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MAGNETO-OPTICAL CURRENT TRANSDUCER (MOCT)
VISVESVARAYA TECHNOLOGICAL
UNIVERSITY
A Seminar Report On
MAGNETO-OPTICAL CURRENT TRANSDUCER
(MOCT)In partial fulfilment of the requirements for the award of the degree of
BACHELOR OF ENGINEERING
InELECTRICAL & ELECTRONICS
Submitted by
HAMID ARIZ 1BI09EE063
Under the guidance of
Mr. N. A. PrashanthAssociate Professor, Dept. of Electrical and Electronics EngineeringBangalore Institute of Technology
K.R. Road, Bangalore 560004
DEPARTMENT OF ELECTRICAL AND ELECTRONICS
ENGINEERING
BANGALORE INSTITUTE OF TECHNOLOGYK.R.ROAD, V.V. PURAM, BANGALORE-560004.
2012-2013
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MAGNETO-OPTICAL CURRENT TRANSDUCER (MOCT)
BANGALORE INSTITUTE OF TECHNOLOGY
K.R.ROAD, V.V.PURAM, BANGALORE-560004DEPARTMENT OF ELECTRICAL AND ELECTRONICS
ENGINEERING
CERTIFICATEThis is to certify that the seminar report entitled MAGNETO-
OPTICAL CURRENT TRANSDUCER (MOCT) has been
successfully completed by HAMID ARIZ USN: 1BI09EE063 of
8TH Semester ELECTRICAL AND ELECTRONICS
ENGINEERING under our supervision and guidance and has
been submitted as per the requirements of the university, as
seminar work for partial fulfilment for the award of BACHELOR
OF ENGINEERING of Visvesvaraya Technological University ,
Belgaum during the academic year 2012- 2013.
Mr. N. A. Prashanth Dr. P.
PRAMILAAssociate Professor Professor& HOD
Dept. of E&EE, BIT Dept. of E&EE,
BIT
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ACKNOWLEDGEMENT
Dr.P.PRAMILA, Professor& HOD, Department of Electrical andElectronics Engineering, has supported me enthusiastically throughout the
seminar work. I am thankful to her for the same.
I am deeply indebted to my seminar guide Mr. N. A. Prashanth, AssociateProfessor, Department of Electrical and Electronics Engineering, for his
invaluable and constant guidance throughout the course of my seminar work. His
exhaustive knowledge has enabled me to find solutions to the problems I faced
and he facilitated me in achieving my goals easily.
I am also thankful to Mrs. Swarnalatha Srinivas, Associate Professor,
Department of Electrical and Electronics Engineering, Bangalore Institute of
Technology, Mrs. P. Pramila, HOD, Department of Electrical and Electronics
Engineering, Bangalore Institute of Technology and Mr. H.B. Nagesh, AssociateProfessor, Department of Electrical and Electronics Engineering, Bangalore
Institute of Technology, for their patient listening and assistance during the
deliverance of my seminar.
I humbly thank the entire faculty of the Department of Electrical and
Electronics Engineering for their full co-operation.
HAMID ARIZ
(1BI09EE063)
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MAGNETO-OPTICAL CURRENT TRANSDUCER (MOCT)
CONTENTS
TOPIC PAGE NO.
1. INTRODUCTION 2-3
2. POLARIZATION 4-5
a. LINEAR POLRIZATION
b. CIRCULAR POLARIZATION
c. ELLIPTICAL POLARIZATION
3. TRANSDUCER 6
4. PIN-PHOTODIODE 7
5. MAGNETO-OPTICAL CURRENT TRANSDUCER 8
6. MOCT-PRINCIPLE 8-13
7. MOCT OPERATION 14
8. DESIGN 15-16
9. SENSORS 17
10. MAGNETO-OPTICAL SENSOR 18
11. ELECRONIC CIRCUIT FOR THE MOCT 19-20
12. APPLICATION 21
13. ADVANTAGES OF MOCT 21
14. DISADVANTAGES OF MOCT 21
15. CONCLUSION 22
16. REFERENCES 23
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INTRODUCTION:
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 reliabilityof 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
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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 transducer
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.
