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Section 4
1MAC050144-MB C
Protection functions
256
615 series ANSI
Technical Manual
If the excitation level increases above the set value when BLK_RESTART is not active
but COOL_ACTIVE is active, the TRIP output is not activated instantly. In this case, the
remaining part of the cooling timer affects the calculation of the trip timer as shown in
Figure 122. This compensates for the heating effect and makes the overall trip time shorter.
Figure 122:
Example of an inverse time counter operation if TRIP occurs when
BLK_RESTART is not active but COOL_ACTIVE is active. (The Restart
Ena level setting is considered to be 40 percent)
4.2.6.6
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 startup 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
PICKUP
TRIP
Cooling time
Trip level
Inverse time
counter
time
20%
100%
PICKUP
TRIP
deactivates
40%
BLK_RESTART
deactivates
COOL_ACTIVE
deactivates
Содержание 615 Series ANSI
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Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
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