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MAGNETO OPTIC

CURRENT

TRANSFORMERS

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INTRODUCTION

An accurate electric current transducer is a keycomponent of any power system instrumentation.

To measure currents power stations and

substations conventionally employ inductive typecurrent transformers with core and windings.

Magneto-optical current transformer technology

provides a solution.

The MOCT measures the electric current by means

of Faraday Effect.

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CONVENTIONALTRANSFORMERS

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OPTICAL CURRENT TRANSFORMERS (OCTS)

A logical step to overcome the problems caused by

electromagnetic interference on the sensor signal is

to use a signal transmission that is immune to

electromagnetic fields.

The advantage of direct magneto-optic transducers

using opto-magnetic active materials is the absence

of additional disturbances.

Disturbances by mechanical or electrical sensor

parts are hysteresis, saturation, induction,

temperature influence and damping.

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Optical Current Transducers can be divided into

five main groups:

OCTs based on the Faraday effect

Interferometric principles

OCTs based on Bragg gratings

Micromechanical sensors with optical readout Other optical current sensing principles

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PRINCIPLE OF MOCT

The Magneto-Optical current transformer is based on the

Faradays effect.

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Generally, this phenomenon can be described as follows:

= V B.dl

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.

The above equation can be rewritten according to

Amperes law as:

=nVI

I is the current to be measured, is the permeability of the material,

n is the number of turns of the optical path.

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The typical application of the Faraday Effect to a

n MOCT is clear from the fig

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The output optical signals from the analyzer can be

described as,

P1 = (P0 / 2) (1 + Sin 2 )

P2 =(P0 / 2) (1 - Sin 2 )

P0 is the optical power from the light source,

is the Faraday rotation angle,

P1 and P2 are the optical power delivered by the detectors.

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DESIGN OF MOCT

The following fig shows the structure of MOCT.

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The two halves can be assembled around the conductor.

The rotation angles from the two halves of the sensor

Fig(a) are added up in the signal processing unit so that thetotal rotation angle (1+2 ) is the same as the rotationangle from the optical path shown in Fig(b), which is twoturns around the conductor.

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Fig shows the structure of the housing for the clamp-on

MOCT.

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MOCT OPTICAL CURRENT TRANSFORMER

SYSTEMFOR METERING

72.5-800 KV SYSTEMS, 50/60 HZ (ABB)

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MAGNETO OPTICAL SENSOR

Almost all transparent material exhibits the magneto-

optical effect or Faraday Effect.

In the MOCT, 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.

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Faraday sensor head

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ELECTRONIC CIRCUIT FOR MOCT

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ADVANTAGES AND DISADVANTAGES OF

MOCT

No fires and explosions.

No need to use metallic

wires to transfer thesignal.

Immune toelectromagneticinterference.

Wider frequencyresponse and largerdynamic range.

Low voltage outputs.

Temperature and stress

induced linear

birefringence causeserror and instability.

Insufficient accuracy.

ADVANTAGES DISADVANTAGES

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APPLICATIONS

The MOCT is designed to operate in a transparent

manner with modern electronic meters and digital relays.

The MOCT system satisfies current sensoring needs for

revenue metering or protective relaying.

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CONCLUSION

Magneto optical current transducer eliminates many of

the drawbacks of the conventional current transformers.

By applying Faradays principle this transducer providesan easier and more accurate way of current measurement.

This MOCT is widely used in power systems and

substations nowadays.

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THANK YOU