APPLICATION

In general, power transformers have different values of current flowing through their primary, secondary,

and tertiary windings. These currents have specific phase relationships depending upon the connections

of the individual windings (e.g., wye/delta). As inputs to a differential relay, these currents must be

compensated or scaled so that the relay can compare the inputs and determine when an unbalance

exists. Under ideal operating conditions, the scaled vector sum of these currents is zero.

Because it is practically impossible to match the magnitudes of these detected currents from the various

power windings using standard CT ratios, the currents are matched within the relay by scaling each of the

applied currents by an appropriate factor called a Tap Setting. This is set by means of the front panel

INPUT dials. By selecting suitable tap ratios, the applied currents are scaled within the relay to achieve

the desired balance for normal operating conditions. BE1-87T relays offer a range of available tap settings

for inputs between 0.4 A to 1.78 A (Sensing Input Range Options 2 and 4 for a 1 A CT) or 2.0 to 8.9 A

(Sensing Input Range Options 1 and 3 for a 5 A CT). These settings are independently adjustable in

increments of 0.02 A for Sensing Input Range Options 2 and 4. or 0.1 A for Sensing Input Range Options

1 and 3. These small increments allow more precise scaling of the applied currents and usually eliminate

the need for installing auxiliary ratio-matching CTs.