ABB RELION REX640, Техническое руководство, Страница: 893 / 1960

Figure 499: Example of an inverse time counter operation if START occurs when
BLK_RESTART is inactive while COOL_ACTIVE is active.
4.5.7.7 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.
Overexcitation protection for the transformer is generally provided by the generator
overexcitation protection, which uses the VTs connected to the generator terminals.
The curves that define the generator and transformer V/Hz limits must be
coordinated properly to protect both equipment.
If the generator can be operated with a leading power factor, the high-side voltage
of the transformer can have a higher pu V/Hz than the generator V/Hz. This needs
1MRS759142 F Protection functions
REX640
Technical Manual
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