Inadvertent Generator Energization - ABB 650 series Applications Manual

Section 6
Multipurpose protection
6.1.2.4
78
• Rotor Overload protection
• Loss of Excitation protection (directional pos. seq. OC protection)
• Reverse power/Low forward power protection (directional pos. seq. OC
protection, 2% sensitivity)
• 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 pos. seq. OC)
• Inadvertent generator energizing

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
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