Vinayan.S Roll no:22 EEE7

 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

 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.

 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: 1) physical design of CT 2) amount of steel in the core 3) the connected burden 4) winding resistance and 5) system X/R ratio

The problem of CT is saturation. These can be avoided altogether by using optical current sensor

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

 A fiber optic current sensor (as shown figure) consist of a light source,photo detector and electronics

 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.

 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

 The electronic circuits present may cause distortions.  The measurement is not much accurate.

 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.

 [1] Interfacing Optical Current Sensors in a Substation,J.D.P. Hrabliuk, IEEE PES Summer Power Meeting,Vancouver, B.C., July 17,  [2] NxtPhase Optical Current System, J. Blake, 2ndEPRI Optical Sensor Systems Workshop January 26–28,2000, Atlanta, Georgia.