Components of a medium head high discharge plant

A medium head (about 30–300 m), relatively high discharge plant is usually a run-of-river or pondage run-of-river scheme with a diversion weir/dam, a long large-diameter water conductor, a surge tank and a powerhouse with Francis turbines.

Layout

 Reservoir/headpond
  ____  Intake
 |    |__[TR]___ Headrace tunnel _________
 |Dam/|  (gate)                            \  Surge
 |weir|                                     \ tank
 |____|--> Spillway        Desander         [ST]
   |                                          |
   +-> Riparian release            Penstock   |
                                       \______|
                                        \
                                   [Powerhouse]
                                   Turbine+Gen
                                        |
                                    Tailrace -> River
                                        |
                              Switchyard -> Grid

Components and functions

A. Civil components

1. Diversion weir / dam: Raises the water level and diverts flow to the intake; may create a small pondage.
2. Spillway and gates: Pass flood water safely; undersluice gates flush sediment near the intake.
3. Intake with trash rack: Draws the design discharge into the conveyance; the trash rack stops floating debris; gates control flow.
4. Gravel trap and settling basin (desander): Remove sand and silt (particles above about 0.2 mm) to protect the turbines from abrasion — very important in Himalayan rivers.
5. Headrace tunnel / canal: Carries the large discharge at a mild slope to the forebay/surge tank with low head loss.
6. Surge tank (or forebay): Absorbs pressure rise (water hammer) during load rejection and supplies water during sudden load increase; protects the tunnel.
7. Penstock / pressure shaft: Carries water under pressure from the surge tank to the turbines; ends in a manifold for each unit, with a valve.
8. Powerhouse: Houses turbines, generators, control and auxiliary systems (surface or underground).
9. Tailrace: Returns water from the draft tube to the river.

B. Mechanical components 10. Main inlet valve (butterfly/spherical): Isolates each turbine. 11. Francis turbine (suitable for medium head, high flow) with spiral casing, guide vanes and draft tube to recover kinetic energy. 12. Governor: Controls guide-vane opening to keep speed/frequency constant. 13. Cooling water, drainage, dewatering and EOT crane.

C. Electrical components 14. Synchronous generator with excitation system and AVR. 15. Generator circuit breaker, generator bus/leads, CTs and PTs. 16. Step-up (generator) transformer: Raises voltage (e.g. 11 kV to 132 kV). 17. Switchyard: Breakers, isolators, busbars, lightning arresters for connection to the transmission line. 18. Control, protection and SCADA; station auxiliary transformer; DC battery system; diesel generator for black start; earthing system.