|
Introduction:
As it is not possible to use line current directly for measurement
or for protective relay due to its high value. Current Transformers
are used to,
1.)
Reduce the line current to a value (e.g. 1 amp.), which is suitable
for standard-measuring instruments, relays, etc.
2.) To isolate the measuring instruments meters, relays, etc. from
direct line.
3.) To protect measuring instruments against short circuit current.
4.) To sense the abnormalities in current signals to protective
relays to isolate the defective system.
There
are two basic types of C.Ts., Bar primary and wound primary C.T.
Generally as far as possible C.Ts. of the former type are selected
because they are mechanically stronger and cheaper. In the bar primary
C.T. the working ampere turns are determined by the primary current
and therefore necessarily, the accuracies that can be offered with
these C.Ts. become progressively inferior as the rated primary current
decreases. In such cases, wound primary C.Ts. are used. A ring type
C.T. is essentially a bar primary C.T., without the primary bar.
These ring type C.Ts. are very easy to connect in existing switchgear
assemblies as these are merely slipped on to the busbars.
The
Indian standards specifications 4201 (application guide for current
transformers) and also various international specifications recommend
the following values of the rated burden of C.Ts. for various types
of instruments and the required class of accuracy for various metering
applications:
Typical
values of VA burden imposed by different meters are given below:
| INSTRUMENT |
VA
BURDEN |
| Ammeters |
3VA |
| Current
Coil of wattmeter, power factor meters |
5VA |
| Recording
ammeters |
3VA |
| Current
Coils of recording power factor meters and watt meters |
3VA |
|
APPLICATION
|
CLASS
OF ACCURACY
As per ISS 2705(part II)/ IEC 60044-1
|
| For
precision testing or as a standard for testing laboratory C.Ts. |
0.1
|
| For
laboratory & test work in conjunction with high accuracy
indicating instruments, integrating meters & also standard
for testing industrial C.Ts. |
0.2
|
| For
precision industrial metering |
0.5
|
| For
commercial & industrial metering |
0.5
or 1
|
| For
use with indicating & graphic wattmeters & ammeters |
1
or 3
|
| For
purposes where the ratio is of less importance, e.g. ammeters
where the approximate values are required |
3
or 5
|
|
