If voltages are not exactly the same in both lines (i.e. in a and a’, b and b’ etc.), a
very large current will flow when the switch is closed. Therefore, to avoid this,
voltages coming from both generators must be exactly the same. Therefore, the
following conditions must be met:
1.The rms line voltages of the two generators must be equal.
2.The two generators must have the same phase sequence.
3.The phase angles of two a phases must be equal.
4.The frequency of the oncoming generator must be slightly higher than the
frequency of the running system.