08-12-2012, 02:23 PM
A Clamp-on Magneto-optical Current Transducer for Power Systems
A Clamp-on Magneto-optical.pdf (Size: 465.1 KB / Downloads: 26)
Abstract
A new type of Faraday effect based magneto-optical current tsansducer
(MOCT) has been developed. This paper presents the theoretical
background, engineering design, and test results of this clampon
MOCT. The sensor consists of two separate clampon parts. ‘This design
avoids the need to break the current carrying circuit in order to
have the conductor enclosed by the optical path while keeping the
same measurement accuracy. A feedback scheme has been designed
to stabilize the light source. The MOCT has two output stages, lKA,
and 20KA, so that it can provide current signals for both power system
metering and relaying devices.
Introduction
An accurate electric current transducer is one of the key components
in power system instrumentations. It is the signal source providing
current signals for revenue metering, control and relaying apparatus.
Conventional current transducers are transformers (CTs) with copper
wire windings and an iron core, which are now widely used in power
systems. With the short circuit capacities of power systems getting
larger, and the voltage levels going higher, the conventional CT becomes
more and more bulky and costly, and the performance limitations,
saturation of the iron core under large fault current, low frequency
response, make it difficult to obtain accurate current signals
under power system transient conditions. With the computer control
techniques and digital protection devices being introduced into power
systems, conventional CTs have caused further difficulties, as they are
likely to introduce electro-magnetic interference through the ground
loop into the digital systems. Because of the advantages of MOCTs
over conventional CTs, there is a growing interest in using magnetooptical
current transducers (MOCTs) to measure the electric current
by means of Faraday effect. [11-[61 These advantages are high immunity
to electromagnetic interference, being able to provide high level
electric insulation, and wider frequency response. MOCTs are ideal
for the interface between power systems and computer systems.
Design
Fig. 6. shows the structure of this MOCT. The device consists of two
separate symmetrical parts. In each part of the device, 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 comer of the prisms, so that the light passing
through the prisms remain linearly polarized. At the other end of the
prism, a metal mirror reflects the light beam so that the light beam
comes back to its sending end via the same route while accumulating
the Faraday rotations. The two halves can be assembled around the
conductor. Thereby, the rotation angles from the two halves of the sensor
[Fig. 7. (a)] are added up in the signal processing unit so that the
total rotation angle is the same as the rotation angle 8 from the
optical path shown in Fig. 7. (b).
Testing Results
The response of the two output stages of the clampon MOCT are designed
as 10 V/KA and 0.5 VKA respectively. Fig. 10. shows the
tested ratio responses and the associated errors of the MOCT. The metering
part of the response was tested on a current loop which is able
to generate 60 Hz constant current up to 1.2 KA. The relaying part of
the response was tested on a high-voltage pulse current generator
which is capable of generating pulse current up to 20 KA. Larger error
at the low current end is due to the background noise of the electronics
which is due mainly to the shot noise of the PIN photo detector. The
tested signal to noise ratio of this device is 64 db at 1 KA. Less than
1.5Oof phase error was observed at 60 Hz for the metering stage. All
the curves shown in Fig. 10. were tested under stable environmental
conditions.
Conclusions
A new type of clampon magnetooptical current transducer has
been successfully demonstrated. It has the advantage of very easy
installation, which make it more suitable for field application.