The major damaging condition that can be imposed on the
synchronous and induction generators by the utility system is
single-phasing. Negative sequence current (46) and voltage
protection (47) is provided to detect this condition. Negative
sequence voltage detection prevents re-synchronizing of the
generator after a tripping if an open conductor condition exists.
Out-of-step (78) protection is used to detect loss of synchronism
for diesel generators, which have very low inertia and can lose
synchronism if subjected to sustained low voltage caused by a
slow-clearing fault.