Conventional gas turbine secondary air system in early stage typically uses a fixed throttle unit between supply side which is compressor bleeding point and demand side which is turbine blade. The cooling air mass flow strongly depends on the extraction pressure characteristics of compressor. Optimal amount of cooling air is supplied only in design point in this way. The cooling air mass flow would be either too much or too less in off design condition.
Recently, heavy duty gas turbine manufacturers introduced an active control method for secondary air system. The main strategy is to adjust the cooling air valve set point as a function of gas turbine load percentage in order to adjust cooling air pressure ratio and cooling air mass flow as well. With this active control strategy, cooling mass flow is separated from compressor extraction pressure characteristics, and it can provide a better way to deal with combustion contaminant issues. But it is still a problem that there is no dependence relationship between cooling air valve set point and operating ambient temperature in that strategy. That is to say, the cooling air pressure ratio is constant while varying ambient temperature at base load.
In order to quantitatively analyze this phenomena, a 1-dimensional integrated gas turbine thermodynamic analysis method is first applied to obtain the extraction pressure characteristics of compressor for all bleeding points. In the meantime, the optimal cooling air mass flow for turbine blades in different operating conditions is evaluated by a 0-dimensional heat transfer assessment method. A 1-dimensional fluid network analysis method is then employed to calculate the cooling air mass flow variation characteristics for 2 typical throttle configurations between compressor bleeding points and turbine blades, the first one is setting a fixed throttle unit, and the second one is setting constant cooling air pressure ratio by a cooling air control valve. Quantitative calculation results show that the cooling air supply will not always meet the optimal requirements at different ambient temperature conditions with neither of the 2 configurations.
This paper further optimized the active control strategy. With the optimized strategy, cooling air supply not only no longer depends on extraction characteristics of compressor, but also could be actively adjusted according to the optimal requirements of turbine blades at different ambient temperature conditions. Performance evaluation results show that the optimized active control strategy could enhance the overall efficiency without exceeding maximum allowable metal temperature of turbine blades.
Advanced control strategy for a digital mass flow controller
