8.6 Hydro-electric Machines and Powerhouse

8.6 Hydro-electric Machines and Powerhouse

Introduction to Hydro-electric Machines and Powerhouse

The powerhouse is the heart of a hydropower plant where the conversion of hydraulic energy into electrical energy occurs. This process is facilitated by a sophisticated assembly of hydro-mechanical and electro-mechanical equipment. The selection, design, and arrangement of these components—turbines, generators, governors, and auxiliary systems—are critical for achieving high efficiency, reliability, and economic viability. This section provides a comprehensive overview of the machinery and the structure that houses them.

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1. Hydro-mechanical Equipment and Their Functions

1. Turbine: Converts the hydraulic energy (pressure and kinetic energy) of water into mechanical rotational energy.

2. Generator: Converts the mechanical energy from the turbine shaft into electrical energy.

3. Governor: The control system that regulates turbine speed and power output by adjusting the flow of water to the turbine (via guide vanes or spear nozzle) in response to changes in electrical load.

4. Main Inlet Valve:

   • Butterfly Valve (medium head) or Spherical Valve (Rotary Valve) (high head).

   • Located on the penstock just before the turbine.

   • Functions: Isolate the turbine for maintenance; emergency shut-off.

5. Draft Tube (for Reaction Turbines): A diverging conduit at the turbine outlet that recovers kinetic energy and reduces the pressure at the runner exit below atmospheric.

6. Scroll Case (Spiral Casing): Distributes water evenly around the periphery of a reaction turbine (Francis, Kaplan).

7. Trash Racks and Cleaning Rakes: Prevent debris from entering the turbine.

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2. Types of Turbines and Performance Characteristics

Turbines are classified based on how water interacts with the runner blades and the head/flow regime.

2.1 Impulse Turbine (Pelton)

• Working Principle: Converts pressure head entirely into kinetic energy (via a nozzle) before the jet strikes the buckets on the runner. The runner operates in atmospheric pressure.

• Head Range: Very High Head (150 m to 2000+ m).

• Flow Range: Low to medium.

• Specific Speed ($N_s$): Low ($N_s$ < 30, SI units).

• Efficiency: High efficiency over a wide load range (flat efficiency curve).

2.2 Reaction Turbines (Francis, Kaplan, Propeller)

• Working Principle: Water pressure changes as it passes through the runner. The runner is fully submerged and operates in a closed conduit. Energy conversion is due to both pressure change and velocity change.

• Types:

  1. Francis Turbine:

     • Head Range: Medium to High Head (40 m to 600 m).

     • Flow Range: Medium.

     • Specific Speed ($N_s$): Medium ($N_s$ ≈ 50 - 300).

     • Efficiency: High peak efficiency, but efficiency drops significantly at part load.

  2. Kaplan / Propeller Turbine:

     • Feature: Adjustable runner blades (Kaplan) or fixed blades (Propeller).

     • Head Range: Low Head (2 m to 70 m).

     • Flow Range: Very high.

     • Specific Speed ($N_s$): High ($N_s$ ≈ 300 - 1000).

     • Efficiency: Kaplan maintains high efficiency over a wide flow range due to adjustable blades.

2.3 Performance Characteristics

• Efficiency ($\eta$): Ratio of shaft power output to hydraulic power input. $\eta = \frac{P_{shaft}}{\rho g Q H}$

• Head ($H$): Net head available at the turbine inlet.

• Discharge ($Q$): Flow rate through the turbine.

• Speed ($N$): Rotational speed of the runner (RPM).

• Power Output ($P$): $P = \eta \rho g Q H$

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3. Selection of Turbine and Their Specific Speed

3.1 Specific Speed ($N_s$)

• A dimensionless number (in SI units) that characterizes the geometry and performance of a turbine. It is the speed at which a geometrically similar turbine would run to produce unit power (1 kW) under unit head (1 m). $N_s = \frac{N \sqrt{P}}{H^{5/4}}$ Where $N$ is in RPM, $P$ is in kW, $H$ is in m.

