Hydro Power Plant

A pumped storage plant is a hydro plant with two reservoirs at different levels. It uses cheap surplus electricity in off-peak hours to pump water from the lower reservoir to the upper one, and releases the same water back through the turbine to generate power during peak hours. It works like a large rechargeable battery for the grid.

  UPPER RESERVOIR (head pond)
  ~~~~~~~~~~~~~~~~~~~~~~~
        |  penstock / pressure shaft
        |  (water flows down when
        |   generating, up when pumping)
        v ^
  +---------------------+      +-----------+
  | reversible          |------| motor-    |---- grid
  | pump-turbine        |      | generator |  (supply
  +---------------------+      +-----------+   or draw)
        | ^
        v |  tailrace
  ~~~~~~~~~~~~~~~~~~~~~~~
  LOWER RESERVOIR (tail pond)

Main parts: upper reservoir, lower reservoir (or river/tail pond), intake, penstock or pressure shaft, power house with a reversible pump-turbine (usually Francis type) coupled to a reversible synchronous motor-generator, and tailrace.

Operation:

1. Generating mode (peak hours): Water from the upper reservoir flows down the penstock, drives the machine as a turbine, and the machine works as a generator feeding the grid. The water is collected in the lower reservoir.
2. Pumping mode (off-peak hours, e.g. night): Surplus power from base-load plants (thermal, nuclear or run-of-river hydro) drives the machine as a motor. The machine rotates in the reverse direction and works as a pump, lifting water from the lower reservoir back to the upper reservoir.
3. The cycle repeats daily (or weekly/seasonally).

Energy relation: Energy stored $E = \rho g V H$. Overall cycle efficiency

$$\eta_{cycle} = \frac{E_{generated}}{E_{pumping}} \approx 0.70 \text{ to } 0.80$$

So about 1.3 kWh of off-peak energy is used to get 1 kWh at peak. It is still economic because peak energy is far more valuable than off-peak energy.

Feasibility in Nepal's power system

Pumped storage is technically feasible and is becoming economically attractive in Nepal, but it should be planned for the near future rather than treated as an immediate priority over storage projects. Reasons:

Points in favour

• Topography: Nepal's hills give high heads over short distances, so small reservoirs can store large energy.
• Run-of-river dominance: Most of Nepal's plants are run-of-river (ROR). They cannot follow the daily peak; the evening peak is hard to meet, while there is surplus at night.
• Wet-season spill: In the monsoon, ROR plants produce more than domestic demand. Off-peak surplus energy that is now spilled or sold cheaply can be used for pumping.
• Cheap import / solar: Off-peak energy can be imported from India at lower rates, and growing daytime solar can also be stored.
• Grid support: Quick start and reserve capacity improve frequency control of a small, weak grid.
• Existing reservoir sites such as Kulekhani (the only storage scheme of NEA) have been studied for adding pumped storage.

Points against (in the current scenario)

• In the dry season Nepal is itself energy deficit; there is little surplus energy to pump with, which is exactly when peaking power is most needed.
• High capital cost, long construction time and difficult access in the hills.
• Cycle losses of 20–30 %; it is economic only if the peak tariff is well above the off-peak price.
• Sediment-laden Himalayan rivers cause abrasion of pump-turbines and siltation of reservoirs.
• Seasonal storage projects (reservoir type) may give more benefit per rupee in a deficit system.

Conclusion: A daily-cycle pumped storage scheme is feasible in Nepal, especially using wet-season and night surplus, cross-border trade and solar. With proper tariff (time-of-day and seasonal pricing) and site selection, it can solve the peaking problem of an ROR-dominated system.

Water hammer is the sudden rise (or fall) of pressure in a penstock when the velocity of water is changed quickly, for example when the turbine gates close quickly after a load rejection. The moving column of water is suddenly stopped; its kinetic energy is converted into pressure energy, and a pressure wave travels up and down the pipe with a hammering effect.

Magnitude (Joukowsky relation): for instantaneous or rapid closure ($t_c \le 2L/a$),

$$\Delta p = \rho\, a\, \Delta v \quad\text{or}\quad \Delta H = \frac{a\,\Delta v}{g}$$

where $a$ = speed of the pressure wave in the pipe (about 1000–1400 m/s), $\Delta v$ = change in velocity, $L$ = pipe length. For slow closure ($t_c > 2L/a$) the rise is smaller, roughly $\Delta p \approx \rho L \Delta v / t_c$.

