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.
A 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 effect.
The 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.
The 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).
These 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.
Researchers 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.
Description of the Optically Powered Data Link (OPDL)
The OPDL system can be divided in to a remote unit at high voltage
potential and a local unit, which is based in the sub station control room or
an existing control enclosure. This unit houses the laser with its associated
laser driver and the data recovery circuitry. The laser system used for this
application can couple a maximum optical power of 1.5 Watt in to the power link
fiber. These lasers are not to be very reliable with a long life time (MTBF:
>100,000 h). A self-check Function supervises all vital functions of the
OPCT. An alarm will be initiated long before the laser reaches the end of its
life time indicating necessary maintenance. A trip signal will be set if the
system has identified a misoperation.
Depending on the metering or relay scheme, this unit can provide a digital
serial output, +/- 10Volts (full scale) or a current loop of 1 amp (nominal) @
maximum 20 or 40 VA. The power to operate this unit can conveniently be
supplied by any station power supply. The OPDL local ground unit is connected
by two optical fibers, a power fiber and a data link, to the remote electronic
board at the high voltage system. The remote unit is shielded against any EMI
or RFI noise and converts the voltage drop across the CT burden resistor in to
digital signals. The electrical power to operate this unit provided by the
photovoltaic power converter that is connected to the laser over one of the
fiber optical links with a conversion efficiency of up to 40 %.
The remote system provides two A/D channels with a sampling rate of 40
kHz each corresponding to a bandwidth of 15kHz (250 Harmonics@60Hz system!).
The performance of this board is below 1% error for protection purposes at
nominal value and a range of 30p u and exceeds Class 0.2 for metering accuracy.
The output of the A/D converter together with some data control and supervisory
signals make up a serial data stream, which is converted in to light pulses and
coupled in to the data fiber. In addition to the data stream, the voltage of
the remote board is monitored for safety reasons and for control of the laser
output.
To ensure the capability for a remote calibration of the electronic
circuitry, a very precise voltage source is incorporated into the design, which
can be connected in to the data path from the local unit while being in a calibration
in a test mode.
CURRENT
TRANSFORMER
The current transformer used for the OPCT can either be a CT designed
for metering or protection class accuracy or a resistive shunt. These
transformers are dry type, out door rated systems. Since a signal column
provides the high voltage isolation, these CT s can be of a low voltage
600V(720V Europe) type.
A high precision, low drift burden resistor together with the CT
provides the voltage input for the OPDL system. The burden resistor and an
input protection filter are housed together with the remote circuitry in a
shielded enclosure to provide immunity against EMI and RFI disturbance. This
combined unit is lightweight (about 15 pounds), which allows easy installation
hence limiting the system outage time to a minimum. The unit is mechanically
protected fiber link connects the unit to the signal column.
The CT and burden resistors are available in all common current ratings.
The output voltage of the burden resistor is adjusted to the full range of
interest (i.e. 30 p. u. for protection).
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