An accurate electric current transducer is a key component of any power system instrumentation. To measure currents power stations and substations conventionally employ inductive type current transformers with core and windings. Conventional current transformers becomes more and more bulky and costly. The MOCT measures the electric current by means of Faraday Effect. Magneto Optic Current Transducer consist of a sensor head located near the current carrying conductor, an electronic signal processing unit and fiber optical cables linking to these two parts.
The sensor head consist of only optical component. The signal is brought down by fiber optical cables to the signal processing unit therefore the insulation structure of an MOCT is simpler. MOCT provide high immunity to electromagnetic interferences, wider frequency response, large dynamic range and low outputs which are compatible with the inputs of analog to digital converters. They are ideal for the interference between power systems and computer systems.
MOCT is based on the Faradays effect, the orientation of 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.
Theta = nµVI where ‘I ‘is the current to be measured, ‘µ’ is the permeability of the material, ‘n’ is the 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. It rejects the magnetic field signals due to external currents which are normally quite strong in power system. A polarizer is used to convert the randomly polarized incident light into linearly polarized light. The orientation of the linearly polarized light rotates an angle after the light has passed through the magneto-optical material because of Faraday Effect. Then another polarization prism is used as an analyzer, which is 45 0 oriented with the polarizer, to convert the orientation variation of the polarized light into intensity variation of the light with two outputs, and then these two outputs are send to photo detectors. The purpose of using the analyzer is that photo detectors can only detect the intensity of light, rather than the orientation of polarizations.
The optical sensor consists of two separate clamp-on parts and linearly polarized light is arranged to pass through the optical glass prism to pick up the Faraday rotation signal. The polarization compensation technique is applied at each corner of the prisms, so that the light passing through the prism remains linearly polarized. At the other end of the prism, a silver mirror reflects the light beam so that light beam comes back to its sending end via the same route while accumulating the Faraday rotations.
The rotation angles from the two halves of the sensor [Fig.4(a)] are added up in the signal processing unit so that the total rotation angle(Theta 1+ Theta 2 ) is the same as the rotation angle Theta from the optical path, which is two turns around the conductor. It avoids the use of magnetic material to concentrate the magnetic field. It is free from the effect of remnant flux, which affect the accuracy of the current measurement.
Almost all transparent material exhibits the magneto-optical effect or Faraday Effect, but the effect of some of the material is very temperature dependent, and they are not suitable for the sensing material. MOCT made out of SF-57 materials can achieve higher sensitivity. Different optical fibers are designed for different usage. The single mode fiber has very wide bandwidth is essential for communication systems, size. Large multimode fiber is convenient for collecting maximum amount of light from the light source, it suffers from the problem of dispersion which limits its bandwidth. In the situation of power system instrumentation, only moderate frequency response is required and in MOCT, The more optical power received by the detectors the better signal to noise ratio can be achieved. Therefore, the large core multi-mode optical fiber is used here to transfer the optical signals to and from the optical sensor
ELECTRONIC CIRCUIT FOR THE MOCT
There are two output stages. One stage, which has 1 KA dynamic range, is for power system current metering, and other stage, which operate up to 20 KA, provides power system current signals for digital relay systems . In each part of the device, the sum of the two receiving channels signals, which have the same DC bias , differenced at junction with a reference voltage Vref from the power level adjustment potentiometer. An integrator is used to adjust the LED driver current to maintain to be the same as the Vref at the junction.
The difference of the two receiving channels signals in each part of the device are added directly and then fed through an amplifier for the small signals. At the same time these two signals are processed digitally to do a sin-1 calculation on each and then summed together for the large signal situation when the non-linearity of the MOCT can no longer be ignored. The ratio responses of the two output stages of the clamp-on MOCT are designed as 10 V/KA and 0.5 V/KA and frequency responses are 4KHZ and 40 KHZ respectively. 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. The design approach is to redefine the interface point as to input the analog to digital conversion function used by each of these measurement systems.