Control of water delivery to turbine

In a hydro station, the water delivered to the turbine is controlled by the governing system. It keeps the speed (frequency) constant by adjusting the wicket (guide) gates in reaction turbines or the spear (needle) valve in Pelton turbines, so that water power matches the electrical load.

     speed set point
          |
          v
 +-------------+   +---------+   +-------------+
 | speed       |-->| pilot / |-->| main servo  |
 | governor    |   | relay   |   | motor (oil  |
 | (fly-ball/  |   | valve   |   | piston)     |
 | electronic) |   +---------+   +-------------+
 +-------------+                     |  moves
      ^                              v
      |                     +------------------+
      |  speed signal       | wicket gates or  |
      |                     | needle/deflector |
      |                     +------------------+
      |                              | water
 +----------+       shaft     +-------------+
 | generator|<================| turbine     |
 +----------+                 +-------------+
      ^ restoring (feedback) link to relay valve

Main components

1. Speed sensor / governor: fly-ball (mechanical) or electronic (PID) unit that senses speed.
2. Pilot (relay) valve: directs pressurised oil to one side of the servomotor.
3. Servomotor: a double-acting oil piston that moves the gate ring or needle; provides the large force needed.
4. Oil pressure unit: pump, accumulator and sump.
5. Restoring (feedback) mechanism: a linkage from the servomotor back to the relay valve so that the gates stop at the correct position (gives droop).
6. Wicket gates / needle valve and deflector, plus the main inlet valve (butterfly or spherical) for shutdown.

Working

1. Suppose the load increases. Speed falls a little.
2. The fly-balls move inward (or the electronic governor detects the error) and move the pilot valve.
3. Oil flows to the servomotor, which opens the wicket gates (or withdraws the needle). More water enters the turbine and torque rises.
4. The feedback link closes the pilot valve when the gates reach the new position, so hunting is avoided. Speed settles at a value fixed by the droop; the load reference can then bring it back to 50 Hz.
5. On load decrease, the action is reversed and gates close. Closing is done slowly to avoid water hammer. In Pelton turbines the deflector first turns the jet away from the buckets, and the needle then closes slowly.

Thus water delivery is matched to load while keeping frequency within limits and penstock pressure safe.