Purpose of governor and governing of Francis turbine

Purpose of a governor

A governor automatically regulates the flow of water to the turbine so that the speed stays constant (synchronous speed, giving 50 Hz) whatever the load on the generator. It:

• keeps the frequency constant when the load changes (voltage is held separately by the automatic voltage regulator);
• prevents runaway on load rejection;
• shares load among units in parallel;
• uses water only in proportion to the power demanded.

Governing of a Francis turbine

In a Francis turbine the discharge is controlled by the movable guide vanes (wicket gates) arranged around the runner. All guide vanes are linked to a regulating ring, which is rotated by an oil-pressure servomotor controlled by the governor.

 fly-ball/speed sensor
      |
   control (relay) valve <-- oil pump + accumulator
      |  pressure oil
   servomotor piston
      |
   regulating ring --> guide vanes (open/close)
      |
   feedback linkage -> back to control valve

Working (load decreases):

1. Turbine speed rises above normal; fly-balls move out (or the electronic sensor detects rising frequency).
2. The control valve opens a port, sending pressure oil to one side of the servomotor piston.
3. The piston moves and rotates the regulating ring, closing the guide vanes. The flow area and discharge decrease, and the power input falls to match the load.
4. Speed returns to normal; feedback brings the control valve back to neutral to prevent hunting.

When the load increases, the guide vanes open in the same way.

Relief (pressure regulating) valve: a Francis turbine has no jet deflector, so rapid closure of guide vanes on sudden load rejection would cause high water hammer in the penstock. A relief valve (synchronous bypass) connected to the spiral casing opens at the same time as the guide vanes close, diverting water to the tailrace. It then closes slowly, so the penstock flow is reduced gradually. On long penstocks a surge tank also helps.

Modern Francis units use electro-hydraulic PID governors, where an electronic controller senses frequency and drives the same oil servomotor.