Unbalanced loading on Generators occurs when the loading on the three phases is not equal. This occurs when the generators are used to power single phase loads such as domestic lighting. Some three phase loads such as induction furnaces and drives can also create unbalanced loading.
When unbalanced loading occurs, the negative sequence component increases. Referred as I2 , this component rotates in a direction opposite to the rotating magnetic field of the alternator. For the alternator rotor, this component rotates at double the frequency of the alternator. This induces heat in the rotor of the alternator.
Every alternator has a limit for unbalanced loading or the negative sequence limit.
This should not be exceeded. The load should be studied prior to selection of the alternator. Negative sequence relays also help trip the alternators if the unbalance limit is exceeded.