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ABSTRACT
Optical current sensors are achieving increased acceptance and use in high voltage
substations due to their superior accuracy, bandwidth, dynamic range and inherent
isolation. Once deemed specialized devices intended for novel applications, optical
sensors have risen to a performance level exceeding conventional magnetic devices. A
specific area where optical current sensors outperform conventional iron core
transformers is the measurement of very high currents that occur during a fault on the
power system. Conventional instrument transformers utilize an iron core and windings
ratio to step down the current measured in the primary to a more manageable current
level for secondary devices such as meters and relays. This signal may be distorted due
to saturation of the magnetic core. In a pure optical current sensor1, no such
mechanism for saturation exists. However, optical sensors must be used and applied properly to provide distortion free signal replication well into the hundreds of kilo amp
region. This paper discusses the characteristics of optical current sensors, specifically
for relaying applications where measurement of faultlevel currents is required.
INTRODUCTION
When faults on a power system occur, they must be isolated quickly to maintain the safe
operation of the system, minimize damage to equipment, and maintain stability of the
system. Therefore, the accurate measurement of fault current is a critical input to
protection relays which monitor the current and/or voltage signals to determine
whether the monitored portion is faulted and should be isolated, or whether conditions
are normal and should remain closed to maintain the flow of power. If protection
relays receive the “true” representation of current flowing on a transmission line, or
into transformers, capacitor banks, or reactor banks, they will make decisions based on
the current that is actually flowing, not based on a distorted representation of the
current which the relay may need to compensate for. An undistorted view could
improve the ability of the relay to trip when it should and to prevent false trips.
Additionally, analyzing the power system as a whole, optical current sensors make
design and analysis easy since no CT saturation will ever be encountered. Optical
sensors behave in a simple and predictable manner known for every situation.
SATURATION IN CONVENTIONAL CURRENT
TRANSFORMERS
During fault conditions a wellknown phenomenon occurs: the iron core in a
transformer “saturates” due to a large magnetic field caused by high fault currents.
This saturation of the iron core prevents the transformer from accurately representing
the primary current in the current transformer secondary, and therefore distorts
current measurement. It is not the intent of this paper to explain saturation or analyze
when and why it occurs. Readers not familiar with saturation should reference the
many papers, books, and standards that deal directly with this subject in detail to fully
understand the phenomena. Additionally, many good reference sources discuss the
problems of CT saturation with respect to relaying, avoidance of saturation and
methods to deal with saturation. The underlying problem surrounding the phenomena
is that essentially all CTs will saturate unless they are built with an excessive amount of
steel to prevent it. This method of mitigation is impractical and must be dealt with by
knowing how, when, and why a CT will saturate, then taking appropriate measures to
prevent any false relay operations. The mechanism for CT saturation is not a simple
relationship. Saturation depends on the physical design of the current transformer, the
amount of steel in the “core” of the transformer, the connected burden, the winding
resistance, the remanence flux in the iron core, the fault level, and the system X/R ratio
(which can cause a larger DC offset to occur). Taken together, these dependencies make
the analysis of CT saturation complex. Figure 1 below shows an example of a CT with a
saturated output against a plot of actual current. Scale is not given on the yaxis since it
could apply to a variety of CTs with various currents. The plots are shown only to
illustrate a saturated CT waveform.
MEASURING CURRENTS USING OPTICAL
TECHNOLOGY
The problem of CT saturation in iron core instrument transformers can be avoided
altogether by using an optical current sensor. Optical current sensors contain no
magnetic components and do not have any saturation effects associated with them.
Optical current sensors also have no iron core to saturate. Depending on the design of
the sensor, these types of sensors have the ability to give a near perfect representation of
the primary current. An optical sensor uses light to measure the magnetic field
surrounding a current carrying conductor and, based on this measurement, electronics
associated with the optics calculate the current flowing in the conductor. If done
optimally, an optical measurement of current has the ability to measure fault currents
exceeding 400 kA peak. Additionally, using advanced techniques, both AC and DC
currents can be measured to this level. An optical current sensor using light to measure
the magnetic field surrounding a conductor has a transfer function with a sine wave
characteristic. With normal load current flowing on the conductor, the measurement of
the magnetic field by light is maintained essentially within the linear portion of the sine
wave. Once the current increases substantially (for example, when a fault occurs) the
transfer function of the light no longer traverses the linear portion of the sine wave, but
enters a nonlinear portion. In this nonlinear portion of the sine wave, the electronics
compensate for the nonlinearity. Since this nonlinear “sine wave” characteristic is
well defined, electronics can easily adjust, in order to maintain overall linearity of the
current measurement throughout the dynamic range. Although this compensation
technique permits excellent accuracy, it has an inherent limit. As the current reaches
the “end” of the sine wave (or at an angle of plus and minus π radians) and continues to
increase, the electronics may interpret the current to be higher than its previously
measured current, or may interpret the current to be at the opposite end of the sine
wave transfer function. The sensor will show a severe jump in the measurement of the
current to a current of negative polarity with respect to its previous value. This
phenomena is illustrated in Figures 2, 3 and 4 which show sensor outputs for 1 fiber
turn, 3 fiber turns, and 5 fiber turns. As more fiber turns are added to the sensor
design, the signaltonoise ratio of the output increases, though not detectable in the
Figures. A better signal to noise ratio has certain distinct advantages, especially in
metering applications [1]. However, as the fiber turns are increased and the fault level is
maintained at a constant level, operating range on the optical transfer function
approaches the limit of plus and minus π radians. If exceeded, the sensor can record a
current “jump”, or move to the next optical “fringe” and thus appear as a different
current value. To avoid this situation, which cannot be tolerated by relays, either special
processing algorithms can be introduced to keep track of which fringe the sensor is on
or the sensor can be designed to reduce possibility of such an occurrence. Fortunately,
for typical fault current levels, reducing the probability of the sensor exceeding the
“fringe” is simple, since the point at which the optical sensor reaches this point is
precisely known based on the number of fiber turns used in an optical current sensor.