 Reservoir
 ~~~~~~~ |
         |\  penstock (length L)
         | \
         |  \  pressure wave
         |   \  <---->
         |    \______[gate]--turbine
                      closes

Effects: very high pressure can burst the penstock, damage valves and turbine casing; negative pressure on opening can collapse the pipe or cause cavitation; vibration and noise; it also slows the governor response.

Methods to minimize water hammer

1. Surge tank near the power house: gives a free water surface close to the turbine, so the length of pipe in which water hammer occurs is reduced and the pressure wave is reflected from the tank.
2. Slow closure of gates: make the closing time longer than $2L/a$ (governor relief/time setting).
3. Relief valve / pressure relief valve on the penstock or casing that opens when pressure rises and lets water escape.
4. Deflector (Pelton) or jet deflector: the jet is deflected away from the buckets at once, while the needle closes slowly.
5. Bypass / synchronous bypass valve that opens as the gate closes so the penstock flow does not stop suddenly.
6. Strong penstock designed for the expected surge pressure, and air valves to prevent vacuum collapse.
7. Keep the penstock as short as possible and the water velocity low (larger diameter).

A pumped storage plant is a hydro plant with two reservoirs at different levels. It uses cheap surplus electricity in off-peak hours to pump water from the lower reservoir to the upper one, and releases the same water back through the turbine to generate power during peak hours. It works like a large rechargeable battery for the grid.

  UPPER RESERVOIR (head pond)
  ~~~~~~~~~~~~~~~~~~~~~~~
        |  penstock / pressure shaft
        |  (water flows down when
        |   generating, up when pumping)
        v ^
  +---------------------+      +-----------+
  | reversible          |------| motor-    |---- grid
  | pump-turbine        |      | generator |  (supply
  +---------------------+      +-----------+   or draw)
        | ^
        v |  tailrace
  ~~~~~~~~~~~~~~~~~~~~~~~
  LOWER RESERVOIR (tail pond)

Main parts: upper reservoir, lower reservoir (or river/tail pond), intake, penstock or pressure shaft, power house with a reversible pump-turbine (usually Francis type) coupled to a reversible synchronous motor-generator, and tailrace.

Operation:

1. Generating mode (peak hours): Water from the upper reservoir flows down the penstock, drives the machine as a turbine, and the machine works as a generator feeding the grid. The water is collected in the lower reservoir.
2. Pumping mode (off-peak hours, e.g. night): Surplus power from base-load plants (thermal, nuclear or run-of-river hydro) drives the machine as a motor. The machine rotates in the reverse direction and works as a pump, lifting water from the lower reservoir back to the upper reservoir.
3. The cycle repeats daily (or weekly/seasonally).

Energy relation: Energy stored $E = \rho g V H$. Overall cycle efficiency

$$\eta_{cycle} = \frac{E_{generated}}{E_{pumping}} \approx 0.70 \text{ to } 0.80$$

So about 1.3 kWh of off-peak energy is used to get 1 kWh at peak. It is still economic because peak energy is far more valuable than off-peak energy.

Scheme to reduce peak generation loss of a steam plant

A steam power plant is efficient only at a steady load near its rating. If it has to follow the daily load curve, it runs at part load at night (poor heat rate) and must be pushed above its economic loading, or backed up by costly peaking units, in the evening. Frequent load swings also cause thermal stress and higher fuel use per kWh.

  Load
  (MW)        peak met by pumped
   ^          storage (generating)
   |          ______
   |         /######\
   |--------/########\------  steam plant
   |  ......            ....   output kept
   |  :pump:            :pump: constant
   |  :mode:            :mode:
   +--------------------------> time (h)
   0        12         18    24
   .... = valley filled by pumping load

Scheme (coordinated operation):

1. The steam plant runs at a constant, near-rated load for all 24 hours, where its efficiency is highest.
