Inadvertent Generator Energization - ABB REG670 Applications Manual

Section 12
Multipurpose protection
12.1.2.4
450
• 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
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.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Application manual