This would eliminate the distortions seen n Figures 2 and 3, provide an accurate current
waveform representation free from saturation effects, and provide a high signaltonoise ratio so the signal is also optimized formetering and power quality analysis applications. To
a user of fiber based optical current sensors, the situation will never be observed unless a
sensor is driven to a value beyond its specifications.
WORKING OF CURRENT SENSOR
A light source sends light through a waveguide to a linear polarizer, then to a
polarization splitter (creating two linearly polarized light waves), and finally to an
optical phase modulator. This light is then sent from the control room to the sensor
head by an optical fiber. The light passes through a quarter waveplate creating right
and left hand circularly polarized light from the two linearly polarized light waves. The
two light waves traverse the fiber sensing loop around the conductor, reflect off a
mirror at the end of the fiber loop, and return along the same path. While encircling the
conductor, the magnetic field induced by the current flowing in the conductor creates a
differential optical phase shift between the two light waves due to the Faraday effect.
The two optical waves travel back through the optical circuit and are finally routed to
the optical detector where the electronics demodulate the light waves to determine the
phase shift. The phase shift between the two light waves is proportional to current and
an analog or digital signal representing the current is provided by the electronics to the
end user.
THE FARADAY EFFECT
The Faraday effect is named after Michael Faraday who discovered this phenomenon in
1845. It describes the rotation of polarisation of light propagating in the direction of a
magnetic field. When a beam of light is sent through a material exhibiting the Faraday
effect, the polarisation of the light will be rotated by the angle θ in dependency of the
magnetic field strength parallel to the light. The Faraday effect is proportional to the
magnetisation of the material.
The Faraday effect arises from the interaction of the electron orbit and the electron spin
with the magnetic field. The general principle can be understood as righthanded and
lefthanded circularly polarized light causing charges in a material to rotate in opposite
senses. Each polarization therefore produces a contribution to theorbital angular
momentum with opposite sign. A magnetic field gives rise to a spinpolarization along
the magnetic field direction and the spinorbit interaction then leads to an energy
contribution for the two circular polarizations having the same magnitude but with
opposite sign [Blun01]. This leads to righthanded and lefthanded polarizations having
different refractive indices in the material. A linearly polarized wave can be seen as the
sum of two circularly polarized waves with equal amplitude but opposite direction of
rotation. As these two waves propagate with different speeds through the material, they
will acquire a phase difference proportional to the travelled distance. In terms of their sum, these two beams, when they emerge, have a phase lag between them implying that
the emerging beam has a rotated plane of polarization by an angle which is equal to half
the phase change. This effect is nonreciprocal, meaning a light beam passing a medium
twice in opposite direction acquires a net rotation twice that of a single pass. It should
be noticed that according to the material, the Verdet constant is temperature and
wavelengthdependent.
ADVANTAGES
immunity to electromagnetic interference (EMI)
high electrical insulation
large bandwidth
potentially high sensitivity
ease in signal light transmission
being compact and lightweight
potentially lowcost
no danger of explosion
ease of integration into digital control systems
no saturation
hysteresisfree
passive measurement
DISADVANTAGES
the electronic circuit present may cause distrotions.
the measurement is not much accurate
CONCLUSION
Optical current sensors provide a reliable method of measuring very high fault currents
with significant DC offsets without any type of saturation, as is understood with
conventional current transformers. Depending on the design of the sensor, several
turns of fiber can be wound around the conductor to increase the signal to noise ratio of
the sensor. This gain in signal to noise ratio is traded with the ability of the sensor to
measure extremely high fault currents without fringe management algorithms.
However, if desired, advanced processing techniques such as fringe management
techniques can be implemented in sensors, and high signal to noise ratios and high fault
current measurements can be achieved simultaneously.