GUID-E62B7A83-72F7-4489-B7BC-DE30D26BF6C7 V1 EN
Figure 324:
Example of an inverse time counter operation if
TRIP
occurs when
BLK_RESTART
is inactive while
COOL_ACTIVE
is active.
4.5.6.6
Application
If the laminated core of a power transformer or generator is subjected to a magnetic flux
density beyond its designed limits, the leakage flux increases. This results in a heavy
hysteresis and eddy current losses in the non-laminated parts. These losses can cause
excessive heating and severe damage to the insulation and adjacent parts in a relatively
short time.
Overvoltage, underfrequency or a combination of the two, results in an excessive flux
density level. Since the flux density is directly proportional to the voltage and inversely
proportional to the frequency, the overexcitation protection calculates the relative V/Hz
ratio instead of measuring the flux density directly. The nominal level (nominal voltage at
nominal frequency) is usually considered as the 100 percent level, which can be exceeded
slightly based on the design.
The greatest risk for overexcitation exists in a thermal power station when the generator-
transformer unit is disconnected from the rest of the network or in the network islands
where high voltages or low frequencies can occur.
Overexcitation can occur during the start-up and shutdown of the generator if the field
current is not properly adjusted. The loss-of-load or load shedding can also result in
overexcitation if the voltage control and frequency governor do not function properly. The
1MAC059074-MB A
Section 4
Protection functions
615 series ANSI
651
Technical Manual
Summary of Contents for Relion 615 series
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