Magneto Optic Current Transformer Technology (MOCT) : Attish Jain
Magneto Optic Current Transformer Technology (MOCT) : Attish Jain
Magneto Optic Current Transformer Technology (MOCT) : Attish Jain
e-ISSN: 2278-1676,p-ISSN: 2320-3331, Volume 12, Issue 1 Ver. IV (Jan. – Feb. 2017), PP 46-50
www.iosrjournals.org
Abstract: 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
.For high voltage applications, porcelain insulators and oil-impregnated materials have to be used to produce
insulation between the primary bus and the secondary windings. The insulation structure has to be designed
carefully to avoid electric field stresses, which could eventually cause insulation breakdown. The electric
current path of the primary bus has to be designed properly to minimize the mechanical forces on the primary
conductors for through faults. The reliability of conventional high-voltage current transformers have been
questioned because of their violent destructive failures which caused fires and impact damage to adjacent
apparatus in the switchyards, electric damage to relays, and power service disruptions. In addition to the
concerns, with the computer control techniques and digital protection devices being introduced into power
systems, the conventional current transformers have caused further difficulties, as they introduce electro-
magnetic interference through the ground loop into the digital systems. Magneto-optical current
transformer(MOCT)technology provides a solution for many of the above mentioned problems. The MOCT
measures the electric current by means of Faraday Effect that is the orientation of polarized light rotates under
the influence of the magnetic fields and the rotation angle is proportional to the strength of the magnetic field
component in the direction of optical path. MOCT is a passive optical current transducer which uses light to
accurately measure current on high voltage systems and determines the rotation angle & converts it into a
signal of few volts proportional to the current.
Keywords: electric current transducer, Faraday effect, power system, switchyards, current transformers
I. Introduction
MOCT-Principle
The Magneto-Optical current transformer is based on the Faradays effect. Michael Faraday discovered
that 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.
Generally, this phenomenon can be described as follows:
= V dl …………Eq (1)
=nVI ………….Eq(2)
The Faraday effect outlined in eq(2) 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 °
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 output optical signals from the analyzer can be described as,
P1 = (1 + Sin 2 )
P2 = (1 - Sin 2 )
is the Faraday rotation angle,
P1 and P2 are the optical power delivered by the detectors.
In order to properly apply Eq(2) in the MOCT design by making the optical path wrap around the
current carrying conductor, the optical path has to be folded by reflections. Total internal reflections and metal
reflections are good ways to achieve this. However reflections introduce phase shift; hence change the
polarization state of the light. The optical prism has to be designed to keep the light going through the MOCT
linearly polarized.
Temperature Characteristics
The first test was to characterize the rotator output with variation in temperature. The rotator and
interface electronics were placed in an environmental chamber that allowed temperature profiles to be run on the
system while it was operating, and the effect of temperature measured. Two temperature effects were studied.
First, the change in the MOCT output that could be related to shifts in the steady state operation due to
temperature excursions possible in field conditions. This sets the extremes of the temperature range. Second, it
was known that temperature induced stress on the optical system could result in short term changes in the output
levels: therefore, a study of the effect of rate of change in temperature was important to characterize the system.
DOI: 10.9790/1676-1201044650 www.iosrjournals.org 47 | Page
Magneto Optic Current Transformer Technology (Moct)
A typical temperature profile for a test run is obtained. This is considered to be an extreme worst case
test condition as it stresses the MOCT system by changing the temperature in a 24 hour period, approximately
10 degrees per hour. Examination of the temperature profiles taken during the field test show this to be a good
assumption for the Tennessee area . Following is the variation in ratio performance of the MOCT system under
the above test condition. Under this extreme test, the device will exhibit no more than 0.1% error from -3O°C (-
220F) to +60°C (+1400F) when compared to the reference CT operating at room temperature.
These test were run for each of the precision current transformer ranges with similar results for all
ranges. Each test in each range was repeated several time with only slight variations in results from test to test.
0 20
10 -30
20 -30
30 -30
40 -20
50 -10
60 0
70 20
80 40
90 60
100 70
120 80
130 95
140 96
150 95
-20 1 1
-10 1 1
0 1 1
10 1 1
20 0.999 0.999
30 0.999 0.998
40 0.997 0.995
50 0.998 0.995
60 0.996 0.996
70 0.997 0.994
Temperature vs stability ratio factor for a MOCT system and conventional ct system
Matlab simulation for Temperature vs stability ratio factor for a MOCT system and conventional ct
system
III. Conclusion
The results have confirmed that the MOCTs are suitable for power system protection and can replace
the current magnetic CTs used in the industries as the variation of stability coefficient/factor analysed is always
close to 1 for various test temperature scale applied on the MOCT apparatus . As can be seen the MOCT system
compares favourably with the conventional system. In terms of comparison of performance against objectives
for the project, the MOCT demonstrate the following applications/advantages over conventional current
transformer:
Optical sensors can be designed to perform reliably in operating system environments
Stable and accurate performance in the substation environment is obtainable with proper design
Effects of temperature, humidity, vibration, and electromagnetic interference are negligible or nonexistent
when the system is compared to a high accuracy conventional system
In future the MOCT technology can be used for advanced tariff measurement and protective relays .
Values of RMS of Output current in conventional CTs and MOCT are measured by less than 0.1% difference
from each other which relates to the high quality of the MOCT output signal MOCT can determine the future
output of the signal and hence can replicate the given signal if a given portion of a signal is lost thereby giving a
continuous and uninterrupted signal at the output end . Rapid advances in the quality of performance and costs
of the optical fibre and electronic equipment encourage development of measuring trances based on new
technology. There is a high scope of application in the field of transmission and distribution for MOCT
technology which would result in safer and more cost efficient systems.
References
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