Current transformers (CTs) are used.
a) to reduce the power system currents to values low enough for safe measurement in
protective relays
b) to insulate the relay circuits from the primary circuit and
c) to permit the use of standardized current ratings for relays.
If the distance between CTs and measuring instruments or relays are appreciable, it is common practice to use the CT whose rated secondary current is equal to 1A. In this case, the secondary circuit resistance of a CT may be 25 times that of the CT whose secondary current is equal to 5A. This will be explained as below:
Saturation of the CT may cause no inconvenience and may even be desirable for its limiting action in the case of switch board instruments (ie. CTs for metering) but obviously it could seriously upset the performance of a time current relay or any form of comparator relay such as differential relay or a distance relay during heavy faults. Saturation can be avoided either by increasing the cross section of iron cores of CT or by reducing the burden. The first method is expensive and the second may be difficult. The burden on the CT is due to the resistance of the relay, the CT secondary and the leads. For a given performance, the relay burden can not be reduced except by changing its design. On the other hand the lead burden can be reduced by using a lower secondary current rating.
For instance in a large station with long runs from the switch yard to the relay panel
(like ERODE 110/22 KV GRID STATION), the lead resistance may be as 6 ohms.
With a 5A CTs the normal burden imposed by the 6 ohm leads is I2 R = 52 x6 = 150 VA . In order not to saturate with a fault current, twenty times normal, the iron circuit would have to be large enough to avoid saturation below. 600 V.
(IR = 20 x 5x6 = 600 Volts). This would require quite a large CT, on the other hand with a 1 A CT secondary rating, the lead burden would be I R = 1 x 6 = 6 VA. The resistance of the secondary of the 1 A CT would be about 5 ohms compared with 0.2 ohm for the 5 A CT.
Dear Alam ,
As much I know about CTs, say for Class 1 accuracy, there is 2% current error in 100/5A CT and 10% current error in 100/1A CT, both referring to 100A primary. Current error is an error that arises when the current value of the actual transformation ratio is not equal to rated transformation ratio. However, the CT burden of a 1A CT will be less compared to a 5A secondary CT. Hence, choosing a CT with accuracy and burden depends upon the real time requirement. Generally we use PS class CT for protection purposes as the accuracy is high.
Thanks & Regards
Srikanth
Dear Pullabhatla
From planning purpose, when one come to a conclusion that he/she have to select CT of either 100/1 or 100/5 ration. Which among the two you will suggest to be selected for protection point of view?
Putting it in a different way! The operation of a CT is between 2 non-linearity. One is the inertia and the other is the saturation. For measurement purposes both the non-linearity need to be avoided for measurement to have meaning. A better range in such a case is spreading out to 0-5A. However, for protection, especially over current as is the lot of CTs the exceeding of a limit may be of most significance than accurate measurement. In that case the spread in the range, avoiding the inertia etc. may be not of much concern in most cases. Hope I have made my point.
Thank you very much Sudha Balagopalan Madam, for giving kind information about current transformer (CT).
It is very important, for C.T to reflect Fault current, Transient current and Overload current Accurately and Quickly as possible, without distortion, for proper discrimination, where C.T time constant play an important role, refer my paper ,"C.T performance for inrush in power transformer".
The range in such a case is spreading out to 0-5A, is practical problem will occur better to choose primary current selection is twice the actual full load current capacity to obtain accuracy with 1 A secondary
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