The authors describe how they are developing technologies of a superconducting linear induction motor (SLIM) for steel making process applications. They have developed and tested a prototype SLIM and this paper presents configurations and test results of the SLIM.<<ETX>>

Study on Method to Suppress Decay of Trapped Magnetic Fluxes in the HTS Bulk Subjected to Perturbation of External Magnetic Field by Use of Shielding Coil Wound of HTS Wire

Trapped magnetic fluxes in HTS bulks are decayed when the bulks are subjected to perturbation of external magnetic fields. In HTS bulk apparatuses such as HTS bulk motors and magnetic levitation bearings, there is a possibility that the trapped magnetic fluxes in the bulks disappear because the bulks are inevitably subjected to perturbations of the external magnetic fields. For those apparatuses to work stably and reliably, it is necessary to develop methods to suppress the decay. It was shown in our previous experiments that the decay is caused by temperature rises of the bulks due to AC losses in the bulks. The authors proposed a method to put a shorted shielding coil wound of HTS wire around the bulk aiming at reduction of the AC losses in the bulk. It was experimentally shown by use of a shielding coil wound of Bi/Ag sheathed wire that the decay of the trapped magnetic flux was effectively suppressed. In this paper, shielding performance of the coil wound of GdBCO wire was experimentally investigated comparing with that of the coil wound of Bi/Ag sheathed wire. AC losses in the bulks and shielding coils are calculated for various arrangements of the shielding coil to find a preferable shielding coil arrangement from stand points of shielding performance and reduction of AC losses.

Suppression of Influence of AC External Magnetic Field on Trapped Magnetic Flux in the HTS Bulk by Use of HTS Shielding Rings

Trapped magnetic fields in high temperature superconductor (HTS) bulks are decayed when the bulks are subjected to perturbation of magnetic fields. This phenomenon is inconvenient for the applications of HTS bulks and it is necessary to develop methods to suppress the decay. It was found in a previous study that the decay is caused by temperature rises of the bulks due to AC losses in the bulks. Therefore, it is necessary to develop a method to reduce the AC losses. In this work, a method is proposed to reduce the AC losses by putting HTS shield rings around the bulk and shielding performance of the rings were investigated. The experimental result shows that the shielding rings can drastically suppress the decay, even if the bulk is used in the same cooling condition and the effectiveness of the method was demonstrated.

Dependence of AC Losses in HTS Coated Conductors With Magnetic Substrates on Tensile Stresses

We investigated uni-axial tensile stress dependence of AC losses (transport current, magnetization and total losses) in HTS coated conductor with magnetic Ni-alloy substrate. The AC loss characteristics are studied by relating to tensile stress dependence of critical currents of the conductor. Experimental results showed that the loss characteristics were different whether the critical current recovered the original value after the release of the stress or not. Dependence of AC loss characteristics on magnetic property of the substrate is also studied.