Hydropower Engineering I-6812(3)PPT

8/16/2019 Hydropower Engineering I-6812(3)PPT

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Classification of Hydropower Plants

H dro ower lants exhibit a reat deal of variet .

.

Almost every hydropower project has some special features uncommon withother projects of the same type.

us, y ropower p ants cou e c ass e on t e as s o :

• The hydraulic features of the plant

• O eratin features of the lant

• Plant Capacity

• Construction Features (layout)

• Location & topographical features

• Presence or absence of storage

• e range o opera ng ea s


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A complete understanding of the types of hydropower

developments requires information under all such

categories.

An im ortant oint which should be borne in mind is that

all those classifications are not mutually exclusive.

us, e presence or a sence o s orage a so o some

extent determines the hydraulic features of the plant.

The operational features of the plant are determined

b resence or absence of stora e.


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.The basic hydraulic principle governs the type of classification in this

category.

. -

Use normally available hydraulic energy of the flowing water of the rivers. e.g. Run-of river plant, diversion plant, storage plant

ii. Pumped storage plants

Use the concept of recycling the same water by using pumping

selectively.

.

It generates energy for peak load, and at off-peak periods water is

pumped back for future use.

A pumped storage plant is an economical addition to a system which

increases the load factor of other systems and also provides additional

capac y o mee e pea oa .


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. -

a) Tidal power plant

se e a energy o e sea wa er.

Very few have been constructed due to structural complication.

b) Wave power plant

c) Depression power plant

Hydropower generated by diverting an ample source of water (e.g. sea

water) in the natural depression which provides operating head for the

plant

Water level in the depression is controlled by natural evaporation


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2. Classification based on actual operation in meeting the demand

isolated plant (SCS)- operating independently

mini and small hydropower scheme serving small community

interconnected into rids ICS

Thus in a grid system, a power station may be distinguished as a

base load plant or peak load plant.

Hydropower plants are best suited as peak load plants, because

hydropower plants can start relatively quickly and can thus accept

load quickly


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.

over a year.

Construction of a dam usually implies a much more

efficient and controlled use of the available water.

Without storage, the plant uses only the natural flow as

best as it can.

In such cases, only a mini-reservoir or a pondage which

a es care o ay- o- ay uc ua ons may e necessary.


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4. Classification based on location and topography

dams;

whereas plants in plain areas may have only weirs for the

main structure.

For plants situated far in the interior and away from load

centers the transmission costs are relativel more.

Thus the knowledge about the location and topography of a

.


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5. Classification based on plant capacity Classification of hydropower plants on the basis of plant capacity changes with time as technology

improves.

Classification according to Mosonyi, and present day trend are:

According to Mosonyi:

ii) Low capacity plant < 1 MW

iii) Medium capacity plant < 10 MW

iv) High capacity plant > 10 MW

Present day classification:

<

ii) Medium capacity plants 5 to 100 MW

iii) High capacity plants 101 to 1,000 MW

iv) Super plants above 1,000 MW


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. While any of the above classifications could be used to describe a

the head operating on the turbine.

n s as s:

i) Low head plants < 15m

ii) Medium head plants 15-50m

iii) High head plants 50-250m

iv) Very high head plants > 250m


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.(layouts)

classified based on the dominant construction features of

e p an as:

Run-off-river plants (low to medium head plants)

Valley dam plants (Medium to high head plants)

Diversion Canal Plants

High head diversion plants

umpe s orage p an s


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Site Selection, Layouts & Arrangementso o ediu Hi h Head Pla ts

1.Run-off-river plants (low to medium head plants)

The normal flow of the river is not materiall disturbed due to the

construction of the plant;

They have small ponds to provide the necessary pondage in order tobalance day-to-day fluctuation;

Such plants neither have a significant storage nor do they have a

A weir or barrage is built across a river & the low head created is used to

generate power.

It also acts as a controlled spilling device


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Preferred in perennial rivers with moderate to high discharge, flat

slo e with low sediment and stable reach of a river.

Water enters the power house through an intake structure

incorporating some or all of the following:

Entrance flume separated by piers and walls for each machine

unit.

The appurtenances of the entrance structure are the sill, fine rack or

screen and gate;

ur ne c am er o scro case w ur ne;

Concrete or steel draft tube;

ower ouse u ng


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Depending on different arrangements, Mosonyi proposed thefollowing groupings of the run-of-river plants:

i. Block power plant,

ii. Twin power plant,

iii. Pier-head power plant,

iv. Submersible power plant

These groupings are mainly on the basis of constructional arrangements of the

power house vis-à-vis the weir.


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is the most widely used arrangement among the above mentioned

layouts.

Power house provided along one bank adjacent to weir and separated

only by a divide wall;

ac ua w s sma may e en arge y excava ng a ay w c

may offer many advantages.

Amon the advanta es is havin sufficient weir len th which can ass

the flood without obstruction.

