Purpose of governing hydraulic turbines

Governing of a hydraulic turbine is the automatic regulation of the flow of water to the turbine (by guide vanes, nozzle spear or runner blades) so that its speed remains constant when the load on the generator changes.

Why governing is necessary

1. Constant frequency: the generator is coupled directly to the turbine, and frequency $f = \dfrac{PN}{120}$. Electrical loads (motors, clocks, electronics) need constant frequency (50 Hz in Nepal), so the speed must stay at synchronous speed.
2. Load changes all the time: consumer demand varies through the day. If the load falls and water input stays the same, the extra energy accelerates the runner; if the load increases, the turbine slows down. Water input must follow the load.
3. Prevent runaway: on sudden full-load rejection (e.g. a line trip), an ungoverned turbine speeds up to runaway speed (1.8–2.5 times normal), causing dangerous centrifugal stresses and damage to generator and runner.
4. Constant voltage and stable parallel operation: speed variation affects voltage and the synchronism of machines running in parallel on the grid; governors share load among units in proportion to their droop settings.
5. Economy of water: water is used only in proportion to the power demanded, which saves stored water.
6. Safety against water hammer: governors close guide vanes or nozzles at a controlled rate, and use deflectors/relief valves, so that pressure rise in the penstock stays within limits.

 load drops -> speed rises -> governor senses ->
 closes guide vanes/spear -> less Q -> speed normal