Experimental Comparison of Conventional and Optical Current Transformers

Several manufacturers offer optical current transformers (OCTs) to replace the conventional magnetic current transformers (CTs). This paper compares the performances of some commercially available optical and conventional CTs. The steady state and transient responses of the two systems are compared in a laboratory. Field records are also obtained and evaluated to verify that the optical systems can replace conventional instrument transformers. In addition, OCT step response as well as the impact of ambient temperature on the OCT performance has been investigated. Results show that optical CTs are more than suitable for replacing conventional CTs. Further, the results also show that OCT has better frequency response than conventional CT. Differences found in the field and laboratory measurements are within experimental errors. Unlike the conventional CT, the OCT can have a digital output according to the IEC 61850 standard. The digital output of the OCT is tested for its frequency-amplitude characteristic. Results confirm that digital output has a narrower bandwidth than the low energy analog output of the OCT due to the limited sampling rate prescribed in a UCA guide to IEC-61850-9-2.

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