1.0 INTRODUCTION An accurate electric electric current current transducer transducer is a key component component of any power syst system em inst instru rume ment ntat atio ion. n.
To
meas measur uree
curr curren ents ts powe powerr
stat statio ions ns and and
subs substa tati tion onss
conventionall conventionally y employ inductive inductive type current transformers transformers with core and windings. 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 reliability of conventional high-voltage current transformers transformers have been questioned because of their violent destructive failures which caused fires and impact damage to adacent apparatus in the switchyards, electric damage to relays, and power service disruptions. !ith short circuit capabilities of power systems getting larger, and the voltage voltage levels going higher the conventional conventional current current transformer transformerss 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 system transi transient ent conditi conditions ons.. "n additi addition on to the concern concerns, s, with with the compute computerr control control techni technique quess and digita digitall protec protectio tion n devices devices being being introd introduced uced into into power power system systems, s, the conventional current transformers have caused further difficulties, as they are likely to introduce electro-magnetic interference through the ground loop into the digital systems. This has required the use of an au#iliary current transformer or optical isolator to avoid such problems. "t appears that the newly emerged $agneto-optical current transformer technology provides a solution for many of the above mentioned problems. The $%&T measures the electric current by means of Faraday 'ffect, which was first observed by $icha $ichael el Fara Faraday day ()* years years ago. ago. The The Fara Faraday day 'ffe 'ffect ct is the the pheno phenome meno non n that that the 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.
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The $%&T measures the rotation angle caused by the magnetic field and converts it into a signal of few volts proportional to the electric currant. "t consist of a sensor head located near the current carrying conductor, conductor, an electronic 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 $%&T is simpler than that of a conventional current transformer, and there is no risk of fire or e#plosion by the $%&T. "n addition to the insulation benefits, a $%&T $%&T is able able to provi provide de high high immu immuni nity ty to elect electro roma magne gneti ticc inte interf rfer eren ence ces, s, wider 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 $%&Ts to measure the electric currents.
1.1 Faraday Effect
Faraday found that when a piece of special glass is affected by a strong magnetic field, it becomes active. And optic surface spin when a flat polarized optic forw forwar ards ds thro through ugh a glas glasss in para parall llel elis ism m with with magn magnet etic ic line lines. s. +ince +ince the the Farad Faraday ays s discovery this phenomenon was seen in many solids, liquids and gases. The amount of whirl in each material is proportional to the amount of magnetic field and the distance that an optic go in a material impractically
2.0 LITERATURE REVIEW
2.1 Physica i!ter"retati#! #f the Faraday effect The linear polarized light that is seen to rotate in the Faraday effect can be seen as consisting of the superposition of a right- and a left- circularly polarized beam, this superposition principle is fundamental in many branches of physics. !e can look at the effects of each component right- or left polarized separately, and see what effect this has on the result. "n circularly polarized light the direction of the electric field rotates at the frequency of the light, either clockwise or counterclockwise. "n a material, this electric field causes a force on the charged particles comprising the material because of their low mass, the electrons are most heavily affected. The motion thus effected will be circular, and circularly moving charges will create their own magnetic field in addition to the e#ternal magnetic field. There will thus be two different cases the created field will be parallel to the e#ternal field for one /circular0 polarization, and in the opposing direction for the other polarization direction 1 thus the net 2 field is enhanced in one direction and diminished in the opposite direction. This changes the dynamics of the interaction for each beam and one of the beams will be slowed down more than the other, causing a phase difference between the left- and right-polarized beam. !hen the two beams are added after this phase shift, the result is again a linearly polarized beam, but with a rotation in the polarization direction. The direction of polarization rotation depends on the properties of the material through which the light is shone. A full treatment would have to take into account the effect of the e#ternal and radiation-induced fields on the wave function of the electrons, and then calculate the effect of this change on the refractive inde# of the material for each polarization, to see whether the right- or left circular polarization is slowed down more.
3
Fi$%re2.1& ' P#ari(ati#! d%e t# Faraday effect
2.2 )OCT'Pri!ci"e The $agneto-%ptical current transformer is based on the Faradays effect. $ichael Faraday discovered that the orientation of linearly polarized light was rotated under the influence of the magnetic field when the light propagated in a piece of glass, and the rotation angle was proportional to the intensity of the magnetic field. The concept of Faraday 'ffect could be understood from the Fig.(.
