When a DG separates from the power system, it generally
cannot be islanded with utility loads external to the DG facility.
This creates restoration problems for the utility as well as potential
power quality issues. The DGs typically cannot maintain voltage,
and in many cases, cannot maintain frequency and harmonics
within acceptable levels at utility loads external to the DG facility.
Reclosing to restore the circuit is also greatly complicated and
would require synchronizing equipment. Fig. 1 illustrates the
islanded operation of a DG with the utility system that is generally
not allowed.