9424200996 84-5
BE1-11m Settings Calculation Examples
System Parameters - Sensing Transformers
Phase CT Ratios
The motor manufacturer data for full-load amps (FLA) is 363 A under normal running conditions. Select
phase CTs so that the FLA is 50 percent or greater than the rated phase CT primary current. To minimize
CT saturation, select the phase CT primary current so the FLA is 100 percent of the phase CT primary or
slightly less, never more. IEEE states that “CT saturation is minimized by keeping burdens low (especially
the dc resistive component), by using high-ratio current transformers, and by selecting current
transformers with a high knee-point (saturation) voltage.” (IEEE Std C37.96-2000, IEEE Guide for AC
Motor Protection). CTs with a ratio of 80 (400:5) are used in this application example. The secondary
current measured by the BE1-11m is 4.54 A when the motor is at full load. Use Equation 84-1 to calculate
the secondary full load amps. A CT with an accuracy class of C400 provides a satisfactory knee-point
voltage for the motor FLA. Enter 80 for CT ratios on the System Parameters, Sensing Transformers
screen shown in Figure 84-5.
363 ( )
80
(
)
= 4.54
Equation 84-1. Secondary Full Load Amps
Phase VT Setup
In this example, the phase VT connection is 3-wire, delta. The system voltage is 4 kV. The phase VT ratio
is calculated in Equation 84-2.
=
4000
120
= 33.32
Equation 84-2. Phase VT Ratio
Use the Settings Explorer in BESTCOMSPlus to navigate to the System Parameters, Sensing
Transformers screen shown in Figure 84-5. Set Phase VT Ratio to 33.20 and set Phase VT Connection to
3W-D.
Figure 84-5. System Parameters, Sensing Transformers Screen
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