Fig.3 An advanced 22kV/ 6.9kV HTS transformer system connected with a commercial electric power line and its characteristic test results

An advanced 22kV/ 6.9kV HTS transformer system connected with a commercial electric power line and its characteristic test results

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As a next step of R&D for HTS distribution transformers, we developed a field-test HTS transformer, which has a level of practical rated voltage and performances to permit inrush current and sudden-short-circuit current, in a collaboration of Kyushu University, Fuji electric Co., Ltd., Kyushu Electric Power Co., Kyushu Transformer Co. Ltd. and Taiyo Nissan Co. Ltd. with a support of Fukuoka Consortium Foundation promoted by NEDO. In the design, the rated primary voltage is 22kV corresponding to a level for spot network distribution systems, and the facility permits AC and lightning impulse voltage tests in accordance with the international standards of test methods and furthermore excess current in a sudden short circuit.

Fig.3 shows an overview photo of the developed field-test model and the cooling system. The device is connected through a step-up conventional transformer with a 6.9kV line of Kyushu Electric Power Co. Ltd. The connection circuit and the characteristic test results are also given. The cooling system is installed in a single cryostat, which is composed of a heat exchange part with two refrigerators and a forced-flow one. In individual tests of the transformer before the connection, the device had no breakdown in a 50kV AC withstand voltage test and a 100kV lightning impulse voltage test. It was furthermore confirmed that the device had no damage for a level of excess current five times as large as the designed rated one by a simulation test using a model winding. In the connection test, the device was excited with no quench for inrush current that attained a level five times as large as the rated one. The successive tests included a no-load test with a current corresponding to 1000kVA for two hours, 20-hour 821kVA and 144-hour 586kVA load tests with no unexpected happening. The cooling condition were continuously steady with 2K increase at the excitation equal to 1000kVA for 10 hours, but somewhat transient at the maximum excitation equal to 1670kVA.

The characteristics of the two test devices showed a high potential of Bi-system HTS wires for the transformer windings and gave a basis for design technology off future HTS transformers.

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