2. During off-peak hours (night), the extra steam-plant output is absorbed by the pumped storage plant working in the pumping mode. This "fills the valley" of the load curve.
3. During peak hours, the pumped storage plant works as a turbine-generator and supplies the peak. This "shaves the peak".
4. The pumped storage plant also takes sudden load changes and acts as spinning reserve, so the steam units are not forced to ramp quickly.

Benefits:

• Higher average efficiency and lower fuel cost of the steam plant; fewer start-stops.
• Load factor of the steam plant rises; the installed capacity needed for peaking falls.
• Even with a cycle efficiency of 70–80 %, the saving is positive because cheap off-peak energy replaces expensive peak generation (gas turbines, diesel, or the steam plant at its worst heat rate).

A pumped storage plant is a hydro plant with two reservoirs at different levels. It uses cheap surplus electricity in off-peak hours to pump water from the lower reservoir to the upper one, and releases the same water back through the turbine to generate power during peak hours. It works like a large rechargeable battery for the grid.

  UPPER RESERVOIR (head pond)
  ~~~~~~~~~~~~~~~~~~~~~~~
        |  penstock / pressure shaft
        |  (water flows down when
        |   generating, up when pumping)
        v ^
  +---------------------+      +-----------+
  | reversible          |------| motor-    |---- grid
  | pump-turbine        |      | generator |  (supply
  +---------------------+      +-----------+   or draw)
        | ^
        v |  tailrace
  ~~~~~~~~~~~~~~~~~~~~~~~
  LOWER RESERVOIR (tail pond)

Main parts: upper reservoir, lower reservoir (or river/tail pond), intake, penstock or pressure shaft, power house with a reversible pump-turbine (usually Francis type) coupled to a reversible synchronous motor-generator, and tailrace.

Operation:

1. Generating mode (peak hours): Water from the upper reservoir flows down the penstock, drives the machine as a turbine, and the machine works as a generator feeding the grid. The water is collected in the lower reservoir.
2. Pumping mode (off-peak hours, e.g. night): Surplus power from base-load plants (thermal, nuclear or run-of-river hydro) drives the machine as a motor. The machine rotates in the reverse direction and works as a pump, lifting water from the lower reservoir back to the upper reservoir.
3. The cycle repeats daily (or weekly/seasonally).

Energy relation: Energy stored $E = \rho g V H$. Overall cycle efficiency

$$\eta_{cycle} = \frac{E_{generated}}{E_{pumping}} \approx 0.70 \text{ to } 0.80$$

So about 1.3 kWh of off-peak energy is used to get 1 kWh at peak. It is still economic because peak energy is far more valuable than off-peak energy.

Advantages:

• Supplies peak load quickly (start-up in a few minutes); also gives spinning reserve and frequency support.
• Lets base-load thermal plants run at a steady, efficient load; raises the system load factor.
• Very little water is consumed; the same water is used again and again.
• Long life and low running cost.

Limitations: high capital cost, need for suitable topography with two reservoir sites, and a net energy loss of 20–30 %.

Water hammer is the sudden rise (or fall) of pressure in a penstock when the velocity of water is changed quickly, for example when the turbine gates close quickly after a load rejection. The moving column of water is suddenly stopped; its kinetic energy is converted into pressure energy, and a pressure wave travels up and down the pipe with a hammering effect.

Magnitude (Joukowsky relation): for instantaneous or rapid closure ($t_c \le 2L/a$),

$$\Delta p = \rho\, a\, \Delta v \quad\text{or}\quad \Delta H = \frac{a\,\Delta v}{g}$$

where $a$ = speed of the pressure wave in the pipe (about 1000–1400 m/s), $\Delta v$ = change in velocity, $L$ = pipe length. For slow closure ($t_c > 2L/a$) the rise is smaller, roughly $\Delta p \approx \rho L \Delta v / t_c$.

 Reservoir
 ~~~~~~~ |
         |\  penstock (length L)
         | \
         |  \  pressure wave
         |   \  <---->
         |    \______[gate]--turbine
                      closes

