Choice of CT’s Primary rating 

The c. t. primary rating is usually chosen to be equal to or greater than the normal full load current o f the protected circuit. Standard primary ratings are given in B.S. 3938:1973. Generally speaking, the maximum ratio of CT’s is usually limited to about 3000/1. This is due to (I) limitation of size of CT’s and more importantly (II) the fact that the open circuit volts would be dangerously high for large CT’s Primary ratings, such as those encountered on large turbo alternators, e.g. 5,000 amperes. It is standard practice in such applications to use a cascade arrangement of say 5,000/20A together with 20/1A interposing auxiliary CT’s
Instantaneous over current relays
When such relays are set to operate at high values of over current, say from 5 to 15 times the rated current o f the transformer, the accuracy limit factor must be at least as high as the value of the setting current used in order to ensure fast relay operation. Rated outputs higher than 15VA and rated accuracy limit factors higher than 10 are not recommended for general purposes. It is possible, however, to combine a higher rated accuracy limit factor with a lower rated output and vice versa. But when the product of these two exceeds 150 the resulting current transformer may be uneconomical, and/or of unduly large dimensions.
Over current relays with Inverse and Definite Minimum Time (IDMT) lag characteristic
In general, for both directional and nondirectional relays class 10P current transformers should be used
Earth fault relays with inverse time characteristic
(1) Schemes in which phase fault
current stability and accurate time grading are not required.
(2) Schemes in which phase fault
stability and/or where time grading is critical. The rated accuracy limit factor is not less than 10 the earth fault relay is not set below 30 % The burden of the relay at its setting does not exceed 4VA The use of a higher relay setting the use of an earth fault relay having a burden of less than 4VA at its setting The use of current transformers having a product of rated output and rated accuracy factor in excess of 150.
Protection current transformers specified in terms of complying with Class ' X I Specification is generally applicable to unit systems where balancing of outputs from each end of the protected plant is vital. This balance, or stability during through fault conditions, is essentially of a transient nature and thus the extent of the unsaturated (or linear) zone is of paramount importance. Hence a statement of knee point voltage is the parameter of prime importance and it is normal to derive, from heavy current test results, a formula stating the lowest permissible value of V_{K} if stable operation is to be guaranteed, e.g.
V_{k} = K I_{n}
(R_{CT} + 2R_{L} + R_{0}) Where K  Is a constant found by realistic heavy current tests? I_{n}  rated current of C.T. and relay R_{CT}  secondary winding resistance of the line current transformers R_{L}  lead burden (route length) in ohms R_{o}  any other resistance (or impedance) in circuit


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