GE 845 Instruction Manual page 270

PROTECTION
4–142
Recommendations for configuring the Differential/Restraint characteristic
The following setpoints define the differential/restraint characteristic:
This setpoint is expressed in xCT of Winding 1 (J1 CT bank for relays
Minimum Pickup:
ordered with voltage, or K1 CT bank for relays ordered without voltage inputs). The
minimum pickup defines the minimum differential current required for operation. The
pickup setting must be higher than the amount of differential current that is seen under
normal transformer loading conditions, and must be higher than the maximum differential
current that is seen during transformer tap changer operation, providing the tap changer
automatic differential current compensation is disabled, i.e. the setpoint TC Winding
Currents from Tap Changer setup set to "None". If this setpoint is set to a current bank (J1,
K1, K2) representing any of the transformer winding currents, the Minimum Pickup can be
set to be more sensitive but not below the differential current caused by CT errors during
normal transformer loading.
For example, for a 20MVA, 25MVA, or 28MVA power transformer with a 138kV/4.16kV ratio,
and class C current transformers CT(w1) 200:5, and CT(w2) 3000:5, the CT errors and the
minimum pickup setting is calculated as follows:
Winding 1 (138kV) CT – reference. Magnitude factor: M(w1) = 1
Winding 2(4.16kV) CT – non reference. Magnitude factor: M(w2) = 4.16 *3000/138*200 =
0.4521
Based on emergency MVA rating:
Winding 1 rated current = 28MVA/(138kV*sqrt(3)) = 117 Amps
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS