•
Special thermal overload protection
•
Open Phase protection
•
Unbalance protection
2.
Generator protection
•
80-95% Stator earth fault protection (measured or calculated 3Uo)
•
Rotor earth fault protection (with external COMBIFLEX RXTTE4 injection unit)
•
Underimpedance protection
•
Voltage Controlled/Restrained Overcurrent protection
•
Turn-to-Turn & Differential Backup protection (directional Negative Sequence.
Overcurrent protection connected to generator HV terminal CTs looking into
generator)
•
Stator Overload protection
•
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
11.1.2.4
Inadvertent generator energization
SEMOD151947-4 v2
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
1MRK 505 343-UEN B
Section 11
Multipurpose protection
497
Application manual
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