84-42 9424200996
Settings Calculation Examples BE1-11m
conductors from both ends of the motor. When this is not possible, another method, percent differential is
used when conductor size prevents the use of flux balance CTs (i.e. motors exceeding 2,500 HP).
Percent differential mode requires a set of CTs on the terminal side and neutral side of the motor. It is
preferred that both sets of CTs are identical. A motor differential connection example is shown in
Figure 84-54.
Figure 84-54. Motor Differential Connection Example
This example uses percent differential mode. The motor must have all six stator leads brought out so that
CTs can be placed on the input and output leads of each stator winding.
The six CTs have matching characteristics, and for smaller operation currents (during running mode),
Restraint Slope 1 % can be less. However, during the starting mode, CT saturation is large. As a security
measure, choose a large Restraint Slope 2 % to avoid false trips during starting.
Set the Minimum Restrained Pickup to 0.1 for maximum differential sensitivity, which provides optimal
motor protection. This setting is in per units of tap. The minimum pickup current sensitivity is calculated in
Equation 84-35.
= = 2.0 0.1 = 0.2
Equation 84-35. Minimum Pickup Current Sensitivity
CT Circuit 1 Tap and CT Circuit 2 Tap are shown here, but calculated from the System Parameters,
Sensing Transformers screen (Figure 84-35). With equal CT ratios, the secondary currents through the
BE1-11m restraint algorithm for external faults and load are similar, and the operating current is very
small. If the differential CTs do not match, use the TAP settings on the System Parameters, Sensing
Transformers screen to balance the differential.
Running Slope
In running mode, the differential can be more sensitive, yet provide security and reliability at 15 percent
as described by IEEE C37.96-2000, IEEE Guide for AC Motor Protection. Set the Restraint Slope 1 %
setting to 15.
Starting Slope
The BE1-11m considers the motor to be starting when current is greater than 2 • FLA in any phase. The
starting mode ends when current in all three phases drops below 2 • FLA. Therefore, the 2
nd
Slope
Pickup (starting) should be set at the point that restraint current is 2 • FLA where motor torques and
heating settle quickly to running mode currents (as observed in typical time-current curves). See Equation
84-36.
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