Optical current transformer and its applications

1. Optical current transformer and
its applications

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’s are :
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 :
1. Linear polarization
2. Circular polarization
3. Elliptical polarization

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DIFFEREN
T
POLARIZA
TION
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
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 discoverd
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
propotional 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.
optical path.
By considering this property , Faraday’s mathematical
equation is given by :
=n VI
I = current to be measured,
V=verdet constant,
= permeability of the material,
n = number of turns of the

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CONVENTI
ONAL
CURRENT
TRANSFO
RMER
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.

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OPTICAL
CURRENT
TRANSFO
RMER
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.

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

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

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

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.
Eg.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.

23. QUESTIONS ???