However, one side bay gives rise to curvilinear flow and adversely

affects the turbine efficiency. The eddies and vortices developed in the bay may also move the bed

load sediment which eventuall enters the machines.


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The consideration of the choice a ba is based on the oint that the width of the river left after theaccommodation of the plant should be sufficient to pass the design flood without creating

unfavorable flow conditions.

, , , .

Twin Power Station:

is similar in arran ement with block ower station exce t that instead of a

single power house, two power houses at the two banks are provided.

If the plant discharge capacity is large at a low head, then it becomes

difficult to achieve satisfactory flow conditions in a single bay and uniform

normal flow to the racks, because of an unusually long power station and a

arge ay.

Under such circumstances a twin power house is preferred by dividing the

.


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the straight short cut of a bend or in bends where the bed load

is not heavy.

more uniform current in all operating conditions compared toblock power stations.

A variation of this type of power house is the island-type

arrangement in which a block type of power station is located

centrally and on both sides of it are the portions of the weir. The twin power station presents some difficulties which

outnumber the advantages of it.

,

higher maintenance and supervision costs and

the practical difficulty of carrying the cables with high voltage are

.


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is one which houses a turbine generator set in each pier (hollowed

out iers of the weir.

Under special requirements, a pier can also house two generating

sets.

A pier head power station is suitable when there is no possibility of

widening the river bed and the river stretch is straight or slightly

curved.

This is one of the advantages of this layout where the valley is

comparatively narrow.

This layout gives the most uniform current distribution in all flow

conditions.


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u mers e power s a on:

In this type of plants, the machine hall is provided under

the body of the weir.

The weir floor serves racticall as the roof of the

machine hall.

s ype o ayou s se ec e or ow ea s – m n

rivers with little bed load transport and large floods.


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2. Valley dam plants (medium to high head plants)

The dominant feature is the dam that creates the required storage (to

balance seasonal fluctuations) and necessary head for the power

house;

Power house is located at the toe of the dam;

No diversion of water away from the main river is involved;

a er ows roug e pens oc em e e n e am or ver e n o

a cannel/tunnel system to deliver flow to the power house;

- -

spillway location. If the spillway is in the central portion of the dam, then

the power house may be located on one of the banks or as twin power

house, one on each bank.


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Single power house

Twin power house


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Important components of a valley dam plant:

The dam with its appurtenance structures like spillway,

energy dissipation arrangements, etc;

, ,

The penstock conveying water to the turbine with inlet

valve & anchorage;

The main ower house with its com onents.


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HW

TW


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,

immediately at the toe of the dam but at some distance

ownstream.

Such arrangement is costlier than the more general dam-

and-power house-together-arrangement and is adopted

onl when it offers some s ecial advanta es

like achieving extra head (e. g. Melka Wakena HP).

The arrangement, however, needs longer conveyances

with consequent losses


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Storage plant (remote development):

a considerable distance over which the water is

conveyed, generally by a tunnel and pipeline, so as to

achieve medium and high heads at the plants ;

The reservoir storage upstream of the dam increases the

firm ca acit of the lant substantiall and de endin on

the annual run-off and power requirements, the plant may

- .


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. vers on cana p an s The distinguishing feature is the presence of power

channel; The power house is provided at suitable location along

the stretch of the canal;

The water often flowing through the turbine is

Diversion canal plants are generally low head ormedium head plants;

They don't have storage reservoir; Pondage requirement is met through a pool called


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The development of the required head in diversion canal plants may be

achieved by:

The head ma be made available due to the flatter bed slo es of ower

canal (as compared with the river); besides, due the river meanders, the length of the river between two points

may be much greater as compared to that of the relatively straight reach of

the channel

,

& locating the power house at the downstream side of the fall provide the

required head;

In inter-basin diversion, water may be diverted from a higher level river to a

lower river through a diversion canal to the power house located at the lower

r ver;


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most suitable on rivers either of steep slopes or meandering reaches.

, ,

have to be moderate.

- - , ,

large discharge development.


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1) Diversion weir with its appurtenant structures;

2) Diversion canal intake with its ancillary works such assills, trash racks, skimmer wall, sluices, settling basin,

de-silting canal, and silt exclusion arrangement is

3) Bridges or culverts of the diversion canal;

4) Forebay & its appurtenant structures.


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arrangement:

1) A diversion weir to create pondage (and no storage). Here likerun-of-river lant the ower roduction is overned b thenatural flow in the river.

2) Storage may be provided on the main river at the point of diversion which feeds into the diversion system. This secondsituation is advantageous since the fluctuation in reservoir leveldoes not materially affect the head and the power output can beadjusted by the controlled flow release from the reservoir.

. . .

This advantage is not available to the valley dam plant in which thepower house is built on the downstream face of the dam.

Under such cases, a change in reservoir level also changes the headpropor ona e y.

If the length of the pressure tunnel is considerable, a surge tank maybe provided upstream of the power station, which may smoothen thefluctuation of flow demand.

