When the power system is island, the P/f algorithm will
determine the total power required to maintain the system
frequency at its nominal value. When however, the power
system is connected to the local utility, the frequency is fixed
by the utility. The reason why the frequency is fixed by the
utility is the fact that the utility has thousands of installed MW
of power generation whereas the industrial plant only a few
hundred. In the connected mode the PMS will determine the
total power required to ensure that the import/export of active
power is within the required limits.