ABB RELION 620 Series Technical Manual page 685

1MRS757644 F
4.5.8.6
620 series
Technical Manual
GUID-7626C453-0A4C-45D1-9850-AFE90F88D0CB V1 EN
Figure 344: Example of an inverse time counter operation if START occurs when
BLK_RESTART
Ena level setting is considered to be 40 percent.
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 low frequency in a system isolated from the main network can result in
overexcitation if the voltage-regulating system maintains a normal voltage.
is inactive while COOL_ACTIVE is active. The Restart
Section 4
Protection functions
679