Speed governing of a hydro generator

The speed governing system of a hydro generator controls the turbine's water flow (wicket gate or needle opening) so that the speed, and hence the frequency, stays constant and load is shared between units running in parallel.

Components

• Speed sensor: a PMG or toothed wheel with a pick-up giving speed/frequency.
• Governor controller: electronic PID; compares the actual speed with the reference and includes droop (permanent speed regulation, usually 4–5 %).
• Electro-hydraulic converter / proportional valve: turns the electrical signal into oil flow.
• Pilot and main distributing valves, servomotor: move the wicket gates (Francis/Kaplan) or the needle and deflector (Pelton).
• Oil pressure unit: pump, accumulator and sump.
• Feedback: gate position transducer.

 f_ref -->( + )--> PID + droop --> E/H valve --> Servomotor
            ^ -                                     |
            |                                 Wicket gates
         Speed sensor <-- Generator <-- Turbine <---+

Working

• Load increases: the speed falls, the error opens the gates, more water flows and the speed recovers.
• Droop makes each unit pick up load in proportion to its rating.
• Hydro units need temporary (transient) droop (dashpot action) for stable control, because the inertia of the water column (water starting time $T_w$) makes the turbine respond slowly and at first in the opposite direction. The gate closing speed is also limited to keep water hammer within safe limits. Pelton turbines use a deflector for quick load rejection.