The MOCT measures the rotation angle caused by the magnetic field and
converts it into a signal of few volts proportional to the electric currant. It consist
of a sensor head located near the current carrying conductor, an electronic signal
processing unit and fiber optical cables linking to these two parts. The sensor
head consist of only optical component such as fiber optical cables, lenses,
polarizers, glass prisms, mirrors etc. the signal is brought down by fiber optical
cables to the signal processing unit and there is no need to use the metallic wires
to transfer the signal. Therefore the insulation structure of an MOCT is simpler
than that of a conventional current transformer, and there is no risk of fire or
explosion by the MOCT. In addition to the insulation benefits, a MOCT is able to
provide high immunity to electromagnetic interferences, wider frequency
response, large dynamic range and low outputs which are compatible with the
inputs of analog to digital converters. They are ideal for the interference between
power systems and computer systems. And there is a growing interest in using
MOCTs to measure the electric currents.
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POLARIZATION:
Polarization is a property of waves that describes the orientation of their
oscillations.
There are basically three types of polarization:
Linear polarization.
Circular polarization.
Elliptical polarization.
LINEAR POLARIZATION:
A plane electromagnetic wave is said to be linearly polarized. In this the
transverse electric field wave is accompanied by a magnetic field wave as
illustrated below.
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CIRCULAR POLARIZATION:
If light is composed of two plane waves of equal amplitude but differing in
phase by 90, then the light is said to be circularly polarized. If you could see the
tip of the electric field vector, it would appear to be moving in a circle as it
approached you. If while looking at the source, the electric vector of the light
coming toward you appears to be rotating counter clockwise, the light is said to
be right-circularly polarized. If clockwise, then left-circularly polarized light. The
electric field vector makes one complete revolution as the light advances one
wavelength toward you.
ELLIPTICAL POLARIZATION:
Elliptically polarized light consists of two perpendicular waves of unequal
amplitude which differ in phase by 90. The illustration shows right- elliptically
polarized light. If the thumb of your right hand were pointing in the direction of
propagation of the light, the electric vector would be rotating in the direction of
your fingers.
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TRANSDUCERS:
A transducer can be defined as a device capable of converting energy from
one form into another. Transducers can be found both at the input as well as at the
output stage of a measuring system.
The input transducer is called the sensor, because it senses the desired
physical quantity and converts it into another energy form.
The output transducer is called the actuator, because it converts the energy
into a form to which another independent system can react. For a biological
system the actuator can be a numerical display or a loudspeaker to which the
visual or aural senses react respectively. For a technical system the actuator could
be a recorder or a laser.
The sensor or the sensing element is the first element in a measuring
system and takes information about the variable being measured and transforms it
into a more suitable form to be measured. The actuator senses these signal and
converts it into the form which can be interpreted by the human.That means the transducer consists of a primary element (sensor) plus a
secondary element (signal conditioning circuit)
Transducer = Sensor + Signal conditioning circuit
Electrical
signal
In MOCT, rotation angle of polarized light caused by the magnetic field is
converted into a signal of few volts propotional to the electrical current by the
help of PIN photodiode.
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PIN PHOTODIODE:
PIN-Photodiode Converts light signal to electrical signal. They are
basically reverse biased diodes.
Under no light- The reverse bias
draws current-carrying electrons and
holes out of the p-n junction region,
creating a depleted region, which
stops current from passing through
the diode.
Under light- Photons will create
electron hole pairs in depletion
region by raising an electron from
the valence band to the conduction
band, leaving a hole behind, so that
current flows proportional to the
light.
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MAGNETO-OPTICAL CURRENT
TRANSDUCER (MOCT):
The MOCT measures the electric current by means
of Faraday Effect, which was first observed by Michael
Faraday 150 years ago. The range for its measurement is in
between 20A to 2000A.
