CURRENT TRANSFORMER ACCURACY CLASSIFICATION FOR RELAYING

Since a current transformer has a magnetic core, it may not have a linear operating characteristic over the desired operating range of primary current. This results in a variation of ratio and possible variation of phase angle between primary and secondary currents for different values of primary current.

This phenomenon is called saturation. In addition to magnitude of ac current, factors that affect this saturation are the following.

Transient Current
The transient dc component of short circuit current is a major cause of current-transformer saturation and consequent misoperation of some bus differential systems. The maximum flux density in the current transformer core, attributable to the dc component of fault current, varies with the time current of the dc transient.

Hence, the L/R ratio of the system impedance which determines the fault current should strongly influence the type of bus protective relaying which is selected. Typically the dc time constants for the different circuit elements can vary from 0.01 s for lines to 0.3 s or more for generating plants.

The nearer a bus location is to a strong source of generation, the greater the L/R ratio and resulting dc component of fault current.

Impedance of Secondary Current Transformer Leads and Associated Relays, Meters, and Auxiliary Current Transformers 
Lead resistance, as well as current transformer winding resistance, contributes to core saturation. Therefore, length of secondary lead runs should be held to a minimum.

However, location of a breaker and its associated current transformer is determined by physical requirements. Hence, the planning stage must ensure that the type and rating of current transformer and its location with respect to the protected bus are comparable with the proposed bus protection scheme.

Where lead runs are excessive, an increase in wire size or use of parallel conductors are means to reduce lead resistance. Location of the differential relay junction points in the breaker yard rather than at the relay location is also practiced.

The preferred practice is to use current transformers dedicated to bus differential protection only. If possible, connection of meters, auxiliary current transformers, and other relays in differential type bus schemes should be avoided since these devices introduce additional burden into the current transformer circuit.

Current Transformer Accuracy Classification for Relaying
ANSI/IEEE C57.13-1978, Requirements for Instrument Transformers, establishes relaying accuracy classifications for relaying current transformers. Because relaying current transformers must operate at high overcurrent levels, ANSI classifications define minimum steady-state performance at those levels.

Performance is described by an identification system which consists of a letter and a number selected from the following: (C, T) (10, 20, 50, 100, 200, 400, 800). The first term describes performance in terms relative to construction.

The C classification covers bushing current transformers with uniformly distributed windings and any other current transformer in which leakage flux has negligible effect on ratio error. Ratio correction at any current can be adequately calculated by knowing the burden and the excitation characteristic.

The T classification covers most wound-type current transformers and any other current transformer in which leakage flux has an appreciable effect on ratio. Ratio correction must, therefore, be determined by test.

The C and T classification is applicable to all tap sections of a current transformer winding. The second term of the classification is the secondary terminal voltage rating.

It specifies the secondary voltage that can be delivered by the full winding at 20 times rated secondary current without exceeding 10% ratio correction. Furthermore, ratio correction is limited to 10% at any current from 10 to 20 times rated current at any lesser burden.


The ANSI secondary voltage rating applies only to the full winding. If other than the full winding is used, the voltage rating is reduced in approximate proportion to turns used.

As an example, relay accuracy Class C100 means that ratio correction can be calculated and that it will not exceed 10% at any current from I to 20 times rated secondary current if the burden does not exceed 1.0 W (1.0 W times 5 A times 20 times rated current equals 100 V).