Selection of Measuring and Protection Of Current Transformers

We all know about various types of relays and protections offered by them. Relays form the brain of the protection system. However, these relays are fed through Current Transformers (CTs) which form the heart of your protection system. I recently received some very valuable inputs about CTs in a seminar. I wish to share the same with you. The correct selection of CTs ensures proper co-ordination of Protection system thereby enhancing system stability. Following are a few points which I would like to bring to your notice.

CTs are different than Power Transformers or Voltage Transformers due to the fact that CT primary loading is independent of Secondary Loading. As CTs step down current, they face extremely high voltages on secondary winding. This can damage the secondary core. Thus, CT Secondary winding should always be shorted when not loaded. CTs can be used for Metering/Protection/Dual purpose.  Let us discuss individual purposes.

Measuring CTs are meant to operate under normal load conditions with an operating range of 5-120% of full load current. Typical accuracy classes are 0.5 and 1.0. This accuracy is maintained between 25-100% of the range with lower accuracy at lower currents and currents beyond this range. These CTs also have an Instrument Safety Factor (ISF) of 3.0 which means CT will be saturated at 3 times full load current. This ensures safety of meters and other measuring devices.

Protection CTs are expected to sense fault currents. Thus they are defined using accuracy class and ALF (Accuracy Limit Factor) e.g. a 5P10 CT means that CT shall have accuracy of 5% up to a current 10 times (ALF) the full load current. Accuracy will improve near the rated current of CT. CT burden also depends on ALF. For economical design, ALF x VA burden should be around 150. Hence, a 5P10 CT (ALF=10) should ideally have a burden of 15VA. 

A dual purpose CT has two separate cores for Metering and Protection. Thus it is specified as below e.g. Core I- Class 0.5, 15VA, Core II- Class 5P10, 15VA.
CTs can be Outdoor or Indoor as per the application needs. 

CT can have secondary current as either 1A or 5A. Choosing secondary current shall depend on the application as follows :
1)      5A CT – a) Typically in MV where currents can be high.
b) Indoor application with Integrated Control and Relay Panels.
c) Not suitable for Low Currents (less than 50A) application
2)      1A CT –a) Preferred in HV/EHV with low currents
b) Outdoor Substations with separated Control and Relay Panels
c) Ideal for low current application

In case of 1A CT, Secondary peak voltage shall be extremely high if open-circuited. Hence, special care must be taken with them.

There are special purpose Current Transformers like PS class CTs and CBCTs which have specific applications which can be discussed at another time.
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