Use of Surge Tank in Hydro Power Plant

Surge tank is a small storage tank or reservoir required in the hydro power plants for regulating the water flow during load reduction and sudden increase in the load on the hydro generator (water flow transients in penstock) and thus reducing the pressure on the penstock. This is the advantage of surge tank.

 

Water Hammering effect in Hydro Power Plant:

When the load on the generator is reduced suddenly (load throw-off), governor closes the turbine gates and thus create an increase in  pressure on the penstock. This may result in water " hammering effect" and may require pipe of extra ordinary strength this pressure, otherwise the penstock may burst. To avoid this positive water hammering pressure some means is provided to take the rejected flow of water. This can be achieved by providing small storage reservoir or tank to accommodate this rejected flow. This small storage reservoir is surge tank and located close to the power station. When the turbine gates suddenly opens due to requirement of more water due to increase in the load demand on the generator, water has to rush through the pipe (penstock) and there is a tendency to cause a vacuum in the pipe supplying the water.
The pipe supplying the water must withstand the high pressures caused by the sudden closing of the turbine gates (known as positive water hammer) and there should not be any vacuum in the pipe line when the gate suddenly opens.
The water hammering is defined as the change in the pressure rapidly above or below normal pressure caused by sudden changes in the rate of water flow through the pipe according to the demad of the prime-mover. The water hammer occurs at all the points in the penstock between the forebay or surge tank and the turbine because of the sudden changes in the demand of the water during the load fluctuations.  

 

Surge Tank in Hydro power Plant:

Surge tank is an open tank which is often used with the pressure conduit of considerable length. The main purpose of providing surge tank is to reduce the distance between the free water surface and turbine thereby reducing the water hammer effect on the penstock and also and also protecting upstream tunnel from high pressure rises. Surge tank also serves as a supply tank to the turbine when the water in the pipe is accelerating during increased load conditions and as storage tank when the water is decelerating during the reduced load conditions. Surge tanks are generally built high enough so that the water cannot overflow evn with a full load on the turbine. 

Use of Surge Tank:

When load demand on the generator decreases, it leads to rise in the water level in the surge tank. This produces a retarding head and reduces water velocity in the penstock. The reduction in the velocity to the desired levels, makes the water in the tank to rise and fall until oscillations are damped out.

When load demand on the generator increases, governor opens the turbine gates in order to allow more water flow through the penstock to supply the increased load demand thereby creating a negative pressure or vacuum in the penstock. This negative pressure in the penstock creates necessary acceleration force and is objectionable for very long conduits due to difficult turbine regulation. Under this condition additional water flows from the surge tank. As a result the water level in the surge tank falls, an acceleration head is created and flow of water in the penstock increases.

Thus surge tank helps in stabilizing the velocity and pressure in the penstock and protects penstock from water hammering and negative pressure or vacuum. 

Function or advantages of Surge Tank:

The functions of the surge tank are listed below:

• It reduces the distance between the free water level of the reservoir  and turbine and also reduces the intensity of the water hammering in the surge chamber to such an extent above the water hammering effect can be neglected in the design of the tunnel. Only relatively short length of the conduit (penstock) below the surge tank must be designed to withstand the water hammering effect
• Other function of surge tank is that it acts as relief valve when the load on the turbine is reduced and the pressure in the pipe suddenly increased by diverting the main conduit flow partly into this tank. The water level in the tank rises until it exceeds the level in the main reservoir thus retarding the main conduit flow and absorbing the surplus kinetic energy
• The tank acts as a temporary reservoir during increased load demand on the turbine. It provides sufficient water to enable the turbine to pickup the new load quickly and safely and to keep it running at the increased load until the water level in the surge tank falls below its original level. Sufficient head is created thereby to accelerate the flow in the penstock until it is sufficient to meet the new load demand

 

Types of Surge Tank:

Some of the types of surge tank used in hydroelectric power plants are:

1. Simple Surge Tank
2. Inclined Surge Tank
3. The expansion Chamber and Gallery type Surge Tank
4. The restricted orifice or Throttled Surge Tank

Pumped Storage Hydro Plant Advantages

Pumped Storage Hydro Plants:

Pumped Storage plants are special type of hydro plants where water driving the turbine to generate electricity is again pumped back in to the reservoir. Hydro plants and gas plants takes very little time to come to hot and generate the power in the power system. These plants are operated when there is peak demand in electricity. Hence these plants are termed as peak load plants.

