Percent Differential - GE 845 Instruction Manual

PROTECTION

Percent Differential

4–142
• Volts Per Hertz
Power Elements
• Directional Power
Frequency Elements
• Underfrequency
• Overfrequency
• Frequency Rate of Change
The 845 relay provides one Percent Differential element per setpoint group. The setpoints
from the protection menu define a dual slope, dual breakpoint differential/restraint
characteristic. The calculation of differential and restraint currents are as shown in the
figure: Filtering and calculating transformer currents on page 5-2.
The 845 relay provides one instantaneous differential element per setpoint group.
The Instantaneous differential protection is not biased protection and operates similar to
the instantaneous overcurrent protection. Inputs to this protection are computed by the
relay per-phase differential currents. Operation occurs if any of the computed phase
differential current is above the instantaneous differential pickup setting.The
instantaneous differential protection is usually set to operate during high fault currents,
and if used, its pickup must be set according to the following criteria:
• The pickup setting (xCT) must be selected higher than the maximum inrush current
during transformer energization.
• The pickup setting must be set higher than the differential current caused CT
saturation during faults outside the zone of protection.
• The pickup setting must be selected lower than the maximum fault current during
internal faults.
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS