Powerhouse

A powerhouse houses the turbines, generators, governors, control and auxiliary equipment. Types are classified by position relative to the ground.

1. Surface (open-air) powerhouse

Built on the ground surface, at the end of the penstock or at the dam toe.

  Penstock
      \
       \   +-------------+
        \  |  Generator  |
         \ |    floor    |
 ground   \|_____________|
 ~~~~~~~~~~|  Turbine    |~~~ tailrace
           |_draft tube__|

Variants: dam-toe / dam-integral (at the foot of or inside a gravity dam, short penstock), river-bed / in-stream (low head run-of-river; the powerhouse forms part of the weir), and canal/penstock-end (conventional, away from the dam).

Suitable where: rock or soil of good bearing capacity near the surface, gentle topography, no landslide, rockfall or flood risk, low to medium head, and cost advantage.

2. Semi-underground (partly buried) powerhouse

The substructure is in an excavated pit or shaft with the superstructure at ground level.

Suitable where: good rock at depth but steep or unstable surface, where the draft tube lies deep, or where protection from floods or snow is needed.

3. Underground (cavern) powerhouse

A large cavern excavated in sound rock, linked by a pressure shaft, access tunnel and tailrace tunnel.

 Surface
 ======= switchyard
   |  cable/vent shaft
   |     ___________
   |    | Generator |   Cavern
 Pressure| Turbine   |   (rock)
 shaft --|___________|--- tailrace tunnel
 access tunnel (adit)

Suitable where: high head and steep valleys, very high rock cover, landslide/avalanche or flood-prone surface, severe climate, security reasons, or where surface land is limited; also for large pumped-storage schemes. Needs good rock and higher investment.

Comparison

General arrangement of a surface powerhouse

A surface powerhouse has a substructure and a superstructure.

• Substructure: below the generator floor. It contains the draft tube, tailrace connection, spiral casing embedded in concrete, turbine pit, scroll case, drainage and dewatering sumps, and the foundation raft.
• Intermediate part: turbine floor, with turbine, governor, valves (main inlet valve) and gallery for pipes and cables.
• Superstructure: above the generator floor. It contains the generator hall with generators, the overhead travelling crane, the erection (service) bay, control room, switchgear and offices, with a roof and ventilation.

Other parts connected to it: penstock and inlet valve, tailrace channel, transformer yard and switchyard, access road and workshop.

   PLAN
 +-----------------------------+
 | Unit1  Unit2  Unit3 | Erect |
 | (G)    (G)    (G)   | bay   |
 +---------------------+ Ctrl  |
 | valves / galleries  | room  |
 +-----------------------------+
   penstocks -> | tailrace ->

Dimensions of the powerhouse

Dimensions are fixed from the following.

• Length $L$ = (number of units $\times$ unit bay width) + erection bay + end clearances. Unit bay width is about 3 to 4 times the runner diameter (governed by the spiral casing/generator size).
• Breadth $B$: width of the generator and spiral casing, valve/gallery space and a control/auxiliary side, usually 3.5 to 5 $D$ for Francis turbines.
• Height $H$: from the draft tube floor to the roof, made up of draft tube depth (about 2.5 to 3 $D$), the distributor/turbine floor, the generator, and the crane hook clearance needed to lift the largest part (generator rotor) with sling.
• Elevation: generator floor is set above the maximum tailwater flood level; the turbine level follows the setting $H_s$.

Section through a powerhouse with a vertical-axis Francis turbine

      roof
   ============================
   ::::  Overhead crane  ::::     <- crane rail
   |                          |
   |        GENERATOR         |   generator floor
   |__________  |  __________|   (G.F.)
   |  exciter / thrust bearing|
   |     Turbine floor        |   (T.F.)
