Dynamic modeling of thermal-supply system for two-by-one combined-cycle gas and steam turbine unit

摘要

The two-by-one (2 x 1) combined-cycle gas turbine (CCGT) unit with condensation-extraction-backpressure (C-E-B) steam turbine has become the major means of peak regulation in China because of its flexible operating mode. However, mode switching of the unit often causes drastic fluctuation of the water level of heating network thermos-exchanger and jeopardizes unit safety. In order to study the suppression mechanism of the water level fluctuation in the transient mode of switching, first a dynamic mathematical model of the thermal-supply system of the C-E-B steam turbine is developed based on a mixed approach of theory and identification. Then field data is used to verify model accuracy. Through step response analysis on the heat-supply system, the relationship between the extraction flow to the thermos-exchanger and its water level is identified along with the major influential factors. On the heat-demand system, the circulating water flow through heating network influences the heat transfer of thermos-exchanger, the exhaust steam pressure of turbine, the extraction flow and the water level in sequel. Combining the factors from the two systems arrives at the conclusion that the circulating water flow stabilizes the water level during mode switching process and provides theoretical basis for water-level controller design.