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1MRK 505 370-UEN D
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10.1.2.4
Inadvertent generator energization
When the generator is taken out of service, and non-rotating, there is a risk that the generator
circuit breaker is closed by mistake.
Three-phase energizing of a generator, which is at standstill or on turning gear, causes it to
behave and accelerate similarly to an induction motor. The machine, at this point, essentially
represents the subtransient reactance to the system and it can be expected to draw from one
to four per unit current, depending on the equivalent system impedance. Machine terminal
voltage can range from 20% to 70% of rated voltage, again, depending on the system
equivalent impedance (including the block transformer). Higher quantities of machine current
and voltage (3 to 4 per unit current and 50% to 70% rated voltage) can be expected if the
generator is connected to a strong system. Lower current and voltage values (1 to 2 per unit
current and 20% to 40% rated voltage) are representative of weaker systems.
Since a generator behaves similarly to an induction motor, high currents will develop in the
rotor during the period it is accelerating. Although the rotor may be thermally damaged from
excessive high currents, the time to damage will be on the order of a few seconds. Of more
critical concern, however, is the bearing, which can be damaged in a fraction of a second due
to low oil pressure. Therefore, it is essential that high speed tripping is provided. This tripping
should be almost instantaneous (< 100 ms).
There is a risk that the current into the generator at inadvertent energization will be limited so
that the "normal" overcurrent or underimpedance protection will not detect the dangerous
situation. The delay of these protection functions might be too long. The reverse power
protection might detect the situation but the operation time of this protection is normally too
long.
For big and important machines, fast protection against inadvertent energizing should,
therefore, be included in the protective scheme.
The protection against inadvertent energization can be made by a combination of
undervoltage, overvoltage and overcurrent protection functions. The undervoltage function
will, with a delay for example 10 s, detect the situation when the generator is not connected to
the grid (standstill) and activate the overcurrent function. The overvoltage function will detect
the situation when the generator is taken into operation and will disable the overcurrent
function. The overcurrent function will have a pick-up value about 50% of the rated current of
the generator. The trip delay will be about 50 ms.
10.1.3
Setting guidelines
Busbar protection REB670
Application manual
Dead-Machine/Inadvertent-Energizing protection
Breaker head flashover protection
Improper synchronizing detection
Sensitive negative sequence generator over current protection and alarm
Phase or phase-to-phase or Negative/Positive/Zero Sequence over/under voltage
protection
Generator out-of-step detection (based on directional positive sequence OC)
Inadvertent generator energizing
When inverse time overcurrent characteristic is selected, the operate time of
the stage will be the sum of the inverse time delay and the set definite time
delay. Thus, if only the inverse time delay is required, it is important to set the
definite time delay for that stage to zero.
Section 10
Multipurpose protection
SEMOD151947-4 v2
IP15009-1 v2
GUID-F7AA2194-4D1C-4475-8853-C7D064912614 v4
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