Optical current transformer and its applications OVERVIEW q

Optical current transformer and its applications

OVERVIEW q INTRODUCTION q WORKING PRINCIPLE q CONVENTIONAL CURRENT TRANSFORMER q OPTICAL CURRENT TRANSFORMER q DESIGN OF OPTICAL CURRENT TRANSFORMER q APPLICATIONS q ADVANTAGES q DISADVANTAGES q CONCLUSION

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

Different types of OCT’s are : Ø Based on Faraday effect Based on interferometric principle Based on Bragg Grating 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 2. Circular polarization 3. Elliptical polarization 1.

DIFFERENT POLARIZAT Linear polarization : ION TYPES 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 Click to edit the outline text format Second Outline Level Third Outline Level Fourth Outline Level Fifth Outline Level Sixth Outline Level Seventh Outline Level Eighth Outline Level a) Linear Polarization � Ninth Outline Level. Click b) Circular polarization Master text styles c) Elliptical polarization � Second level to edit

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. 2. Linear birefringerance Circular birefringerance

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=I 0 cos 2 (�- 45)

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 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.

CONVENTI ONAL The most used device for the measurement of alternating current. CURRENT Construction consists TRANSFOR of a ferromagnetic core entirely enclosing the MER conductor. The 10 current is Second Outline Level Third Outline Level 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. Click to edit the outline text format � Fourth Outline Level Fifth Outline Level Sixth Outline Level Seventh Outline Level Eighth Outline Level Ninth Outline Level. Click to edit Master text styles � Second level

Click to edit the outline text OPTICAL format CURRENT MOCT ‘s are another type of Second Outline Level transducers , which are highly immune to EMI TRANSFOR Third Outline Level MER Fourth Outline Level MOCT uses optical sensing methods to measure electric currents. Fifth Outline Level Sixth Outline Level Seventh Outline Level Eighth Outline Level In the sensors several properties of the signal carrier(light) can be influenced Ninth Outline Level. Click to edit MOCT’s are mainly based on. Master text styles � Faraday Effect. � Second level

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.

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

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

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)

The optics model is developed using Jones Calculus method E 0 and Ei represents the output and input energy. In Jones calculus method polarised light is represented by a corresponding Jones Vector

APPLICATION S 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.

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.

OCT analog output may have significant white noise , but the white noise does not affect the accuracy or protection performance. Temperature up to 500 c 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.

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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