MOCT-PRINCIPLE:
The Magneto-Optical current transducer is based on the Faradays effect.
Michael Faraday discovered that the orientation of linearly polarized light was
rotated under the influence of the magnetic field when the light propagated in a
piece of glass, and the rotation angle was proportional to the intensity of the
magnetic field. The concept of Faraday Effect could be understood from the
Figure.
(Concept of Faraday Effect)
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Generally, this phenomenon can be described as follows:
= V .
dl Eq(1)
is the Faraday rotation angle,
V is the Verdet constant of magneto-optical material
B is the magnetic flux density along the optical path
l is the optical path.
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Glass type Verdet Constant
(radians/amp turns)
SF-59 2.08 x 10-5
SF-58 1.86 x 10-5
SF-57 1.61 x 10-5
SF-6 1.39 x 10-5
SF-5 0.91 x 10-5
SF-2 0.84 x 10-5
F-2 0.77 x 10-5
BK-7 0.27 x 10-5
QUARTZ 0.31 x 10-5
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Verdect constant for several optical glasses
(Wavelength = 820nm)
When the linearly polarized light encircles a current carrying conductor
eq(1) can be rewritten as according to Amperes law as
=nVI..Eq(2)
I is the current to be measured,
is the permeability of the material,
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n is the number of turns of the optical path.
The Faraday Effect outlined in eq (2) is a better format to apply to an
MOCT, because the rotation angle in this case is directly related to the enclosed
electric current. It rejects the magnetic field signals due to external currents which
are normally quite strong in power system.
The typical application of the Faraday Effect to an MOCT is clear from
figure. A polarizer is used to convert the randomly polarized incident light into
linearly polarized light. The orientation of the linearly polarized light rotates an
angle after the light has passed through the magneto-optical material because of
Faraday Effect. Then another polarization prism is used as an analyzer, which is
450
oriented with the polarizer, to convert the orientation variation of the
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polarized light into intensity variation of the light with two outputs, and then
these two outputs are send to photo detectors. The purpose of using the analyzer
is that photo detectors can only detect the intensity of light, rather than the
orientation of polarizations. The output optical signals (vectors) from the analyzer
can be described as,
Let,
A = Intensity vector
= Angle difference b/w polarizer and analyzer.
The resultant vector is given as:
From the law of Malus, which states that the transmitted optical intensity
varies with the square of the cosine or sine between the two planes of
polarization, the transmitted optical power is then:
Then including the optical modulation(M) due to sensor material
Where
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Pin = Optical power input.
Both can also be expressed as:
Now to determine the value of so that when modulation is equal to zero
then there will be an equal distribution of light b/w Pp and Ps. Substituting
M = 0 in above equations and equating them, we get = /4.
There are two ways to do this. The first is to actually rotate the analyzer
/4 around the optical axis. The second is to rotate both, one clockwise and the
other counterclockwise 22.50 off optical axis.
Inserting = /4 into above equations, we get-
Pin is the optical power from the light source,
is the Faraday rotation angle,
Pp and Ps are the optical power delivered by the detectors.
In order to properly apply Eq (2) in the MOCT design by making the
optical path wrap around the current carrying conductor, the optical path has to be
folded by reflections. Total internal reflections and metal reflections are good
ways to achieve this. However reflections introduce phase shift; hence change the
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polarization state of the light. The optical prism has to be designed to keep the
light going through the MOCT linearly polarized.
In order to stimulate the behavior of the polarized light reflect through the
glass prism of an MOCT, ie to maintain the light traveling through the glass prism
to be linearly polarized and also for the analysis of the effects of dielectric and
metal reflections on the linearly polarized light, a computer programme is written
in FORTARN language. Stimulation results include information such as
polarization state change at each reflection and the overall responsibility of the
optical sensor.
MOCT SYSTEM OPERATION:
Figure given shows the functional block diagram for the MOCT system.