Fi$%re 2.2& ' R#tati#! #f a Pa!e "#ari(ed i$ht *y a !ic#e "ris+
4
5enerally, this phenomenon can be described as follows
θ 6 7
dl
8888'q/(0
9θ: is the Faraday rotation angle,
97: is the 7erdet constant of magneto-optical material 92: is the magnetic flu# density along the optical path 9l: is the optical path
!hen the linearly polarized light encircles a current carrying conductor eq/(0 can be rewritten as according to Ampere:s law as
θ 6nµ7"
888888888888888888.. 'q-
9" 9is the current to be measured, 9µ: is the permeability of the material, 9n: is the number of turns of the optical path.
The Faraday effect outlined in eq is a better format to apply to an $%&T, because the rotation angle in this case is directly related to the enclosed electric current. "t reects the magnetic field signals due to e#ternal currents which are normally quite strong in power system.
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Fi$%re 2.,& ' Arra!$e+e!t #f )a$!et#'#"tic c%rre!t tra!sd%cer
The typical application of the Faraday effect to an $%&T is clear from fig/0. 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 θ after the light has passed through the magneto-optical material because of Faraday 'ffect. Then another polarization prism is used as an analyzer, which is 4) polarizer, to convert the orientation variation of the polarized
*
oriented with the 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 from the analyzer can be described as,
;( 6
/( < +in θ 0
=
; 6
/( - +in θ 0
;* is the optical power from the light source, θ is the Faraday rotation angle, ;( and ; are the optical power delivered by the detectors.
"n order to properly apply 'q/0 in the $%&T 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. >owever reflections introduce phase shift? hence change the polarization state of the light. The optical prism has to be designed to keep the light going through the $%&T linearly polarized. "n order to stimulate the behavior of the polarized light reflect through the glass prism of an $%&T, ie to maintain the light traveling through the glass prism to be linearly polarized and also for the analysis of the effects of dielectric and metal reflections on the linearly polarized light, a computer programme is written in F%@TA@ language. +timulation results include information such as polarization state change at each reflection and the overall responsibility of the optical sensor.
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2., Desi$!
Fi$%re 2.-& ' tr%ct%re #f the )OCT
Figure.4 shows the structure of this $%&T. The optical sensor consists of two separate clamp-on parts. "n each part of the device, linearly polarized light is arranged to pass through the optical glass prism to pickup the Faraday rotation signal. The polarization compensation technique is applied at each corner of the prisms, so that the light passing through the prism remains linearly polarized. At the other end of the prism, a silver mirror reflects the light beam so that light beam comes back to its sending end via the same route while accumulating the Faraday rotations. C
Fi$%re2./& ' T# "aths f#r Li$ht t# diert a!d ret%r!
The two halves can be assembled around the conductor. Thereby, the rotation angles from the two halves of the sensor DFig.4/a0E are added up in the signal processing unit so that the total rotation angle /θ(<θ 0 is the same as the rotation angle θ from the optical path shown in Fig4/b0, which is two turns around the conductor.
Fi$%re 2.& ' 3#%si!$ #f the ca+"'#! )OCT
Figure .) shows the structure of the housing for the clamp-on $%&T. The optical glass prism polarizes, and lenses are completely sealed in the housing by epo#y, 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 $%&T design and >all 'ffect current measurement devices. There for it is free from the effect of remanent flu#, which could affect the accuracy of the current measurement.
2.- )a$!et#'O"tica e!s#r Almost all transparent material e#hibits the magneto-optical effect or Faraday 'ffect, 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 7erdet 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 +F-)B
is the best choice, as it has
larger 7erdet constant than most of the other optical glasses. And $%&T made out of these materials can achieve higher sensitivity. "n the $%&T, from 'q /0, the total internal rotation angle is,
θ
≈ θ(< θ ≈ µ7" !here " is the current to be measured, µ 6 4π # (*-B >Gm 76B.B # (* degreesGTm at a wavelength of C*nm Therefore θ 6 (. degreesG HA.
