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We commercialized a large-size optical current transformer (OCT) using the Faraday effect for the Cable Head Station of Hokkaido-Honshu HVDC Link for cable protection. The rated current and voltage of the HVDC are ±1200 A and ±250 kV, respectively. The diameter of the sensing head of the OCT is about 2 m and it is applicable not only to newly constructed cable heads but also to existing cable heads without disassembling a cable head bushing. The field test of the OCT was conducted at Furukawa Cable Head Station for more than one year and we confirmed long-term stability. In the field test, ambient temperature changed in the range from -15degC to 27degC. Primary current measured by the OCT was compared to that measured by a Kraemer-type DCCT, and the data were consistent. Reflecting the field test results, we manufactured the OCT for commercial installation. Factory tests of the OCT for commercial installation were conducted. Ratio error, frequency response, and the results of other tests satisfied their criteria. In particular, we measured temperature characteristics precisely and confirmed that the ratio error of the total system at any ambient temperature, including dispersion, satisfied the required specification, which is equivalent to IEC 60044-8 class 1.0.