| Magneto-optical
current transformer technology (MOCT) |
Definition
An accurate electric current transducer is a key component of any power system
instrumentation. To measure currents power stations and substations conventionally
employ inductive type current transformers with core and windings. For high voltage
applications, porcelain insulators and oil-impregnated materials have to be used
to produce insulation between the primary bus and the secondary windings. The
insulation structure has to be designed carefully to avoid electric field stresses,
which could eventually cause insulation breakdown. The
electric current path of the primary bus has to be designed properly to minimize
the mechanical forces on the primary conductors for through faults. The reliability
of conventional high-voltage current transformers have been questioned because
of their violent destructive failures which caused fires and impact damage to
adjacent apparatus in the switchyards, electric damage to relays, and power service
disruptions. With short circuit capabilities
of power systems getting larger, and the voltage levels going higher the conventional
current transformers becomes more and more bulky and costly also the saturation
of the iron core under fault current and the low frequency response make it difficult
to obtain accurate current signals under power system transient conditions. In
addition to the concerns, with the computer control techniques and digital protection
devices being introduced into power systems, the conventional current transformers
have caused further difficulties, as they are likely to introduce electro-magnetic
interference through the ground loop into the digital systems. This has required
the use of an auxiliary current transformer or optical isolator to avoid such
problems. It appears that the newly emerged
Magneto-optical current transformer technology provides a solution for many of
the above mentioned problems.
The MOCT measures the electric current by means
of Faraday Effect, which was first observed by Michael Faraday 150 years ago.
The Faraday Effect is the phenomenon that the orientation of polarized light rotates
under the influence of the magnetic fields and the rotation angle is proportional
to the strength of the magnetic field component in the direction of optical path.
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