Governing of hydraulic turbines

Governing is the automatic regulation of the turbine speed to keep it constant (so that the generator frequency stays constant) when the electrical load varies. When load falls, the speed rises, and the governor reduces the water flow; when load rises, it increases the flow.

Principle

Output power $=\rho g Q H\eta$. With the head fixed, power is changed by changing the discharge $Q$:

• Pelton turbine: by moving the spear (needle) in the nozzle, and by the jet deflector for sudden load drops.
• Francis and Kaplan turbines: by changing the opening of the guide vanes (wicket gates); in Kaplan, runner blade angle is also adjusted.

Oil-pressure relay governor

 Speed        Pilot valve     Servomotor (relay cylinder)
 sensor  ->   +--+          +--------------------+
 (fly-ball   -|  |<-oil in  |  piston ->  rod ---+--> spear /
 or electric)  +--+         +--------------------+    gate ring
   ^             |  |                                     |
   |             v  v                                     |
   +---- feedback linkage (restoring) <-------------------+
   Oil sump <- gear pump -> oil under pressure

1. A fly-ball (or electronic) speed sensor driven from the turbine shaft senses the speed.
2. If the load decreases, the speed increases; the balls move outward and raise the sleeve, which moves the pilot (control) valve.
3. The pilot valve admits high-pressure oil (from a pump driven by the turbine) to one side of the servomotor piston; oil on the other side goes to the sump.
4. The piston moves and, through the linkage, moves the spear forward (Pelton) or closes the guide vanes (Francis, Kaplan), reducing discharge and bringing the speed back.
5. A feedback (restoring) mechanism returns the pilot valve to the neutral position as soon as the correct opening is reached, which prevents overshoot and hunting.
6. For a sudden large load rejection on a Pelton, the jet deflector acts first to divert the jet, and the spear closes slowly, so water hammer in the penstock is avoided. For reaction turbines, a surge tank and slow closing of gates do the same.

Because oil pressure supplies the force, the governor can handle a much larger power than a direct mechanical linkage could.

Qualities of a good governor

• Stability: returns the speed to normal without hunting (continuous oscillations).
• Sensitivity: responds to small speed changes.
• Quick response (promptness): small time lag in changing the gate opening, but not so fast that water hammer is caused.
• Small speed regulation (droop): the change of speed between no load and full load should be small, typically 3 to 5 %.
• Isochronism: ability to give exactly constant speed at all loads (zero droop). This is an ideal case. A permanent restoring (feedback) linkage gives some droop, so true isochronism needs temporary (dashpot) feedback or integral action, as in electronic governors. An isochronous governor cannot be used for parallel operation of generators.
• Reliability, simple maintenance and the ability to be used for parallel operation of generators (which needs a small droop).