Operational Features of Combined-Cycle Power Plants at Large Frequency Excursions

Abstract

Currently, the share of combined-cycle power plants (CCPP) in the total installed power of Russia has increased due to their advantages over conventional power plants. Therefore, it is necessary to have a clear idea of how CCPPs operate in different modes. Most of the CCPPs installed in Russia were produced by foreign companies, which increases the uncertainty of data on the operation of CCPPs. Modeling of CCPPs and their operational features at large frequency excursions are discussed. Low-frequency events are considered in detail. If the compressor and the turbine are direct coupled, a frequency decrease leads to a lower compressor speed and, consequently, to reduced air supply to the combustion chamber. As a result, the air/fuel ratio is distorted, which can cause an increase in the gas temperature. To recover the temperature, the temperature controller signals to reduce the fuel supply. Thus, upon frequency decrease, the CCPP can start reducing power, which can lead to an increase in power deficit and an emergency. These processes are modeled mathematically. To prevent emergencies, an algorithm for power boosting during low frequency events is proposed. It is shown that this algorithm helps to increase the permissible power deficit, keeping the gas temperature within the permissible range.