• Significance: It is the key parameter for turbine selection. Each turbine type operates efficiently within a specific $N_s$ range.

3.2 Turbine Selection Chart & Process

Selection is primarily based on Head ($H$) and Discharge ($Q$) or Power ($P$).

1. Determine Operating Head ($H$):

   • H > 150 m: Pelton turbine (or Turgo for slightly lower head).

   • 40 m < H < 600 m: Francis turbine.

   • H < 70 m: Kaplan (if large flow variation) or Propeller (if constant load).

2. Calculate Specific Speed ($N_s$) for preliminary selection:

   • Choose a suitable synchronous speed ($N$) based on grid frequency ($f$) and generator poles ($p$): $N = \frac{120f}{p}$.

   • Use the $N_s$ formula. The calculated value should fall within the typical range of the turbine type suggested by the head.

3. Consider Other Factors:

   • Part-load Operation: If plant will operate frequently at part load, Kaplan or Pelton are better than Francis.

   • Sediment: High sediment content favors Pelton (less erosion on buckets) over Francis.

   • Cavitation: More critical for reaction turbines (Francis, Kaplan), requiring careful setting level.

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4. Preliminary Design of Francis and Pelton Turbines

4.1 Francis Turbine Preliminary Dimensions

1. Runner Inlet Diameter ($D_1$): Estimated from specific speed and discharge. $D_1 \approx \frac{84.5 \phi \sqrt{H}}{N}$ where $\phi$ is speed ratio (0.6-0.9).

2. Runner Outlet Diameter ($D_2$): $D_2 \approx (0.4 \text{ to } 0.7) D_1$.

3. Runner Width at Inlet ($B_1$): Related to flow: $Q = \pi D_1 B_1 V_{f1}$, where $V_{f1}$ is flow velocity.

4. Number of Runner Blades: Typically 13 to 17.

4.2 Pelton Turbine Preliminary Dimensions

1. Jet Diameter ($d$): $d = \sqrt{\frac{4Q}{z \pi \sqrt{2gH}}}$ where $z$ is number of jets (1 to 6).

2. Bucket Width ($B$): $B \approx 3d$.

3. Runner Pitch Diameter ($D$): $D \approx \frac{60 u}{\pi N}$ where $u$ is bucket speed ratio ($u = \phi \sqrt{2gH}$, $\phi \approx 0.45$).

4. Number of Buckets: Empirical: $Z \approx 15 + \frac{D}{2d}$.

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5. Scroll Case and Draft Tubes

5.1 Scroll Case (Spiral Casing)

• Function: To distribute water from the penstock with uniform velocity and pressure around the entire circumference of the Francis/Kaplan turbine guide apparatus.

• Design: Cross-sectional area decreases uniformly from the inlet (largest) to the end (smallest) to maintain constant velocity. Often made of fabricated steel for high head, or concrete for low head.

5.2 Draft Tube

• Function (for Reaction Turbines):

  1. Recover kinetic energy at runner exit by decelerating flow.

  2. Allow the turbine to be set above tailwater level without losing head, by creating a suction head.

• Types:

  1. Conical (Straight Diverging): Simple, used for vertical shafts.

  2. Elbow-Type: Common for horizontal Francis turbines. Has a vertical inlet and horizontal outlet.

  3. Moody Spreading Tube: For very low head plants.

• Efficiency ($\eta_{dt}$): $\eta_{dt} = \frac{\text{Kinetic Energy Recovered}}{\text{Kinetic Energy at Inlet}}$

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6. Generators: Types, Rating; Governors

6.1 Generators

• Type: Synchronous Generator (Alternator) is universally used.

• Rating:

  • MVA (Apparent Power): $S = \sqrt{3} V_L I_L$.

  • MW (Active Power): $P = S \times \text{Power Factor (pf)}$.