 --+---   shaft  |   --- Governor
   |      Guide vanes         |
   |  Spiral    ( Runner )    |
   |  casing                  |
   |           \  /           |
   |  Draft tube \/           |   draft tube
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tailwater
 foundation raft
 Penstock --> Main inlet valve --> Spiral casing

Parts shown

1. Penstock and main inlet valve (butterfly or spherical valve) at the entry to the spiral casing.
2. Spiral (scroll) casing embedded in concrete, supplying water around the guide vanes.
3. Stay ring and guide vanes controlling the flow, operated by servomotors from the governor.
4. Runner (Francis) mounted on the vertical shaft.
5. Draft tube under the runner discharging to the tailrace under tailwater.
6. Turbine shaft, thrust and guide bearings.
7. Generator (umbrella or suspended type) on the generator floor, with exciter above or below.
8. Overhead travelling crane on rails spanning the hall.
9. Governor, oil pressure unit, cooling water, drainage and dewatering in the galleries below.
10. Superstructure (roof, walls) and the control room beside the generator floor.

Dimensioning of a powerhouse

The size of the powerhouse is determined by the equipment, its erection, maintenance and operation needs. The process:

1. Fix the number, type and rating of units and their principal sizes (runner diameter $D$, spiral casing, generator and draft tube dimensions) from manufacturers' data or empirical relations.
2. Fix the unit spacing (bay width) from the spiral casing and generator clearances, usually about 3 to 4 $D$.
3. Add the erection (service) bay and end clearances to get the length.
4. Fix the breadth from the width of the generator/turbine section plus galleries and auxiliary side.
5. Fix the height from the draft tube floor up to the crane rail and roof.
6. Check the levels against tailwater flood level and the turbine setting.
7. Check the layout for access, safety and expansion, and balance cost and operation.

Functional requirements fixing the dimensions

• Hydraulic and mechanical: sizes of spiral casing, draft tube, valves and runner; the setting level of the turbine (cavitation); minimum water cover and tailwater level.
• Generation: generator floor space, clearance for rotor removal, bearings and exciters.
• Erection and maintenance: an erection bay large enough for the heaviest part (the rotor or runner) and space for dismantling a unit; crane capacity and hook height to lift the largest piece over equipment.
• Control and switching: control room, switchgear, batteries and cable galleries close to the machines.
• Auxiliary services: cooling water, drainage, dewatering, compressed air, oil handling, ventilation and fire protection.
• Access and safety: corridors, stairs, emergency exits, loading/unloading space for trucks, and workshops.
• Structural: loads from machines and crane, vibration, flood protection and foundation conditions.
• Future expansion: extra bay for additional units.

Types of powerhouse

1. Surface (open-air) powerhouse: on the ground, at the dam toe or at the end of the penstock.
2. Semi-underground powerhouse: substructure in a pit or shaft, superstructure at ground level.
3. Underground (cavern) powerhouse: excavated in rock, connected by shaft and tunnels.

Plan of a typical powerhouse with three units

  <----------- Length ----------->
  +---------------------------------+  ^
  |  Unit 1   Unit 2   Unit 3  Erect| Breadth
  |  ( G )    ( G )    ( G )   bay  |  |
  |   ||       ||       ||    (EOT  |  |
  |  [MIV]    [MIV]    [MIV]  crane |  |
  +--||--------||-------||---------+  |
     Penstock branches        Control |  v
     (manifold)               room
  ------------------------------------
   Draft tubes -> tailrace ->

G = generator on a vertical shaft, MIV = main inlet valve. The three units sit side by side with equal spacing; the erection bay (with the loading area and the overhead travelling crane) is at one end; the control room, switchgear and offices lie along one side; the transformer yard is outside near the tailrace.

Surface powerhouse: suitable conditions

• Sound rock or good soil foundation near the surface, with enough bearing capacity.
• Gentle or moderate topography with enough space for the building, transformer yard and access road.
• Low to medium head, or a short penstock (dam-toe or river-side schemes).
• No risk of landslide, rockfall, avalanche or flooding beyond what can be protected against.
• Easy access and transport of heavy machines; cost, construction time and ease of maintenance matter most.
• Typical for most Nepali run-of-river schemes.

Underground powerhouse: suitable conditions

• Steep valley slopes where the surface has no flat area or is unstable (landslide, rockfall, avalanche or glacial hazard).
• Good, massive, strong rock with high rock cover and low groundwater inflow (self-supporting cavern).
• High head with a long, steep penstock or pressure shaft, so the cavern shortens the waterways.