When the demand for electricity is high pumped storage hydro plants generate electricity and supply the power to the grid. During low load periods (late nights where load is less) water will be again pumped back to the reservoir. During this period Turbine-Generator acts as motor and pumps the water back.

Advantages:

• Pumped storage plants can contribute to the overall electrical system in performing one or more functions
• Load regulation
• Quick response reserve capacity to offset short term generation or transmission outages
• Increase in overall system energy

Pumped plans can also be constructed for multiple uses such as water supply projects, flood control and nuclear cooling capacity.

The draft tube for discharging the water in to the tail race when generating power acts as a suction tube for pumping the water from the tailrace and guiding it to the impeller runner. Impeller runner for reversible pump/turbine is similar to centrifugal pump impeller but modified to produce optimum performance in both the operations. Conventional turbine runners with shorter blades are not suited for pumping operation. An impeller runner for reversible turbine/pump is usually larger in diameter than Francis runner for the same generating capacity.

Salient Pole Generators in Hydro Power Plant

Salient pole Synchronous generators and induction generators are used in hydroelectric power plant to convert the mechanical energy output of the turbine to electrical energy. Normally in hydro plants synchronous generators are employed for generating electricity, however for small power plant generator applications (plants less than 5 MVA generation capacity) Induction generators are used because of  their lower cost due to the absence of the exciter, voltage regulator, and synchronizer which should be associated  with synchronous generators.

Salient pole Synchronous generators are used in hydoelectric  plant because:

The majority of hydroelectric installations utilize salient pole synchronous generators. Salient pole machines are employed  because the hydraulic turbine in the hydroelectric plants operates at low speeds compared to steam plants, therefore generators in hydro plant requires  large number of field poles to produce the rated frequency. Rotor of synchronous generator requires  salient poles is mechanically best suited for low-speed applications, compared to round rotor machines which are employed in horizontal axis high-speed applications in turbo-generators. Round rotor or non salient pole synchronous generators are used in Thermal power plant because of high speed turbine. 

Interview Questions : Hydroelectric Power Plant (Set 2)

What is Forebay and its importance?

Answer: Forebay serves as regulating reservoir storing water temporarily during the light load period and providing the water for initial increase of the load during which water in the canal is being accelerated. In simple words Forebay may be considered as enlarged body of water just above the intake to store water temporarily to meet the hourly load fluctuation. When the hrdroelectric plants are located just at the base of the dam, no forebay is required because the reservoir itself serves the purpose of forebay. However, where the plants are situated away from the storage reservoir forebay is provided.

What is Penstock?

Answer: It is the closed conduit or pipe which connects the forebay or surge tank to the scroll case of the turbine. Penstocks are built of steel or reinforced concrete. The thickness of the penstock must be adequate to withstand both the normal hydrostatic pressure and also sudden surges both above and below normal caused by fluctuations in the load and in emergency conditions.

 What is water hammering phenomenon?

Answer: When the load on the generator reduces suddenly, governor mechanism will close the turbine gates and thus create an increased pressure in the penstock. This results in ' water hammering phenomenon' and may need pipe (penstock) of extra ordinary strength to withstand the pressure otherwise the penstock will burst. To avoid this positive water hammering pressure some means are provided to take the rejected flow. This job will be carried out by small storage tank or reservoir for receiving the rejected flow. This storage reservoir or tank is called as 'surge tank'.

What is Trash Racks?

Answer: Trash Racks are long, flat vertical bars spaced accordingly such that it provides the minimum passage width to the water through the turbine. They are employed to prevent the ingress of floating and other material to the turbine

What is Tailrace?

Answer: Water after doing the useful work in the turbine is discharged to the tail race. It is the path through which water will flow down stream after running the turbine.Tailrace storage will acts as reservoir for the down stream projects for generating electricity or for irrigation purposes.

Interview Questions: Hydroelectric Power Plant 1

What are the essential elements of the hydroelectric power plant ?

Answer: Some of the essential elements of hydroelectric power plant are:

(1).Catchment area (2).Reservoir (3).Dam (4).Spillways (5).Conducts (6).Surge tanks (7).Draft Tubes (8).Power House (9).Switchyard for transmission of power

 What is the significance of Dam ?

Answer: Dam performs the following tow basic functions

• It develops a reservoir of the desired capacity to store water
• It builds up the head for power generation.

