Schematic of a PROR hydropower plant

A Peaking / Pondage Run-of-River (PROR) plant is a run-of-river scheme with a small daily storage (pondage) behind the diversion dam. Water is stored during low-demand hours and used to generate full power during peak hours. It combines the low cost of ROR with peaking capability.

Schematic

  River ->[ Diversion dam + radial gates ]-> Spillway
           |  Pondage (few hours storage) |
           +------------------------------+
                 |                 |
         [Intake + trash rack]   Riparian
                 |               release
        [Gravel trap / Desander]
                 |
       Headrace tunnel -------> [Surge tank]
                                     |
                              Penstock / shaft
                                     |
              +----------[ Powerhouse ]----------+
              | MIV -> Turbine -> Generator      |
              | Governor, Exciter, Control room  |
              +----------------------------------+
                 |                     |
              Tailrace           Transformer
                 |                     |
               River        Switchyard -> Line

Description of components

1. Diversion dam/weir: Concrete or rockfill structure across the river that raises the water level and creates the pondage. Gates control water level.
2. Pondage: Small reservoir that stores water for a few hours (e.g. 4–6 h) so that the plant can run at full capacity during the evening peak.
3. Spillway and undersluice: Spillway passes flood flow safely; undersluice flushes sediment from the pondage.
4. Intake: Admits the design discharge into the water conductor; has a trash rack to stop debris and gates for closing.
5. Desander (settling basin): Settles sand and silt so that turbine runners are not eroded.
6. Headrace tunnel/canal: Conveys water from intake to surge tank at a gentle slope.
7. Surge tank: Vertical shaft that absorbs water hammer when turbines close suddenly and supplies water when load increases.
8. Penstock/pressure shaft: Steel or lined pipe that carries water under high pressure to the turbines; branches into each unit.
9. Powerhouse: Building housing main inlet valves, turbines (Francis/Pelton depending on head), generators, governors, excitation, control and protection equipment, cranes and auxiliaries.
10. Turbine: Converts the energy of water into mechanical rotation.
11. Generator: Synchronous machine converting mechanical power to electrical; excitation and AVR control voltage.
12. Tailrace: Channel or tunnel that returns water to the river.
13. Transformer and switchyard: Step up the generator voltage (e.g. 11 kV to 132 kV) and connect the plant to the transmission line with breakers, isolators and arresters.
14. Auxiliary systems: Station service transformer, DC battery, diesel generator for black start, cooling water, fire protection, SCADA.

Examples in Nepal: Kaligandaki A (144 MW), Middle Marsyangdi (70 MW), Upper Tamakoshi (456 MW) are PROR plants.