11-10-2016, 10:45 AM
1458558230-MOCT.doc (Size: 499 KB / Downloads: 4)
INTRODUCTION
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. 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.
With short circuit capabilities of power systems getting larger, and the voltage levels going higher the conventional current transformers becomes more and more bulky and costly also the saturation of the iron core under fault current and the low frequency response make it difficult to obtain accurate current signals under power system transient conditions. 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 are likely to introduce electro-magnetic interference through the ground loop into the digital systems. This has required the use of an auxiliary current transformer or optical isolator to avoid such problems.
It appears that the newly emerged Magneto-optical current transformer technology provides a solution for many of the above mentioned problems. The MOCT measures the electric current by means of Faraday Effect, which was first observed by Michael Faraday 150 years ago. The Faraday Effect is the phenomenon that 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.
The MOCT measures the rotation angle caused by the magnetic field and converts it into a signal of few volts proportional to the electric currant. It 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 such as fiber optical cables, lenses, polarizers, glass prisms, mirrors etc. the signal is brought down by fiber optical cables to the signal processing unit and there is no need to use the metallic wires to transfer the signal. Therefore the insulation structure of an MOCT is simpler than that of a conventional current transformer, and there is no risk of fire or explosion by the MOCT. In addition to the insulation benefits, a MOCT is able to 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. And there is a growing interest in using MOCTs to measure the electric currents.
MAGNETO-OPTICAL SENSOR
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. The optical glasses are good candidate for the sensing material, because the Verdet constants are not sensitive to the temperature changes, and they have good transparency properties. They are cheep and it is easy to get large pieces of them. Among the optical glasses SF-57 is the best choice, as it has larger Verdet constant than most of the other optical glasses. And MOCT made out of these materials can achieve higher sensitivity. In the MOCT, from Eq (2), the total internal rotation angle is,
1+ 2 2VI
Where I is the current to be measured,
= 4 x 10-7 H/m
V=7.7 x 102 degrees/Tm at a wavelength of 820nm
Therefore = 1.9 degrees/ KA.
Different optical fibers are designed for different usage. The single mode fiber has very wide bandwidth, which is essential for communication systems, but it is difficult to launch optical power into the single mode fiber because of it’s very thin size. While 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 sensors.
APPLICATION
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. Typically, 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.
ADVANTAGES OF MOCT
1. No risk of fires and explosions.
2. No need to use metallic wires to transfer the signal and so simpler insulation structure than conventional current transformer.
3. High immunity to electromagnetic interference.
4. Wide frequency response and larger dynamic range.
5. Low voltage outputs which are compatible with the inputs of digital to analog converters.
DISADVANTAGES OF MOCT
1. Temperature and stress induced linear birefringence in the sensing material causes error and instability.
2. The accuracy of MOCT is so far insufficient for the use in power systems.
CONCLUSION
This paper presents a new kind of current transducer known as magneto optical current transducer. This magneto optical current transducer eliminates many of the drawbacks of the conventional current transformers. In an conventional current transformers, there is a chance of saturation of magnetic field under high current, complicated insulation and cooling structure, a chance of electro magnetic interference etc.
By applying Faraday’s principle this transducer provides an easier and more accurate way of current measurement. This MOCT is widely used in power systems and substations nowadays. And a new trend is being introduced, which known as OCP based on adaptive theory, which make use of accuracy in the steady state of the conventional current transformer and the MOCT with no saturation under fault current transients.