GE 745 Instruction Manual page 160

S4 ELEMENTS
5–64
• W1(3) HARMONIC DERATING CORRECTION: Select "Enabled" to enable automatic
harmonic derating correction of the winding 1(3) phase time overcurrent curve. The
745 calculates the derated transformer capability when supplying non-sinusoidal
load currents (as per ANSI / IEEE C57.110-1986) and, when this feature is enabled,
automatically shifts the phase time overcurrent curve pickup in order to maintain the
required protection margin with respect to the transformer thermal damage curve, as
illustrated below.
time
FIGURE 5–15: Harmonic derating correction
FIGURE 5–16: Phase time overcurrent scheme logic
Transformer thermal
damage curve
Selected relay time
overcurrent curve
Transformer thermal
protection margin
current
Pickup setting based
rated load capability
Pickup shifted based
on harmonic derating
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS