Fig.2 The 500kVA-class HTS transformer cooled by a subcooled liquid nitrogen system

The 500kVA-class HTS transformer cooled by a subcooled liquid nitrogen system

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Cooling with subcooled liquid nitrogen below 70 K may give the following advantageous aspects in characteristics of HTS power devices: (i) the coolant of subcooled nitrogen itself is an excellent insulating material corresponding to insulating oil and, at the same time, non-flammable, (ii) the critical current of the Bi-2223 tape is enhanced in comparison with that in saturated liquid nitrogen at 77 K, (iii) Disadvantage due to the refrigeration penalty is suppressed because we can make use of a single-stage refrigerator with high efficiency. We prepared a continuous flow system of subcooled nitrogen to cool down the windings of the transformer.

Fig.2 shows the HTS transformer cooled by subcooled liquid nitrogen. The cooling system is composed of additional cryostats for heat exchange and forced flow of subcooled nitrogen to the main cryostat of the transformer. The warmer nitrogen from the main cryostat is forced out from the vessel with a LN2 pump. The forced-flow nitrogen is cooled down in the vessel of heat exchange that is evacuated by a rotary vacuum pump at a level of 155 Torr. The temperature of saturated liquid nitrogen in the heat exchanger was adjusted around 65 K. The cryostats for the transformer and the LN2 pump were maintained at an atmospheric pressure. The steady flow rate of the liquid nitrogen is about 60 l / hr. In steady operations of the transformer, the cooling capacity was about 20 deltaT [W] delt

 DeltaT < 5 K, where delta T is the temperature difference between the subcooled nitrogen in the main cryostat and the saturated nitrogen in the heat exchanger.

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