GE 869 Instruction Manual page 244

PROTECTION
4–112
Each pair of measured restraining and differential currents represents a point on the
previous figure: Percent Differential Characteristic. If this point is located above the limit
line (area labeled "Operate"), then the differential flag is set (DIF=1). If the point is located
below or on the limit line (area labeled "Block"), then the differential flag is reset (DIF=0).
Core Balance Method
The Percent Differential element with core balance method selected does not use the dual
slope dual break point differential/restraint characteristic. Instead, unrestraint differential
measured directly from a core balance CT is used in this element. The differential element
operates as per the equations shown below, where r is the restraint factor on motor
starting, a user setting, and STARTING is a flag (either equal to 1 or equal to 0), which is an
indication whether the motor is starting. Note that the setpoints related to biased
differential with internal summation method, including slopes and break points, are not
used in core balance method.
Where I is the terminal side CT current phasor in per unit based on the terminal CT
p
nominal; I is the core balance CT differential current phasor in per unit based on the
dcb
core balance CT nominal. They are calculated as shown:
Where,
Setpoints
Path:
Setpoints > Protection > Group 1 > Motor > Percent Differential
FUNCTION
Range: Disabled, Trip, Configurable
Default: Disabled
TYPE
Range: Internal Sum, Core Balance
Default: Internal Sum
The setting defines the method used for differential protection and requires proper CT
connections per method employed. Refer to the wiring diagram in the Electrical
Installation chapter.
PICKUP
Range: 0.05 to 1.00 x CT in steps of 0.01
Default: 0.20
The setting is applicable to both the internal summation method and core balance
method. In addition, this setting defines the minimum differential current required for
operation. The setting is based on the amount of differential current that can be seen
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
Eq. 4
Eq. 5