Optical current transformer and its applications

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

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.

 Different types of OCT’s are : 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 : 1. Linear polarization 2. Circular polarization 3. Elliptical polarization

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  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level  Fifth level 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 phase and amplitude of the x and y components are different. a) Linear Polarization b) Circular polarization c) Elliptical polarization

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

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)

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

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  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level  Fifth level 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. 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.

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  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level  Fifth level 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. The Optical Current transformer basically consists of: light source, fibre optic cable, polarizer, modulator and electronics part.

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 magnetic field. Finally,two waves again reach the quarter wave plate and linear polarisation gets regained.

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

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 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

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.

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 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.

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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