Iifferent 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 it:s very thin size. !hile large multimode fiber is convenient for collecting ma#imum amount of light from the light source, it suffers from the problem of dispersion which limits its bandwidth. "n
(*
the situation of power system instrumentation, only moderate frequency response is required and in $%&T, the more optical power received by the detectors the better signal to noise ratio can be achieved. Therefore, the large core multi-mode optical fiber is used here to transfer the optical signals to and from the optical sensors.
2./ Eectr#!ic Circ%it f#r the )OCT
Fi$%re 2.4& ' Eectr#!ic dia$ra+ #f the eectr#!ic circ%it f#r )OCT
Fig. .= shows the schematic diagram of the electronic circuit for the clamp-on $%&T. "n 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. %ne stage, which has ( HA dynamic range, is for power system current metering, and other stage, which operate up to * HA, provides ((
power system current signals for digital relay systems. "n each part of the device, the sum of the two receiving channels signals, which have the same I& bias α"*, differenced at unction with a reference voltage 7ref from the power level adustment potentiometer. Then an integrator is used to adust the J'I driver current to maintain α"* to be the same as the 7ref at the unction. 2ecause the reference voltage 7ref is the same for both the sides, the I& bias α"* and the sensitivities α"* of the two halves of the clamp-on $%&T are considered to be stable and identical. The difference of the two receiving channels signals α"* or +inθ( and α"* +inθ in each part of the device are added directly and then fed through an amplifier for the small signals. At the same time these two signals a re processed digitally to do a sin-( calculation on each and then summed together for the large signal situation when the non-linearity of the $%&T can no longer be ignored. The ratio responses of the two output stages of the clamp-on $%&T are designed as (*7GHA and *.)7GHA and frequency responses are 4H>K and 4* H>K respectively.
2. )aterias a!d )eth#ds 2.1 O"tica C%rre!t Tra!sf#r+er %ptical-&T was proposed by the creators of several methods by using
common theory (. &onventional &T with optical readout D(, )E "n this type, one channel of optical and completely insulated information connects to the output of &T so instead of the typical copper wire is used the optic fiber in the output data transmission. The methodology of converting the &T output to optical signal form is out of the discussion of this article, but we can say that do not open the output heads of &T is the benefit of this &T that it is the most important factor in the e#plosion of the &T. A magnetic circuit arises around the conductor by ferrous core. The difference with the traditional &T is it that an air gap is generated in the core and magnetic field in the core measure in this air gap with optical instruments. The advantage of this design is that the path of light is short and simple, and smaller optical elements are required
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Fi$%re 2.5& ' che+atic #f Faraday c%rre!t se!s#r %si!$ +a$!etic c#!ce!trat#r
2.2 O"tica "ath ar#%!d the c#!d%ct#r
"f the path is put around the conductor carrying of current that through it the magnetic field effects on the light ray. This optical package path around the conductor measures current similar to normal &T core. "n our Iivision, this is the first plan that is not including the Ferromagnetic component. This type includes two alternatives A "iece #f i$ht'se!sitie
"n this alternative of light path, actually, a piece of optical active materials that one round surrounds around the conductor according to Figure.
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Fi$%re 2.6& ' che+atic #f faraday *%7 se!s#r %si!$ Fi*re O"tics 2., Fi*er O"tics
>ere the light path around the conductor consists of an optical fiber that it is wrapped to the number of rounds that it required to achieve the desired sensitivity.
Fi$%re 2.10&' Fi*re #"tics *ased c%rre!t +eas%re+e!t
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2.- Wit!ess e!s#r
This converter is the latest type in our assortment and it is only type that measurement of it does not include the surrounding of conductor completely. "nstead, as shown in Figure =, the magnetic field at a point closer to the conductor affects on the light distribution. And therefore, it is not a real current transformer. Although it can be said a field constant distribution around the conductor is a function of its current. "t can be said that light with arbitrary polarization is composed of two independent components. "n the case of linearly polarized components can be simply said that two components are perpendicular to each other
. Fi$%re 2.11&' che+atic #f faraday effect se!s#r %!i!7ed ty"e $e#+etry
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,.0 APPLICATION The $%&T 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.
