Technology in Current Measurement
INTROUCTION to Optical Technology in Current Measurement
Over the past 15 years, optical
current sensors have received significant attention by a number or search groups
around the world as next generation high voltage measurement devises, with a view
to replacing iron-core current transformers in the electric power industry. Optical
current sensors bring the significant advantages that they are non-conductive
and lightweight, which can allow for much simpler insulation and mounting the
designs. In addition, optical sensors do not exhibit hysteresis and provide a
much larger dynamic range and frequency response than iron-core CT's.
common theme of many of the optical current sensors is that they work on the principle
of the Faraday effect. Current measurement plays an important role in protection
and control of electric power systems. With the development of the conventional
CT, the accuracy of the CT is up to 0.2% in the steady state power system. However
many disadvantages of the conventional CT appear with the short circuit capacities
of electric power systems getting larger and the voltage levels going higher for
example, saturation under fault current conditions, ferroresonance effects, potential
for catastrophic failure etc. Today there is number of interest in using optical
current transformer (OCT) to measure the electric current by means of Faraday
benefits of an OCT are the inverse of the conventional CT's problems. That is,
no saturation under fault current conditions, with out iron core and there fore
no ferroresonance effects, with out oil and there fore cannot explode, light weight,
small size, etc.
A common theme of many of the optical current sensors is that they work on the
principle of the Faraday effect. Current flowing in a conductor induces a magnetic
field, which, through the Faraday effect, rotates the plane of polarization of
the light traveling in a sensing path encircling the conductor. Ampere's law guarantees
that if the light is uniformly sensitive to magnetic field all along the sensing
path, and the sensing path defines a closed loop, then the accumulated rotation
of the plane of polarization of the light is directly proportional to the current
flowing in the enclosed wire.
sensor is insensitive to all externally generated magnetic fields such as those
created by currents flowing in near by wires. A measurement of the polarization
state rotation thus yields a measurement of the desired current. The technology
originated 8 years ago to measure currents in Series Capacitor installations.
Since then, it has been introduced not only to Series Capacitor and Thyristor
Controlled Series Capacitor installations (FACTS), but also into High Voltage
Direct Current Systems (HVDC).
FACTS & HVDC systems gain their very high availability and reliability using
the optically powered CT technology. Further integration of the optically powered
technology has led to an economical and solid metering and protection current
transformer without any of the known environmental problems associated with the
oil or SF6-gas filled technology.
have perfected the OPCT to measure currents and transmit the data from high voltage
system to ground potential using state of the art Laser technology. The fundamental
of this technology includes the idea of using fiber optic cables to isolate the
current transformers from ground potentials. The advantages of the optically powered
scheme compared to the conventional, high voltage, free standing magnetic CT include
an environmentally friendly, light weight, non seismic critical composite signal
column together with proven, conventional, low voltage rated 'dry type' CT technology.
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