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

Class P method of specification will a suffice. A secondary accuracy limit current greatly in excess of the value t o cause relay operation serves no useful purpose and a rated accuracy limit of 5 will usually be adequate.

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 non-directional 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.
     Class 10P current transformers are generally recommended in which the product of rated
     output and rated accuracy limit fact or approaches 150 provided that the earth fault relay is
     not set below 20% of the rated current of the associated current transformer and that the
     burden of the relay at its setting current does not exceed 4VA.

(2) Schemes in which phase fault stability and/or where time grading is critical.
     Class 5P current transformers in which the product of rated output and accuracy
     limit factor approaches 150 should be used.

They are in general suitable for ensuring phase fault stability up to 10 times the rated primary current and for maintaining time grading of the earth f a u l t relays, up to current values of the order of 10 times the earth fault setting provided t h a t the phase burden effectively imposed on each current transformer does not exceed 50% of it s rated burden.

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.

Class “X” Current Transformer
 

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 + R₀)
 

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