Effects: very high pressure can burst the penstock, damage valves and turbine casing; negative pressure on opening can collapse the pipe or cause cavitation; vibration and noise; it also slows the governor response.

Surge tank

A surge tank is an open-top vertical tank (or shaft) connected to the conduit just before the penstock, close to the power house. Its water level rises and falls with the load.

 Reservoir     low-pressure tunnel     surge tank
 ~~~~~~~ |____________________________ | ~~~~ |
                                       |      |
                                       |__  __|
                                          \ penstock
                                           \
                                            [turbine]

Advantages of a surge tank

1. Reduces water hammer: the free surface reflects the pressure wave, so only the short penstock between tank and turbine is exposed to high pressure. The long tunnel is protected and can be thinner and cheaper.
2. On load rejection: when gates close, the water coming down the tunnel enters the tank and its level rises, storing water instead of creating high pressure.
3. On load increase: the tank supplies the extra water immediately until the tunnel flow accelerates. So the turbine does not starve of water and the governor responds faster.
4. Reduces the effective water starting time ($T_w$), giving better speed regulation and stability.
5. Reduces pressure fluctuation in the tunnel, prevents negative pressure and collapse.
6. Allows faster gate operation and therefore better frequency control.

Types: simple tank, restricted orifice (throttled) tank, differential tank, closed (air-cushion) tank and inclined tank.

A surge tank is an open-top vertical tank or shaft placed on the water conduit between the low-pressure tunnel and the penstock, as near to the power house as possible. Its water level rises and falls as the turbine load changes.

 Reservoir   headrace tunnel    surge tank
 ~~~~~~ |______________________ |  ~~~~  |
                               |        |
                               |__    __|
                                  \ penstock
                                   \__[turbine]

Functions:

1. Protects against water hammer: when the turbine gates close suddenly (load rejection), the water in the tunnel flows into the tank and the level rises. The pressure wave is reflected at the free surface, so only the short penstock sees high pressure.
2. Supplies water on load increase: when gates open suddenly, the tank gives the extra water at once (its level falls) while the long tunnel column accelerates slowly.
3. Reduces the effective length of pipe whose water must be accelerated, i.e. reduces water starting time $T_w$, which improves governing and stability.
4. Allows the tunnel to be designed for lower pressure, saving cost.
5. Prevents negative pressure and vacuum in the conduit when the load increases.

Types: simple, restricted orifice, differential, inclined and closed (air cushion) surge tanks.

In steady state, a hydro unit runs at constant speed with constant water flow and gate opening, and the mechanical power from the turbine exactly equals the electrical power plus losses ($P_m = P_e$).

