Hydraulic turbine transient performance

[kippie]

Hi

I am having difficulty explaining to my mechanical engineering supervisors the following:

When a pumped storage unit is in generation mode, synchronised to the grid and in steady state and the guide vanes/wicket gates are opened more to allow more water flow and therefore more power generation, the pressure across the Francis turbine initially decreases before increasing. The pressure is related to the power so therefore there is a power dip first then a increase in power.

Similarly, when the guide vanes are closed to decrease power, the power/pressure first increases before decreasing after a few seconds.

The transfer function of a hydraulic turbine shows this phenomenon with the use of initial value theorem and final value theorem.

Can you help to explain why this happens?
Thanks

 

[AlephZero]

I'm not an expert on hydraulic turbines, but one possible cause is that to get an increased flow, you have to accelerate the water that is already flowing through the inlet pipes (the penstock). While that is happening, the pressure at the inlet of the turbine will be reduced. The reverse would happen when you reduce the flow.