The LED provides the light source which is transmitted through the optical fibre
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link to the polarizer. After the light travels around the primary conductor through
the Faraday sensor, the plane of the polarized light is rotated by the magnetic field
of the primary conductor. The light them exists through the analyzer which
converts the amount of rotational shift into a proportional amount of light
intensity. This intensity modulated light is conducted through a second optical
fibre to a PIN diode. The PIN photodiode demodulates the light and after being
amplified and filtered is a scaled voltage that represents the amount of current
flow in the primary conductor.
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MAGNETO-OPTICAL CURRENT TRANSDUCER (MOCT)
DESIGN:
Figure shows the
structure of this MOCT.
The optical sensor consists
of two separate clamp-on
parts. In each part of the
device, linearly polarized
light is arranged to pass
through the optical glass
prism to pickup the Faraday
rotation
signal. The polarization
compensation technique is applied at each corner of the prisms, so that the light
passing through the prism remains linearly polarized. At the other end of the
prism, a silver mirror reflects the light beam so that light beam comes back to itssending end via the same route while accumulating the Faraday rotations.
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(Power line and Optical Path of the Optical
Sensor)
The two halves can be assembled around the conductor. Thereby, the
rotation angles from the two halves of the sensor are added up in the signal
processing unit so that the total rotation angle (1+2) is the same as the rotation
angle from the optical path shown in above figure, which is two turns around
the conductor.
Structure of the Housing of the Clamp-on MOCT
Figure shows the structure of the housing for the clamp-on MOCT. The
optical glass prism polarizes, and lenses are completely sealed in the housing by
epoxy, so that they are free of environmental hazards such as dust and moisture.
This structure avoids the use of magnetic material to concentrate the magnetic
field as found in some other MOCT design and Hall Effect current measurement
devices. There for it is free from the effect of remanent flux, which could affect
the accuracy of the current measurement.
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SENSORS:
The chosen technique to measure the magnetic field and thus the current is
to use a magnetic field sensor exploiting the Faraday effect. These sensors use a
folded design with two multimode fibres (Cladding=140m, Core=100m) fixed in
a plastic jacket. The polarizers are fixed at the ends of the fibres. A gradient index
lens collimates the polarized light to a reflective gold layer on the backside of the
Faraday film. The reflected light beam again transverses the Faraday film and the
second polarizer and is focused into the second multimode
Fibre.
The most important properties of the sensor are its
Sensitivity:- The sensitivity of a sensor is the ratio of output signal or
response of the instrument to a change of input or measured variable. Here, theinput variable is the magnetic field strength or magnetic flux density and the
output signal is the corresponding change of the output voltage.
Temperature dependency:- In order to determine the influence of thetemperature on the sensor signal, the output signal for both sensors was measured
at different ambient temperatures. Therefore the sensor was placed in a
temperature-controlled chamber. The output voltage of both sensors was
measured for a temperature range from T=10C to T=60C at zero field. For the
actual application a wider temperature range has to be covered.
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MAGNETO-OPTICAL SENSOR:
Almost all transparent material exhibits the magneto-optical effect orFaraday Effect, but the effect of some of the material is very temperature
dependent, and they are not suitable for the sensing material. The optical glasses
are good candidate for the sensing material, because the Verdet constants are not
sensitive to the temperature changes, and they have good transparency properties.
They are cheep and it is easy to get large pieces of them. Among the optical
glasses SF-57 is the best choice, as it has larger Verdet constant than most of the
other optical glasses. And MOCT made out of these materials can achieve higher
sensitivity. In the MOCT, from Eq (2), the total internal rotation angle is,
1+ 2 2VI
Where I is the current to be measured,
= 4 x 10-7 H/m
V=7.7 x 102 degrees/Tm at a wavelength of 820nm
Therefore = 1.9 degrees/ KA.
Different optical fibers are designed for different usage. The single mode
fiber has very wide bandwidth, which is essential for communication systems, but
it is difficult to launch optical power into the single mode fiber because of its
very thin size. While large multimode fiber is convenient for collecting maximumamount of light from the light source, it suffers from the problem of dispersion
which limits its bandwidth. In the situation of power system instrumentation, only
moderate frequency response is required and in MOCT, the more optical power
received by the detectors the better signal to noise ratio can be achieved.
