29-10-2010, 12:01 AM
Optical Current Transformers
SREEKANTH.S.B
S7E2
College Of Engineering, Trivandrum
2007-11 batch
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STRUCTURE OF PRESENTATION
Theoretical Concepts
Transducers using Faraday effect
Field trial of O.C.T
Performance of O.C.T v/s C C.T
Merits & Demerits of O.C.T
Manufacturers of O.C.T -AREVA ;ABB
Wrapping
THEORETICAL CONCEPTS
In O.C.T, optical sensors are used to measure electrical currents.
These are called magneto-optic transducers which replaces the secondary in C.C.T
OCT’s give better performance than C.C.Ts due to better safety, lesser size and absence of core loss, saturation, insulation failure.
TYPES OF OPTICAL SENSING METHODS
Faraday effect
Interferometric principles
Bragg gratings
Micromechanical sensors
WHAT IS FARADAY EFFECT ?
Michael Faraday discovered this phenomenon in 1845
The Faraday effect is a magneto-optical effect that causes a change in the state of polarisation of light
When a beam of polarised light is sent through a Faraday material, the polarized light is rotated by the angle θ, in proportion to strength of the magnetic field parallel to the light.
PRINCIPLE OF THE FARADAY EFFECT
FARADAY MATERIALS
Diamagnetic (e.g. SF-57, SiO2, BK-7) and paramagnetic (e.g. TGG (Terbium gallium garnet), FR-5). They have relatively low rotations
Ferrimagnetic and ferromagnetic materials give higher rotations . Widely-used ferrimagnetic materials are RIGs (Rare Earth Iron Garnets) & YIG (Yttrium Iron Garnet)
POLARISATION
It refers to time varying behavior of electric intensity (E) of light
Oscillation of E in a single plane- Linear Polarisation
Locus of rotation of E is a circle - Circular Polarisation
Locus of movement of E is an ellipse- Elliptical Polarisation
a) Linear b) circular c) elliptic polarization
MATHEMATICAL REPRESENTATION OF FARADAY EFFECT
Mathematically , Faraday effect is
θ = ∫ k H dl where
θ is the angle of rotation of polarized light
H is the Intensity of magnetization
l is the length of the light path
k is a constant dependent on the propagating material
TRANSDUCERS USING FARADAY EFFECT
Magnetic concentrator
Optical fibre sensing element
MAGNETIC CONCENTRATOR
OPTICAL FIBRE SENSING ELEMENT
CURRENT SENSOR DEVELOPED BY ABB
FIELD TRIAL OF O.C.T BY TOSHIBA
PERFORMANCE OF OCT- RATIO ERROR
MEASUREMENT OF SHORT CIRCUIT CURRENT (maximum peak instantaneous error – 1.3 %)
EFFECT OF LIGHTNING
MERITS OF O.C.T v/s C.C.T
High accuracy
Reduced size and weight
No insulation failure
High bandwidth (0.5 Hz to 6 kHz)
Low maintenance
No effect of saturation
Immune to E.M.I
explosion free (avoids oil or SF6)
LEADING MANUFACTURES OF O.C.T
CONCLUSION
Fibre-optic based current and voltage sensors are an increasingly being used in developed countries
In the long term OCTs will be potentially cheaper
due to innovation in optical, electronics and power engineering fields
When total costs equipment, maintenance, replacement upon failure etc are considered
REFERENCES
T.Kumai. H. Nakabayashi, Field trial of OCT using silica optical fibre, IEEE Trans , vol.12 ,pp 920-925, 2002
J.D.P Hrabliuk, Optical sensors eliminate CT saturation, IEEE, Trans 2002
Masao Takashashi ,Kiyohisa Terai, Optical Current Transformer for 245 kV integrated air insulated switch gear, IEEE xplore, pp 389-392
Andrew Klimek ,Optical technology –A new generation of instrument transformer, Electricity today ,issue 2 ,2003
Sascha Liehr, Optical measurement of current in power convertors, Report No . XR-EE-MST 2006:001 ,Royal institute of technology ,Stockholm, March 2006
areva.com
abb.com