Hybrid optical current transformer with optical and power-line energisation

The hybrid optical current transformer (HOCT) has previously been proposed as a suitable device to replace a conventional open-terminal post-type current transformer. The HOCT is much smaller than the conventional device leading to lower manufacturing costs. There is also a degree of electrical insulation inherent as optical fibre is used for connections between the line-mounted HOCT and ground level instrumentation; this reduces installation costs. A performance specification for a HOCT is presented: a HOCT unit satisfying this specification could, in principle, be a suitable replacement for a conventional device for power system protection purposes without loss of system quality or reliability. A prototype HOCT has been designed and constructed; initial tests show that it meets the specified accuracy requirements over the full operating temperature range. As far as can be determined, this is the first alternative to conventional current transformers that has been shown to meet these stringent requirements.