The $agneto-%ptic &urrent Transformer /$%&T0 is a passive optical current transducer which uses light to accurately measure current on high voltage systems. The $%&T system is suitable for outdoor application and has an accurate metering current range from less than ) A to 3()* A using the same sensor. >igher current rated designs are available upon request. $a#imum rated current is 3()* A. $etering accuracy e#ceeds class *. per "'& (C) standards. The system is provided with one electronic module that is located in the control house. The $%&T system provides a (.* amp current output for metering. &ontact the factory for application assistance. L Three phases of $%&T sensors mounted on polymer insulator columns with pre-terminated fiber optic cable in the insulator. L 'lectronic $%&T signal processing module (**-" suitable for installation in the substation control house. L Fiber optic cables for transmission of the light signals between the optical sensors and the $%&T electronic module.
(=
,.1 Ada!ta$es #f )OCT (. o risk of fires and e#plosions. . o need to use metallic wires to transfer the signal and so simpler insulation structure than conventional current transformer. 3. >igh immunity to electromagnetic interference. 4. !ide frequency response and larger dynamic range. ). Jow voltage outputs which are compatible with the inputs of digital to analog converters. =. The subect of creating noise is ruled out due to the use of optical fiber. B. @esistance to Acoustic and 'lectromagnetic parasites is e#cellent that it plays an important role in protecting. C. .The internal problems of $%"Ts and I%"Ts cause connecting with lines and substations so it causes the traditional equipment to e#plode very dangerous. "n addition, it can damage surrounding equipment. . . "n terms of size and weight, they have small size and low weight. The size of equipment can be a great help in substations that have land problems and they cause the foundations and structures to remove and they have high costs. The probable installation and relocation does not require heavy machinery that it is huge economy. (*. Iuring installation, do not require cutting the insulated conductors. "t causes re-insulating to prevent. ((. >aving sufficient electrical insulation resistance (. Jack of magnetic saturated. Iue to Jack of core, there is no saturation that it solves many protection and $easurement problems.
,.2 Disada!ta$es #f )OCT (. Temperature and stress induced linear birefringence in the sensing material causes error and instability. (B
. The accuracy of $%&T is so far insufficient for the use in power systems. 3. "f the magnetic field induced by the currents through the other conductor is sufficiently high, the fault current measured will have some errors. +ince many conductors in the substation adopted air insulation, it is possible for the Faraday sensor to detect the fields resulting from the fault current by other conductors. 4. The effect of temperature on glass sensitive sensor causes duality of the refractive inde# and it causes polarized light to distort and Jight with linear polarization has become into elliptical and it causes unwanted disturbances to create, refractive inde# becomes various amounts at different temperatures. %f course, with modern methods, this problem has been solved. %ne of these methods is the use of diamagnetic glass. "t is independent of the effects of temperature and it can be used without danger from )* to ((*M & ). Faraday 'ffect is related to the wavelength of light in the system. To remain constant wavelength of led, a temperature controller is used. Thermal e#pansion and vibration effect on the magnetic field adversely and this problem is solved by loop system. =. 2ecause output quantities of $%"Ts are negligible, despite the simplicity of the building they need to have very high accuracy when they design and build.
-.0 CONCLUION @apid advances in the quality of performance and costs of the optical fiber and electronic equipment to encourage development of measuring trances based on new technology. 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. "n an conventional current transformers, (C
there is a chance of saturation of magnetic field under high current, complicated insulation and cooling structure, a chance of electro-magnetic interference etc. 2y applying Faraday:s principle this transducer provides an easier and more accurate way of current measurement. This $%&T is widely used in power systems and substations nowadays. And a new trend is being introduced, which known as %&; based on adaptive theory, which make use of accuracy in the steady state of the conventional current transformer and the $%&T with no saturation under fault current transients.
/.0 Refere!ces (. . 3. 4. ).
N. !orld 'lectric Tech. 4/(0, www.science-line.com. www.nature.comGarticles.html www.en.wikipedia.orgGwikiGmagneto-opticeffect.html www.ece.mtu.edu A ew Faraday @otation $easurement $ethod for the +tudy on $agneto
%ptical ;roperty of ;b%-2i%3-2%3 5lasses for &urrent +ensor Applications, www.+ci@;.org. =. www.weet.science-line.com. B. $agneto optics by +. H. +uame and koima. (
C. '. $unin, A. 2. 7illaverde, et al., O$agneto-%ptical @otation of >eavy-$etal %#ide 5lasses,P Nournal of on-&rystalline +olids, 7ol. 3(, o. (-, (C, pp. (34-(4.
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