• Surface exposed to extreme cold, snow or flood, or where security, environmental and land reasons favour hiding the plant.
• Large installed capacity (e.g. pumped-storage), where the extra excavation cost is justified.
• Where surface land is limited or valuable.

The parts of a powerhouse are grouped by what they do.

1. Water passage (hydraulic) parts

• Penstock and main inlet valve bring water to the machine and allow isolation.
• Spiral casing, stay ring and guide vanes distribute the water to the runner.
• Draft tube and tailrace discharge the water after the runner, recovering head.

2. Energy conversion parts

• Turbine and shaft: convert water energy to mechanical energy.
• Generator and exciter: convert mechanical to electrical energy.
• Governor and oil pressure unit: control speed by moving the guide vanes.

3. Electrical and control parts

• Transformers, switchgear, busbars and switchyard: step up and transmit power.
• Control room, relays, instrumentation, batteries and cables: monitoring, protection and operation.

4. Auxiliary services

• Cooling water, drainage and dewatering, compressed air, lubricating oil, fire protection, ventilation and lighting.

5. Handling and service parts

• Overhead travelling (EOT) crane and erection bay: for installation and maintenance of heavy parts.
• Workshop, store, offices, staff rooms, access roads and parking.

6. Structural parts

• Substructure (foundation, draft tube concrete), intermediate floors (turbine and generator floors) and superstructure (walls, roof, crane beams).

Assumptions

Total 10 MW with 2 units of 5 MW each, net head 80 m, vertical-shaft Francis turbines, $\eta_o = 0.88$.

$$Q = \frac{5000}{0.88 \times 9.81 \times 80} = 7.24\ \text{m}^3/\text{s per unit}$$

Rated speed 500 rpm ($N_s \approx 148$) and runner diameter $D \approx 1.06$ m.

Plan

  <-------------- 22 m --------------->
  +-----------------------------------+ ^
  | Unit 1     Unit 2     Erection   | |
  |  (G)        (G)         bay      | 11 m
  |  MIV        MIV      (loading)   | |
  +--||----------||-------------------+ v
   Penstock     Control + switchgear room
   -> Draft tubes -> tailrace

Section A-A (through one unit)

   roof truss  ========================
   crane rail  --- EOT crane ---
   |            GENERATOR             |  generator floor
   |__________  exciter  ____________|
   |      Turbine floor / governor    |
   |   Spiral casing ( runner )       |
 ~~|~~~~~~~~~~~ draft tube ~~~~~~~~~~~|~~ tailwater
   foundation raft
 penstock -> MIV -> spiral casing

Components shown

• Penstock, main inlet valve, spiral casing, stay ring, guide vanes, runner and draft tube.
• Generator, exciter and governor.
• Overhead crane (about 30 t, lifting the rotor), erection bay and loading area.
• Control room, switchgear, battery room, workshop and office; transformers outside.
• Approximate dimensions: length about 22 m, breadth about 11 m, height about 14 m (draft tube floor to roof).

A vertical-shaft arrangement places the turbine below and the generator directly above on the same vertical shaft. It is preferred, especially for medium and large Francis and Kaplan units, because:

1. Compact plan area: the units need less floor space, so the powerhouse is shorter and narrower, reducing excavation and concrete cost.
2. Better hydraulics: the spiral casing and draft tube are symmetric about the shaft, with a smooth vertical draft tube giving less loss and more uniform flow.
3. Better setting: the turbine can be kept low relative to tailwater, giving a larger submergence and reducing cavitation risk.
4. Direct, rigid coupling: the generator is placed above the flood level, safe from water and damp, and the weight of the rotating parts is carried by a single thrust bearing.
5. Easy erection and maintenance: generator and turbine can be lifted vertically by the overhead crane; the runner can be removed through the generator stator or draft tube access.
6. Suitable for large sizes and low speeds: large-diameter, low-speed generators suit a vertical shaft, with less bending in the shaft.
7. Less wear on bearings and shaft: no side thrust from rotor weight.

The disadvantages are a taller building and a higher cost of the thrust bearing, but these are accepted for large units. A horizontal shaft is generally used only for small units, Pelton wheels and bulb turbines.