ABB RET670 Applications Manual page 560

Section 3
IED application
554
• Turn-to-Turn & Differential Backup protection (directional Negative
Sequence. Overcurrent protection connected to generator HV terminal CTs
looking into generator) (67Q)
• Stator Overload protection (49S)
• Rotor Overload protection (49R)
• Loss of Excitation protection (directional pos. seq. OC protection) (40)
• Reverse power/Low forward power protection (directional pos. seq. OC
protection, 2% sensitivity) (32)
• Dead-Machine/Inadvertent-Energizing protection (51/27)
• Breaker head flashover protection
• Improper synchronizing detection
• Sensitive negative sequence generator over current protection and alarm (46)
• Phase or phase-to-phase or Negative/Positive/Zero Sequence over/under
voltage protection (27x, 59x, 47)
• Generator out-of-step detection (based on directional pos. seq. OC) (78)
• Inadvertent generator energizing
Inadvertent generator energization
When the generator is taken out of service, and stand-still, 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.
1MRK504116-UUS C
Application manual