What means of Isolated in generation system?

When more than one generating unit is paralleled to supply a load, none of the units can be truly "isolated". They all work together at the same voltage and frequency constraints.

A utility typically has a large number of paralleled units, providing a large power base. Local facilities may have their own co-generation equipment and/or emergency backup generation.

If the facility generation system operates while the transmission grid is also supplying power, then the facility units are in parallel with the utility. This is NOT an "isolated" or "islanded" operation mode.

If the facility generation system is not normally connected to the transmission grid - but CAN be - then the facility system is considered to be in "isolated" operation mode. Note that if the facility generation system comprises more than a single generating unit, NONE of the individual units can be considered to be "isolated" or "islanded" within the facility distribution network.

If the facility generation system cannot be connected to the transmission grid the facility system is considered to be in "islanded" operation mode. Note that if the facility generation system comprises more than a single generating unit, NONE of the individual units can be considered to be "isolated" or "islanded" within the facility distribution network.

In large grids, almost constant voltage and constant frequency is assumed, because impact on U and f from single small generator is negligible. You are dealing with similar situation very large power plants have. Although they do operate on grid, but their impact on U and f is observed. Governing such small system is hard, because it's not single machine isolated (you keep f and U constant) and it's not full paralleling (you keep P and Q constant), it's something in between. In old days all systems were such, few power plants paralleling on same grid, so expertise how to do it exists.