What is Hydrograph  and its significance?

Answer: Hydrograph is a graphical representation between the discharge or flow with time. It is plotted with flow as ordinates (Y-axis)  and time intervals as abscissas ( X-axis). The ordinates may be plotted in terms of the gauge height, the number of cubic meters per second per square kilometer, the power in kW that can be developed. The abscissas may be taken in terms of hours, days, weeks or months. It also indicates the power available from the stream at different times of the day or year (particular period). Hydrograph is similar to load curve and used to study the electric power.

A Hydrograph provides the following information:

• The discharge at any time during the period under consideration
• The maximum and minimum runoff during  the period
• The average run-off during the period
• Total volume of the discharge up to any time, given by the area under the curve up to that time

What is the use of Spillway?

Answer: When the water level in the reservoir basin rises, the stability of the dam structure is endangered. To relieve the reservoir of this excess water, a structure is provided in the body of the dam. This structure is called spillway.This is constructed to act as safety value. This provides structural stability of the dam under conditions of the floods without rising the reservoir level above High Flood Level.

What is Mass Curve?

Answer: Mass curve indicates the total volume of run-off in m³ up to certain time. The abscissae may be day, month or year. This curve is obtained from the records of average monthly flow. The slope of curve at any point gives the rate of flow at that time. These curves are used to determine the capacity of the storage reservoirs for hydro projects.

Hydro Electric Power Plants Advantages and Disadvantages

Hydro power plants plays an important role in generation of electrical power. Along with the power generation hydro power plants have other advantages such as providing irrigation and drinking water. Disadvantages of hydro power plants exist such as cost, construction period and dependent in the availability of the water. Some of the advantages and disadvantages of hydro electric power plants are given below:

 

Advantages or Merits of Hydro power Plant:

 Hydro electric power plants offer some inherent advantages or merits over other types of power plants which makes hydro power plants more attractive. Some of the advantages of hydro power are given below:

• Hydel energy is the renewable energy source
• Hydro electric power plants does not pose any air pollution problem as in the case of thermal power plants and radiation hazard and waste disposal in the case of nuclear power plants.
• Used as Peak load plant and it responds quickly to change in load. Hydro power plants are well suited for use as peak load plants as it responds quickly to the changing loads and as its part load efficiency is considerably higher compared to other power plants. Reduction in the pat load efficiency of hydro power plants is considerably less whereas there is a rapid fall of the efficiency with reduction in the load for all the types of thermal power plants. Therefore it is more advantage to operate the hydro power plant at different loads depend on the requirement compared to other power generation power plant types. When the peaking power is to be met for a fraction of the day (peak load duration), there is nothing like hydro-power plants which perform more efficiently. The extra-ordinary facility of switching on and off almost instantly of hydro power plant is great asset in maintaining the grid stability.
• Life of hydro power plant is very long (few centuries) compared to thermal and nuclear power plants (lifetime will be few decades). This advantage of hydro power plant longer life is because operation of the hydro power plants will be at atmospheric pressure. On the otherhand operation of the thermal and nuclear power plants will be at higher temperature ad pressure. Therefore life of the operating equipment falls down significantly.
• Hydro power plants have advantages of no standby losses which are unavoidable for thermal and nuclear power plants
• Hydro-power projects are provide additional benefits or advantages like irrigation, fishery and flood control
• The efficiency of hydel plant does not fall with time unlike thermal and nuclear power plants

 

 Disadvantages or Demerits of Hydro Power Plant:

Hydro power plants also have some disadvantages or demerits compared to other type of power generation sources. Some of the disadvantages are listed below:

• Hydro power plants requires Huge capital and long period for construction (around 10 to 15 years) compared with the thermal power plants having about 3 to 5 years gestation period and lower cost. This is one of the major disadvantage of hydro power plants
• In Hydro power plants power generation quantity is totally dependent on availability of water which depends on the natural phenomenon. The dry year affects the power generation considerably.
• Sites of hydro plants are away from the load centers. For transmission of power to load centers require large investment in transmission lines and it causes loss of power during transmission

Comparing the advantages and disadvantages of the hydro power plant it should be concluded that hydro power plants should act as complement to the thermal and nuclear power sources and should not be used as competitor where the water sources are not abundantly available. Hydro power plants should play a key role in acting as a peak load plant.