 Reservoir (head H)
 ~~~~~~~ |___tunnel___[surge tank]
                         \ penstock (Q)
                          \
               gate/  +---------+   +-----+
             governor>| turbine |===| gen |==> Pe
                      +---------+   +-----+
                          | tailrace

Power relation: the hydraulic power converted is

$$P = \eta\, \rho\, g\, Q\, H_{net}$$

where $Q$ = discharge (m³/s), $H_{net}$ = gross head minus head losses in tunnel and penstock, $\eta$ = overall efficiency of turbine and generator.

Steady state conditions:

• Water velocity in the penstock is constant, so there is no water hammer and the surge tank level is steady.
• Gate (or needle) opening is fixed at the value set by the governor for the present load.
• Speed is constant at synchronous value; frequency is steady. The accelerating power $P_m - P_e = 0$.
• Generator voltage is held by the AVR at its set value.

Change of operating point: if the load changes, the governor senses the speed change and moves the gate until a new steady state is reached where $P_m = P_e$ again (with a small frequency change set by the droop). The power output is proportional to $Q$, so for a fixed head the gate opening sets the power.

A pumped storage plant is a hydro plant with an upper and a lower reservoir. In off-peak hours it uses surplus grid power to pump water up; in peak hours the water flows back through the turbine to generate power. The same reversible pump-turbine and motor-generator usually do both jobs.

 Upper reservoir
     |  penstock
     v ^
 [pump-turbine]--[motor-generator]--grid
     | ^
 Lower reservoir

Cycle efficiency is about 70–80 %, so it is a net consumer of energy. It is economically feasible when:

1. Large price difference between peak and off-peak energy. The value of peak energy must exceed the cost of pumping energy divided by the cycle efficiency: $C_{peak} > C_{off\text{-}peak}/\eta_{cycle}$.
2. Cheap surplus energy is available in off-peak hours from base-load plants (thermal, nuclear, run-of-river hydro) or from solar and wind that would otherwise be spilled.
3. Base-load plants must run steadily, e.g. a system with many steam plants, where peak-following causes poor efficiency and wear.
4. Good site: two reservoirs close together with a high head (short penstock per metre of head), water available, firm foundation and low sediment.
5. Need for fast reserve and regulation: grids that need spinning reserve, frequency control and black-start capability value the plant beyond its energy.
6. Peak load lasts only a few hours, so a modest storage volume is enough.
7. Existing reservoirs can be used (lower cost civil works).

In a pumped storage plant the Francis turbine is used as a reversible pump-turbine. One machine works as a turbine when water flows down and as a centrifugal pump when it is rotated in the opposite direction. It is coupled to a reversible synchronous machine that works as a generator or as a motor.

 Upper reservoir
   |   generating: water down
   v ^ pumping: water up
 +-----------------+   +------------------+
 | Francis pump-   |===| motor-generator  |== grid
 | turbine (spiral |   | (reverses for    |
 | casing, runner, |   |  pumping)        |
 | wicket gates)   |   +------------------+
 +-----------------+
   | ^ draft tube
 Lower reservoir

Why a Francis machine is used

• The Francis runner is a mixed/radial flow reaction runner, so its shape is very close to that of a centrifugal pump impeller. When driven backwards it pumps water efficiently.
• It covers the head range of most pumped storage sites (about 30 m to 700 m).
• One machine replaces a separate pump and turbine, so the power house, valves and civil works are smaller and cheaper.
• Wicket gates control flow in turbine mode and are set for best efficiency in pump mode.
• Mode change is quick (a few minutes), so it gives fast peaking and spinning reserve.

Operation

1. Turbine mode (peak hours): Water from the upper reservoir enters the spiral casing, passes through the wicket gates and runner, and leaves through the draft tube. The machine generates power.
2. Pump mode (off-peak): The machine is started as a motor (by a starting motor, back-to-back start or static frequency converter) in the reverse direction with the runner dewatered by compressed air, then water is admitted and pumped up.
3. Efficiency is slightly less than a separate pump and turbine (about 90 % in each mode, 70–80 % for the cycle).

Feasibility in Nepal

It is feasible in Nepal, mainly for the reasons below, though high cost and dry-season energy deficit are the main obstacles.

• For: high heads in hilly terrain; run-of-river plants give night and monsoon surplus that can be stored; time-of-day peak in the evening; possibility of cheap off-peak import from India and solar surplus; better frequency and reserve support for the grid; sites near existing reservoirs like Kulekhani have been studied.
• Against: large investment and long construction; in winter there is little surplus to pump with; 20–30 % energy loss; heavy sediment in rivers wears Francis runners; reservoir projects may be a higher priority.

Overall, with time-of-day tariffs and power trade, a pumped storage plant using Francis pump-turbines is a sound option for meeting Nepal's peak demand.

In a hydro station, the water delivered to the turbine is controlled by the governing system. It keeps the speed (frequency) constant by adjusting the wicket (guide) gates in reaction turbines or the spear (needle) valve in Pelton turbines, so that water power matches the electrical load.

     speed set point
          |
          v
 +-------------+   +---------+   +-------------+
 | speed       |-->| pilot / |-->| main servo  |
 | governor    |   | relay   |   | motor (oil  |
 | (fly-ball/  |   | valve   |   | piston)     |
 | electronic) |   +---------+   +-------------+
 +-------------+                     |  moves
      ^                              v
      |                     +------------------+
      |  speed signal       | wicket gates or  |
      |                     | needle/deflector |
      |                     +------------------+
      |                              | water
 +----------+       shaft     +-------------+
 | generator|<================| turbine     |
 +----------+                 +-------------+
      ^ restoring (feedback) link to relay valve

Main components

1. Speed sensor / governor: fly-ball (mechanical) or electronic (PID) unit that senses speed.
2. Pilot (relay) valve: directs pressurised oil to one side of the servomotor.
3. Servomotor: a double-acting oil piston that moves the gate ring or needle; provides the large force needed.
4. Oil pressure unit: pump, accumulator and sump.
5. Restoring (feedback) mechanism: a linkage from the servomotor back to the relay valve so that the gates stop at the correct position (gives droop).
6. Wicket gates / needle valve and deflector, plus the main inlet valve (butterfly or spherical) for shutdown.

Working

1. Suppose the load increases. Speed falls a little.
2. The fly-balls move inward (or the electronic governor detects the error) and move the pilot valve.
3. Oil flows to the servomotor, which opens the wicket gates (or withdraws the needle). More water enters the turbine and torque rises.
4. The feedback link closes the pilot valve when the gates reach the new position, so hunting is avoided. Speed settles at a value fixed by the droop; the load reference can then bring it back to 50 Hz.
5. On load decrease, the action is reversed and gates close. Closing is done slowly to avoid water hammer. In Pelton turbines the deflector first turns the jet away from the buckets, and the needle then closes slowly.

Thus water delivery is matched to load while keeping frequency within limits and penstock pressure safe.