Application - ABB 615 series Technical Manual

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1MAC059074-MB A

615 series ANSI

Technical Manual

GUID-E62B7A83-72F7-4489-B7BC-DE30D26BF6C7 V1 EN

Example of an inverse time counter operation if TRIP occurs when

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

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

is inactive while COOL_ACTIVE is active.

Protection functions

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