Optical technology in current measurement
Vinayan.S
Roll no:22
EEE7
Sree Buddha College Of Engineering, Pattoor

Contents
Introduction
Saturation in conventional current transformers
Saturated & Non-saturated CT outputs
Problem in Coventional CT
Measuring currents using optical technology
Primary current comparison
Working of current sensor
Advantages
Disadvantages
Conclusion
Reference

Introduction
When fault on power system occur, they must be isolated quickly maintain stability of a system.
If protection relays receives the “true” representation of current flowing on a transmission line ,or into transformer ,capacitor banks, or reactor banks.etc.
An undistorted view could improve the ability of the relay to trip when it should and to prevent false trips.
Optical sensors behave in a simple and predictable manner known for every situation.
Saturation in conventional current transformer
During fault conditions a well-known phenomenon occurs: the iron core in a trans former ‘’saturates” due to a large magnetic field caused by high fault current.
This saturation of the iron core prevents the transformer from accurately representing the primary current in the current transformer secondary.
So there fore distorts current measurement.
Saturation depends on:
physical design of CT
amount of steel in the core
the connected burden
winding resistance and
system X/R ratio
Saturated and non-saturated CT secondary output
Problem in Conventional CT
The problem of CT is saturation. These can be avoided altogether by using optical current sensor

Measuring current using optical technology
Optical current sensors contain no magnetic components so do not have affected saturation effects.

The optical sensor is used to measure the electro- magnetic field surrounding a current carrying conductor.

The electronic circuits are used to connect optics and calculate the corresponding current .

If we use this method to measure the current upto 400 KA peak
Additionally, using advanced techniques, both ac and dc currents can be measured this level.


The optical sensor determined magnetic field is a transfer function of sine wave characteristics of CTs.

The measurement of magnetic field is linearly with the sine wave.ie, the linear portion of the sine wave.

So we can measure the phase difference between that magnetic wave and orginal sine wave by using several electronic circuits
Primary current comparison
Current sensor technology
A fiber optic current sensor (as shown figure) consist of a light source ,photo detector and electronics
Working of current sensor
Light passes through phase modulator reaches the sensor head.cal waves travel

The quarter wave plate creates circularly polarised light.

FARADAY EFFECT: the magnetic field induced by the current in conductor creates optical phase shift between them.

Optical waves travel back to the detector where the circuitry determines the phase shift.

The phase shift is proportional to the current and the electronics provide corresponding analog or digital signal.
Advantages
No losses can be determined by using the techniques.
size can be reduced by these techniques.
It is highly efficient than CT.
High electrical insulation.
Large bandwidth.
Potentially high sensitivity.
Low cost
No danger of explosion


Disadvantages
The electronic circuits present may cause distortions.


The measurement is not much accurate.
Conclusion
Optical current sensors provide a reliable method of measuring very high fault currents with significant DC offset without any type of saturation.


Advanced processing techniques can be implimented in sensors and high fault current measurement can be achieved.
References
[1] Interfacing Optical Current Sensors in a Substation,J.D.P. Hrabliuk, IEEE PES Summer Power Meeting,Vancouver, B.C., July 17, 2001.
[2] NxtPhase Optical Current System , J. Blake, 2ndEPRI Optical Sensor Systems Workshop January 26–28,2000, Atlanta, Georgia.


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