GE 745 Instruction Manual page 179

CHAPTER 5: SETPOINTS
5.6.11 Overexcitation
5.6.11.1 Main Menu
 OVEREXCITATION [
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
• OVERFREQUENCY DELAY: Enter the time that the frequency must remain above the
pickup level before the element operates.
FIGURE 5–32: Overfrequency scheme logic
PATH: SETPOINTS S4 ELEMENTS

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]  5th HARMONIC [
LEVEL
 VOLTS PER
MESSAGE
HERTZ 1

VOLTS PER
MESSAGE
HERTZ 2
A transformer is designed to operate at or below a maximum magnetic flux density in the
transformer core. Above this design limit the eddy currents in the core and nearby
conductive components cause overheating which within a very short time may cause
severe damage. The magnetic flux in the core is proportional to the voltage applied to the
winding divided by the impedance of the winding. The flux in the core increases with either
increasing voltage or decreasing frequency. During startup or shutdown of generator-
connected transformers, or following a load rejection, the transformer may experience an
excessive ratio of volts to hertz, that is, become overexcited.
When a transformer core is overexcited, the core is operating in a non-linear magnetic
region, and creates harmonic components in the exciting current. A significant amount of
current at the 5th harmonic is characteristic of overexcitation.
OVEREXCITATION
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See page 5–84
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See page 5–85
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See page 5–85
S4 ELEMENTS
5–83