At present time the combined cycle power plants (CCPP) share increases in Russian total installed power due to some advantages in comparison with traditional power installations. In connection with this fact it is necessary to have clear idea about CCPP operation in different modes. The most part of CCPP installed in Russia is produced by foreign companies, that increases information uncertainty regards to CCPP operation. The paper contains data about CCPP modelling and operation features under large frequency excursions. Low frequency events are considered in detail. If the compressor is on the one shaft with turbine, frequency decrease result in compressor speed decrease and, consequently, in reduced air supply to combustion chamber. Then the air/fuel ratio is broken, that results in gas temperature increase. The temperature controller will give a signal for fuel decrease for the purpose of temperature value restoration. Thus, CCPP will decrease power that results in deficit increase and emergency development. The math model of these processes is presented in the paper. For emergency development prevention the power boosting algorithm under low frequency events is proposed. It is shown that such algorithm helps to increase the permissible deficit. Herewith the gas temperature is inside permissible range.