Current Transformer Question

CURRENT TRANSFORMERS - SECONDARY CURRENT RATING - 1A OR 5A? - WHAT TO CHOOSE?

by - K.Sivakumar, Manager-Training, Larsen & Toubro Limited, Switchgear Training Centre, Coonoor (T.N.)

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Introduction: As we all know, current transformer is an important link in power system that helps us to measure, monitor & protect the power system. As such, the correct selection and specification of a current transformer also becomes a very important task. CT ratio is one such important specification. As far as the primary current rating is concerned, the user has no choice but to go for a primary current rating as close to the load current of the feeder. The Indian Standard (IS 2705) also recognizes this when it says that the rated primary current of a CT can be 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75A and their decimal multiples or fractions. But, when it comes to the rated secondary current of the CT, IS gives two choices: 1A & 5A. Now, which one to select, 1A or 5A?

First let us analyse a 100/1A, bar primary CT. As you must be aware a bar primary CT will have only one primary turn. Now, when 100A is flowing in the primary, the primary ampere-turns is 100A x 1 T = 100 AT. In any transformer, the primary ampere-turns must be balanced by the secondary ampere-turns. In this case, as the CT secondary has a current of 1A, to achieve ampere-turns balance, we need to have 100 turns of secondary winding. Moreover, as we discussed in an earlier article, the voltage output of a CT secondary for a given primary current is constant. And, when this voltage has to circulate only 1A in the CT secondary, the resistance of the CT secondary wire has to be more. It means that we have to use a finer wire (i.e.) wire with lesser cross-sectional area for the CT secondary winding. As the number of turns in secondary is more, the core area required to wind the same is also more. So, the core size & weight too will increase.

Also, the emf induced across a CT secondary is given by:

e α 4.44 φ f N₂ Volts

(i.e.) the secondary emf is proportional to the number of turns in the secondary. As the number of secondary turns in a CT with a 1A secondary current is more, so also will be the open circuit voltage of the CT secondary. Hence, the CT needs to be insulated for a higher voltage. This increases the insulation thickness and hence the size of the CT.

Thus, a finer wire with more number of turns, increased core area and increased insulation thickness will all make a CT with a 1A secondary costly and bulky.

On the other hand, had it been a CT with a 5A secondary in the above case, one would only need 20 Turns in the CT secondary side to achieve AT balance. Also, as the amperage is more, resistance needs to be less, implying that one can go for a thicker wire. Also, with reduced secondary turns, the open circuit voltage will also reduce fivefold, as also the insulation thickness and size. Less number of turns also means that reduced core size. Hence, for a given CT ratio and burden, a CT with a 5A secondary will be cheaper, smaller and lighter.

Thus, on the outlook it definitely appears that CTs with 5A secondary current are far more superior to those with 1A secondary. Is it not? But then, a question that would appear in one's mind is that if a CT with 1A secondary is that bad, why then that the Standard has to recognize it?

For the moment, let us conclude that because of the advantages of a CT with a 5A secondary, we will only use it in all our applications. Let us anlayse another case, wherein the CT is mounted at one place - say, a switchyard, and the relay or the meter is located at another place - say, the switchgear room or the control panel room. Consider that the two are inter-connected by 100 meters of 2.5 sq.mm copper cable. The relay or meter burden is, say, 5VA. But, the connecting lead will also impose a burden on the CT. The two-way lead resistance of the above cable is assumed to be 0.8 Ohms. Then, the lead burden in VA will be (I_s)² x (R_L) (i.e.) (5)² x (0.8) = 20VA. Now, the CT has to be rated for lead burden and the load burden.

In this case, the CT rating would be 30VA (nearest standard burden value as per IS 2705). Imagine, just for supplying a 5VA load, we have to go for a 30VA CT here. Also, please remember the size, cost & weight of the CT is also proportional to burden, like it is in Power Transformers. So, under the given conditions, a CT with a 5A secondary would be costly & bulky. All the advantages of the CT with a 5A secondary discussed above are lost.

Now, if one considers a CT with a 1A secondary in this case, the lead burden would only be 0.8VA and together with the load burden, the CT may have to be rated only for 7.5VA, thereby reducing the cost & size of the CT. Since the CT will be mounted outdoors, insulation will not be an issue, as the increased outdoor air-clearances would take care of the increased insulation needs of the CT as well.

Whereas in the case of Indoor CTs mounted in Switchboards, size & weight definitely matters. Also, the lead lengths are very small, as the CT would be mounted in the rear cable chamber and the relay or meter in the front instrument chamber. The lead lengths would hardly be more than 5 or 10m. Lead burden is not an issue here. So, a CT with a 5A secondary would be the solution here.

Concluding, selection of CT secondary current rating shall be based on the location of the CT, the lead burden, the load burden, dimensional constraints, etc.

To put it in a nutshell:

"ALL BOARD MOUNTED CTs SHALL BE WITH A 5A SECONDARY CURRENT

and

"ALL YARD MOUNTED CTs SHALL BE WITH A 1A SECONDARY CURRENT".