Dear Bandi Ravi Kumar

Unlike a voltage transformer, the primary current of a current transformer is not dependent of the secondary load current but instead is controlled by an external load. The secondary current is usually rated at a standard 1 Ampere or 5 Amperes for larger primary current ratings.

For further illustration and understanding, have a look:

1. https://www.electronics-tutorials.ws/transformer/current-transformer.html

2. https://eapsm.net/current-transformer-sizing/

3. https://forums.mikeholt.com/threads/ct-secondary-current-rating-limited-to-20-times-the-rated-current.129884/

Kind Regards

Qamar Ul Islam

To measure accurate air core bar primary current with out saturation, in linear region of B-H curve for air core …….

Current transformers are used in the protection systems to feed the overcurrent relays with their standard rated current which is 20A.

Dear Bandi Ravi Kumar

Current transformers are divided into 3 classes. Measurement, Protection and Special protection. For all these functions the CT should work in the linear portion of the B-H curve. For the CT used for measurement, this linear portion should be limited within the measurement range. Beyond that it should get saturated. This is to avoid over loading of the meter which is connected in the secondary of the CT during over load or short circuit fault in the system. But for protection purpose, the saturation point should be at a much higher level.

For Restricted Earth Fault and Differential protection, where multiple CTs are used, special protection class come in. Here the requirement is to avoid activation of the protection during a through fault because of different saturation point for different CTs.

CT saturation point is defined through knee point voltage (KPV) in the V-I characteristic of the CT. For measurement CT, KPV will be small where as protection this will be higher. For special protection, KPV is calculated from the fault level of the system and the loop resistance in the protection circuit. Based on the required KPV the special protection CT to be designed. 20 times the rated current is sometimes required for protection but it is not hard and fast.

Dear Ravi,

Number 20 is good enough to accommodate most of the fault levels that are beyond load current (5P20). This is the reason, CTs can offer the correct replica of fault current in the linear region (without saturation) even if the fault current is up to 20 times the rating of CT without saturation. Certainly, there are also some CTs that can offer a correct replica of fault towards its secondary side up to 10 times only (5P10). Such CTs are used in the system where fault currents are not going to be very high (beyond 10 times its rating). There are some other protection class CTs as well such as 5P30, but such CTs are not often needed. Mostly, 5P20 type CTs (protection class CTs) are used as in offer accuracy level of within 5% deviation (maximum error), and cannot saturate even if the fault is up to 20 times its rated primary current. Short circuit studies is a planning problem and we are required to do it for protection planning such as selection circuit breakers, CTs and calculating relay settings.

Hoe that you go the point.

Hello my Libyan brother nacer:

I am very happy to answer you.

the philosophy of scientific research is an ideology that makes it possible to create a unique technical language, so that researchers understand each other well, that's why organizations have set standards to facilitate research, sometimes these standards are mandatory by what they have a science and physics you can't change them.

sometimes we find standards for technical adaptation between international services (for example the network frequency 50 Hz and 60 Hz), and for scientific research there are several standards as you indicated current transformer problem .

According to my experience in an industrial site (international company) with high technology, the standard of 20 times does not exist, we have several types of sensors (Please see the document attached with the characters of a current transformer)

Your saturation proposal. where in the industry this standard was taken into consideration in several cases;

practical experience gave us an idea to understand these problems