Optical Current Transformer is an alternative to the existing conventional current transformers, providing an advanced measurement solution for both metering and protection applications, based on a cutting-edge patented optical sensing technology.

1. (OCT)
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2. OVERVIEW
 INTRODUCTION
 WORKING PRINCIPLE
 CONVENTIONAL CURRENT TRANSFORMER
 OPTICAL CURRENT TRANSFORMER
 DESIGN OF OPTICAL CURRENT TRANSFORMER
 APPLICATIONS
 ADVANTAGES
 DISADVANTAGES
 CONCLUSION

3. INTRODUCTION
 An accurate electric current transducer is a key component of any
power system instrumentation.
 OCT’s defined as sensors that directly or indirectly use optical sensing
methods to measure electric currents .

4. Different types of OCT
Based on Faraday effect
Based on interferometric principle
Based on Bragg Grating
Micromechanical sensors with optical readout

5. WORKING PRINCIPLE
Magnetic OCT measures the electric current by means of faraday effect.
Faraday effects comprises of two principles :
Polarization of light
Polarization is a property of certain types of waves that describes the
orientation of their oscillations.
Polarization of light are mainly of three types :
 Linear polarization
 Circular polarization
 Elliptical polarization

6. Different Polarization Types
Linear polarization : when x and y
components of the electric field vector
E are in phase.
Circular polarization : when x and y
components of the field vector E have
same amplitude and are 900 phase
out.
Elliptical polarization: when phase
and amplitude of the x and y
components are different. (a) Linear Polarization
(b) Circular polarization
(c) Elliptical polarization

7. Birefringerence (or Double Refraction)
It is the phenomenon of splitting up of a light ray into o-ray and e-ray when
passed through an anisotropic material.
Birefringerence are of two types :
1. Linear birefringerance
2. Circular birefringerance

8. FARADAY EFFECT :
“Michael Faraday discovered that the
orientation of a 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”
Intensity, I=I0 cos2 ( - 45)

10. The Faraday effect outlined in equation is a better format to apply to an MOCT,
because the rotation angle in this case is directly related to the enclosed electric
current.
By considering this property , Faraday’s mathematical equation is given by :
μ0 = the free-space permeability
I = current to be measured,
V=verdet constant,
P = permeability of the material,
N = number of turns of the optical path.

11. Conventional Current Transformer
The most used device for the measurement
of alternating current.
Construction consists of a ferromagnetic
core entirely enclosing the conductor.
The secondary winding has to be isolated
from high voltages to prevent short circuits.
Therefore the transformers are filled with
oils. It also helps in dissipating the heat.
The 10 current is translated to the 20
current by the turns ratio (n).
The 20 is measured with an ampere meter
in most of the cases.
The basic principle is the Amperes law.

12. Optical Current Transformer
MOCT ‘s are another type of transducers
,which are highly immune to EMI
MOCT uses optical sensing methods to
measure electric currents.
In the sensors several properties of the
signal carrier(light) can be influenced
MOCT’s are mainly based on Faraday
Effect.
The Optical Current transformer basically
consists of: light source, fibre optic cable,
polarizer, modulator and electronics part.

13. The MOCT consists mainly of electronic and optic part.
Light from a source enters an optical fibre polariser, which splits into two linear orthogonally
polarized light.
Light enters into the sensing head that includes a quarter wave plate .
Quarter wave plate splits the two linear orthogonal waves into right and left hand circularly
polarized waves.

14. Two waves travel at different speeds around the sensing fibre.
The difference in speed is proportional to the strength of the magnetic field.
Two waves reflects of a mirror and circular polarization of two waves gets reversed .
They travelled in the opposite direction of the magnetic field.
Finally ,two waves again reach the quarter wave plate and linear polarisation gets regained.

15. Design of Optical Current Transformer
Design is divided into two parts: the electronics part and the optical
Design of the electronics part again divided into analog model and digital model
The analog model of the electronics part is modelled with complex curve fitting method
This method is basically to obtain the transfer function model

16. The transfer function obtained is iterated by the Gauss-Newton method to increase the
accuracy
The above analog model is converted to the digital model by bilinear transformation
The bilinear transformation changes stable continuous time system to stable discrete time
system
Thus the digital model H(z) is obtained from the analog model H(s)

17. The optics model is developed using Jones Calculus method
E0 and Ei represents the output and input energy.
In Jones calculus method polarised light is represented by a corresponding Jones Vector
And the optical element is represented by a corresponding Jones Matrix

18. APPLICATIONS
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.
Makes the output signal insensitive to shock & vibration.
Most suited for outdoor type applications. E.g. substations
Accurate measurement of fault currents.

19. ADVANTAGES
No risks of fires & explosions.
High immunity to Electromagnetic interferences.
Wide frequency response.
Larger Dynamic Range.
Low voltage outputs-compatible with the inputs of digital to analog converters.
No need to use metallic wires to transfer the signal and so simpler insulation structure than
conventional current transformer.

20. OCT analog output may have significant white noise , but the white noise does not affect the
accuracy or protection performance.
Temperature up to 500c do not adversely affect the performance of the OCT.
No requirement for oil or gas insulation system , environmentally safe.
No magnetic core ferroresonance or saturation limits.
Total isolation from surges for microprocessor based meters and relays.

21. CONCLUSION
A new type of current transducer is introduced known as magneto optical
current transducer.
This magneto optical current transducer eliminates many of the drawbacks of
the conventional current transformers.
By applying Faraday’s principle this transducer provides an easier and more
accurate way of current measurements. This MOCT is widely used in power
systems and substations nowadays.

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