4.6 TURBINE MODEL

For a simple non-reheat type turbine, the model is given by a single time constant but reheat turbines have more than one time constant. If a reheat unit has two-stage steam turbines, the dynamic response will be influenced by:

1. The entrained steam between the steam inlet valve and first stage of turbine and
2. The storage action in the reheater which causes the outlet of the L.P stage to lag behind that of the H.P stage (See Fig. 4.6).

Fig 4.6 Two-stage re-heater unit

The turbine transfer function is characterized by two time constants. For the purpose of analysis, it can be assumed that the turbine in modeled by a single equivalent time constant T_t, which lies between 0.2 to 2.5 sec. Block diagram representation of the turbine model is shown in Fig 4.4.