Therefore, the large core multi-mode optical fiber is used here to transfer the
optical signals to and from the optical sensors.
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ELECTRONIC CIRCUIT FOR THE MOCT:
(Electronic Circuit for the MOCT)
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Above figure shows the schematic diagram of the electronic circuit for the
clamp-on MOCT. In order to make use of the dynamic range of the digital system
as well as the different frequency response requirements of metering and relaying,
metering signal (small signal) and relaying signal (large signal) are treated
differently. Two output stages have been designed accordingly. One stage, which
has 1 KA dynamic range, is for power system current metering, and other stage,
which operate up to 20 KA, provides power system current signals for digital
relay systems.
In each part of the device, the sum of the two receiving channels signals,
which have the same DC bias I0, differenced at junction with a reference voltage
Vref from the power level adjustment potentiometer. Then an integrator is used to
adjust the LED driver current to maintain 2I0 to be the same as the Vref at the
junction. Because the reference voltage Vref is the same for both the sides, the DC
bias I0 and the sensitivities 2I0 of the two halves of the clamp-on MOCT are
considered to be stable and identical.
The difference of the two receiving channels signals 2I0 (2Sin1) and 2I0
(2Sin2) in each part of the device are added directly and then fed through anamplifier for the small signals. At the same time these two signals are processed
digitally to do a sin-1 calculation on each and then summed together for the large
signal situation when the non-linearity of the MOCT can no longer be ignored.
The ratio responses of the two output stages of the clamp-on MOCT are designed
as 10V/KA and 0.5V/KA and frequency responses are 4KHZ and 40 KHZ
respectively.
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APPLICATION:
The MOCT is designed to operate in a transparent manner with modern
electronic meters and digital relays, which have been adopted for a low energy
analog signal interface. Typically, the design approach is to redefine the interface
point as to input the analog to digital conversion function used by each of these
measurement systems.
ADVANTAGES OF MOCT:
1. No risk of fires and explosions.
2. No need to use metallic wires to transfer the signal and so simpler
insulation structure than conventional current transformer.
3. High immunity to electromagnetic interference.
4. Wide frequency response and larger dynamic range.
5. Low voltage outputs which are compatible with the inputs of digital to
analog converters.
DISADVANTAGES OF MOCT:
1. Temperature and stress induced linear birefringence in the sensing material
causes error and instability.
2. Accuracy of MOCT is less than conventional transformer.
3. The accuracy of MOCT is so far insufficient for the use in power systems.
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CONCLUSION
This paper presents a new kind of current transducer known as magneto
optical current transducer. This magneto optical current transducer eliminates
many of the drawbacks of the conventional current transformers. In an
conventional current transformers, there is a chance of saturation of magnetic
field under high current, complicated insulation and cooling structure, a chance of
electro magnetic interference etc.
By applying Faradays principle this transducer provides an easier and
more accurate way of current measurement. This MOCT is widely used in power
systems and substations nowadays. And a new trend is being introduced, which
known as OCT based on adaptive theory, which make use of accuracy in the
steady state of the conventional current transformer and the MOCT with no
saturation under fault current transients.
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REFERENCES
Farnoosh Rahmatian ;patric p. chavez & Nicholas A.F
Optical voltage transducers using multiple electric field sensors .
IEEE transactions on power delivery ,vol.17 april 2002
J C Santos, M.C Taplama Ciogle and K Hidak
Pockels High Voltage Measurement Systems
IEEE transactions on power delivery ,vol.15 jan 2000
http://www.iop.org/EJ/article
http://www.cris-inst.com/publication/bejing
Advanced Engineering Physics by Premlet
Published by- Phasor Books, Kerala.
Physics for engineers by M.R. Srinivasan
Published by- New Age International Publication, New Delhi.