1bi09ee063 hamid ariz report

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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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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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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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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.

1bi09ee063 hamid ariz report

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