  • Voltage: Typically 6.6 kV, 11 kV, or higher for large units.

  • Frequency: 50 Hz (Nepal, India) or 60 Hz.

  • Speed: Synchronous speed $N = \frac{120f}{p}$.

• Types based on Shaft Orientation:

  • Vertical Shaft: Common for Francis and Kaplan turbines. Requires a thrust bearing to support the weight of the rotating parts and water pressure.

  • Horizontal Shaft: Common for Pelton and small Francis turbines.

6.2 Governors

• Function: To maintain constant generator speed (and thus frequency) despite load changes, by regulating water flow to the turbine.

• Components: Speed sensor, controller, hydraulic servomotor (which moves guide vanes or spear).

• Droop Characteristic: A small, deliberate reduction in speed as load increases, allowing for stable load sharing between multiple generators on the grid.

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7. Pumps and Their Performance Characteristics

7.1 Types

• Centrifugal Pumps: Most common for general plant service (cooling water, drainage).

• Positive Displacement Pumps: Used for lubrication oil, governor oil pressure.

• Deep-well Pumps: For dewatering.

7.2 Performance Characteristics

• Head ($H$): Energy imparted to fluid per unit weight (m).

• Discharge ($Q$): Flow rate (m³/s).

• Power Input ($P_{in}$): $P_{in} = \frac{\rho g Q H}{\eta}$ where $\eta$ is pump efficiency.

• Net Positive Suction Head (NPSH): Critical to avoid cavitation.

  • NPSH Available (NPSHa): Must be greater than NPSH Required (NPSHr) by the pump.

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8. Powerhouse: Types, General Arrangements, Dimensions

8.1 Types of Powerhouse

1. Surface Powerhouse:

   • Built on the surface, on the riverbank or downstream of a dam.

   • Most common type.

2. Underground Powerhouse:

   • Excavated inside a rock mass.

   • Advantages: Security, protection from avalanches/rockfall, minimal surface disturbance, often shorter penstock.

   • Disadvantages: Higher excavation cost, ventilation and access challenges.

   • Common in Himalayan projects.

3. Semi-Underground Powerhouse: Partially embedded in a hillside.

8.2 General Arrangement (for a Vertical Francis Unit)

The layout is organized in bays or levels:

1. Generator Floor (Main Floor):

   • Houses the generator stator, control panels, switchgear, and gantry crane rails.

   • Key Dimension: Clearance for crane hook to lift heaviest part (usually generator rotor).

2. Turbine Floor (Intermediate Floor):

   • Located below generator floor. Provides access to turbine cover, governor, and auxiliary equipment.

3. Spiral Case & Draft Tube Level:

   • The scroll case is embedded in concrete. The draft tube extends down to the tailrace.

4. Draft Tube Gate Gallery: For maintenance gates at draft tube outlet.

5. Tailrace Level: Outlet channel returning water to the river.

8.3 Preliminary Dimensioning

1. Unit Spacing (Center-to-center, $L_c$): $L_c = D_{sc} + \text{Clearance}$ where $D_{sc}$ is spiral case inlet diameter. Clearance is typically 2-5 m for access and structural piers.

2. Powerhouse Length: $\text{Length} = N_{units} \times L_c + \text{End Bays}$

3. Powerhouse Width:

   • Dictated by the size of the generator, spiral case, and draft tube, plus access galleries on both sides.

4. Powerhouse Height:

   • Sum of heights from tailrace floor to generator floor, plus crane hook height and crane girder depth.

5. Crane Capacity: Must exceed the weight of the heaviest component (generator rotor or turbine runner).

Conclusion: The powerhouse and its machinery represent the culmination of a hydropower project. Optimal selection of turbines based on specific speed, careful design of water passages (scroll case, draft tube), integration of robust generators and control systems, and logical arrangement within a well-dimensioned powerhouse structure are all imperative for efficient, reliable, and safe power generation. This integration of civil, mechanical, and electrical engineering defines the functional core of the plant.