Optical current transformer and its applications

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An accurate electric current transducer is a key component of any power system instrumentation. Optical current transformer(OCT) is defined as sensors that directly or indirectly use optical sensing methods to measure electric currents. OCT’s are based on Faraday effect, interferometric principle, Bragg Grating and Micromechanical sensors with optical readout.

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 - 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.
Different Polarization Types

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

  • Linear birefringerance
  • Circular birefringerance

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. By considering this property, Faraday’s  mathematical equation is given by 

Faraday Effect
I = current to be measured,
V = Verdet constant,
P = permeability of the material,
n = number of turns of the optical path

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. 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. MOCT 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.

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