In all electrical measurements and protection purposes Current transformers and Voltage transformers are used. You can’t measure current upto 100A by directly connecting to Ammeter as Ammeter required for that purposes will be very much large in size which could not be installed. So CT’s are used for that purposes as CT will feed the current to Ammeter or any other measurement devices within limits of that measurement equipment.
Same is the case of relays , relays can’t handle very high value of current and voltages. CT and PT are used for these purposed as these make current and voltages within limits of relay.
CT and VT function like ‘ears' and the ‘eyes' of the protection system. Relay processes these signals and accordingly give commands to circuit breakers and other protection systems.
Classification of CTs
The CTs can be classified into following types:
1. Measurement CTs
2. Protection CTs
A measurement grade CT has much lower VA capacity than a protection grade CT.
These types of CT’s are explained as below:-
1. Measurement CT’S
A criteria for measurement CT is that they should be accurate over its complete range i.e. from 5% to 125% of normal current. Which means that its magnetizing impedance at low current levels should be very high, which is done in order to accurate measurement of small currents. Note that due to non-linear nature of B-H curve, magnetizing impedance is not constant but varies over the CT's operating range. It is not expected to give linear response during large fault currents.
2. Protection CT’s
In protection type CT’s there is ultimate requirement that these CT’s should have linear response up to 20 times the rated current, it should be so as that relay could operate accurately. As in case of measurement CT’s magnetizing impedance is high at low current levels but for protection grade CT's magnetizing impedance should be maintained to a large value in the range of the currents of the order of fault currents.
Sometimes CTs can be used for both purposes i.e. for measurement and protection in such cases it has to be of required accuracy class to satisfy both accuracy conditions of measurement CTs and protection CTs. In other words, it has to be accurate for both very small and very large values of current.
Most commonly CT secondary is always rated in range from 1A to 5A.
We are assuming that CT should have linear response but practically it’s not possible as linear response is dependent on the net burden on the CT secondary. Net impedance on the secondary side is assumed as the CT burden.
If burden increase on CT then this will leads to increase in voltage, if voltage exceeds the set limits, then the CT core will saturate and hence linear response will be lost.
Thus we see that CT will give linear response up to 20 times the rated current, there is also an implicit constraint that the CT burden will be kept to a low value. In general, name-plate rating specifies a voltage limit on the secondary (e.g., 100 V) up to which linear response is expected. If the CT burden causes this voltage to be exceeded, CT saturation results.
Classification of CTs
CT’s are classified into two types namely:-
Class T CT
Class C CT
Class T CTs
One or more primary turns are wound on a core in Class T CT as Class T CT is a wound type CT. These CT’s are associated with high leakage flux in the core. Due to these higher leakage fluxes, the only way to determine CT performance is by test. Standardized performance curves cannot be used with these types of CTs.
Figure above shows one such tested and calibrated curve for a CT. The letter ‘B' indicates the burden in ohms to which the CT is subjected. From curve we will see that when when burden is less than say 0.1 ohms, CT meets the linear performance criterion.
Now you can see from the curve that as the burden increases to 0.5 ohms, the corresponding linearity criteria is not met till the end. Now when burden is increased to 4 ohm there is high deviation from linearity.
Thus it is very clear that keep burden as low as possible so as to attain linearity.
CT performance Is measured from the ratio error.
Now what is Ration Error?
The ratio error is the percentage deviation in the current magnitude in the secondary from the desired value.
It can explained as :- Let secondary current is Is, and actual value is Ip/N, where N is nominal ratio and Ip is the primary current then ratio error is given by Ip/N-Is X100.
When the CT is not saturated ratio error is due to of magnetizing current IE since Ip/N-Is =Ie.
Therefore, % ratio error = Ie/IsX100 .
When there is Saturation in CT then coupling between Primary and Secondary get reduced and Hence large ratio errors are expected in saturation. The current in the secondary is also phase shifted.
For measurement grade CTs, there are strict performance requirements on phase angle errors also.
Error in phase angle measurement affects power factor calculation and ultimately real and reactive power measurements.
It is expected that the ratio error for protection grade CTs will be maintained within +10%.
Class C CT
In Class C CT are more accurate CT’s. Where 'C' letter indicates that the leakage flux is negligible. These type of CT’s are usually bar type CT’s.
In such CTs the leakage flux from the core is very small. Performance of such CT’s can be evaluated from the standard exciting curves.
Ratio error is maintained within for limits for standard operating conditions for such CT’s.
Voltage rating on the secondary is specified on CT’s for which linear response is guaranteed. For example, a class C CT specification could be as follows: 500:5 C 100.
This tell us that 500:5 is the CT ratio and C indicate that it’s curve will be linear up to 100 times rated current provided the burden on the secondary is kept below 100/(5X100) = 0.40 ohm.
For class C CTs, standard chart for versus excitation current on the secondary side is available.
This provides the protection engineer data to do more exact calculations e.g., in determining relaying sensitivity.