Primary frequency regulation stability of parallel turbo-generators

It is a noticeable phenomenon in power system frequency regulation that a turbine generator being unstable in island condition always becomes stable when paralleled to a power system. After the detailed investigation of this phenomenon, it is found that an unstable generator becomes stable, when and only when connected to a system with enough stability margins in the primary frequency regulation. As a result, the stability margin of the system will decrease to a certain extent. It is indicated that the stability margin of a power system mainly comes from the reheat units, the units that do not join in the primary frequency regulation and the units using turbine follow-up control.