This purpose was served in the canal plants by the forebay.


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Canals follow the contours of the terrain and thus

may not have the shortest route from the intake to

the power house.

unne s, owever, can ma e roug e r way y

the shortest distance and thus create enormousheads apart from enabling to divert water of one

basin to another


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Main Com onents of hi h head diversion lants: Diversion weir with appurtenant structures;

ana unne ;

Head race either open cut or tunnels with its

structures;

Penstock;

Power house;

Tail race.


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to the low head diversion canal plants.

e ma n po n o erence s, owever, e e a ora e

conveyance system for the high head plants (diversion

tunnel plants).

In the diversion tunnel type plant; the dam replaces a diversion weir,

reservoir intake is used instead of a canal intake and

a surge tank is employed in place of a forebay


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-. Pumped storage plants are special types of power

plants which work as ordinary conventional

h dro ower stations for art of the time.

Pumped storage plant is suitable where:

the natural annual run-off is insufficient to justify a

conventional hydroelectric installation;

It is possible to have reservoir at head & tail water

.


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This kind of plant generates energy for peak load, & at off

peak period water is pumped back for future use.

During off peak periods excess power available from some

the lower reservoir.

Various arrangements are possible for higher and lower

1) Both reservoirs in a single river;

2) Two reservoirs on two separate rivers close to each other andflowing at different elevations;

3) Higher reservoir on artificially constructed pool on a high level

plateau or on a leveled hilltop and the lower reservoir on natural

river;4) The lower reservoir in a natural lake while the higher one is on

artificially created reservoir.


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-classify them as pure or mixed operation.

A ure um ed stora e lant is a closed c cle lant with the

volume of water flowing to the lower reservoir being equal to thevolume pumped to the higher reservoir in one cycle of operation.

In such a system, same water is circulated again and again and

thus except for make-up quantity of water for seepage andevaporation losses, the plant does not need any fresh water flow.

In mixed plants the total generation in one cycle is greater than

the total pumping during that period. In mixed type of plants, the

higher reservoir has to be necessarily on a natural stream so as

to provide greater flow during generation.


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- on the basis of cycle of operations.

Some lants are o erated on a dail c cle of um in

and generation;

ome are p anne on a wee y cyc e w ere e

pumping is confined to slack weekend periods only;

A few pumped storage plants have been built on a

seasonal cycle where the pumping is done during

seasons of lean demand and generation during high

.


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relative arrangements of turbines and pumps.

Four-unit installation - pump, motor, generator, turbine;

Three-unit installation - pump, turbine and generator

which can also function as a motor – both the pump and

.

In this case, when the turbine runs, the unit operates as a

generator and when the pump is operated the same unit operates

Two-unit installation - generator, turbine or reversible

pump-turbine installation.

The modern trend is to use only a two-unit installation, namely, agenerator which operates as a motor coupled to a turbine which in

turn also operates as a pump when rotating in reverse direction.

This arrangement is called reversible pump-turbine installation.


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-

locked together and the pump can be coupled during the pumping

phase

Reversible pump-turbines: Any reaction turbine can, technically

speaking, work as a pump if the direction of rotation is reversed.

, ,

runners and the versatile Francis turbines, all can be used as

reversible machines.

The salient design features of reversible pump-turbines are not

markedly different from those of conventional turbines.

Lar e ca acit units are usuall Francis t e reversible um -

turbines. For low head developments, propeller/Kaplan turbinesare suitable


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e opera ng c arac er s cs o e revers e mac nes

are different when it runs as a turbine and as a pump.

If the rotational speed is kept constant during both modes,

the discharge during the pumping phase is less than the

discharge during the turbine operation.

The maximum efficiency of the pump-turbine as a pump

occurs at a different speed as compared to its running as a

turbine.

In order to obtain good efficiencies at the same head, some

.


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any es gns, owever, rom e s mp c y po n o

view, keep the same rotational speed during both

phases.,

different heads.

Problems of operation: The main problem of a high head

pump is cavitation.

Cavitation is the phenomenon which manifests in the flow when

the pressures are nearing vapour pressure of water.


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Thoma has su ested a cavitation arameter σ for turbines and

pumps as;

Where hb, hs, and h are the barometric head, the suction head (or

head on the pump, respectively.

According to Thoma, for cavitation free running, σ, has to be greater

For high values of head h, hs comes out to be negative and hence it

becomes necessary to provide the pump with negative suction head.


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,fixed that the pump operates under submerged condition.

The magnitude of submergence depends upon the

specific speed and the net head.

If the submergence required is high, the power house

.

As a result, many of the pumped-storage plants have

underground power houses.


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. ea : eservo r storage requ rements are nverse yproportional to head (Figure below),

so reservoir costs can be minimized b selectin a site with

a high head.

Hydraulic capacity is also inversely proportional to head.

so pens oc ame er, an ence pens oc cos s, can a so

be minimized by maximizing head.

For a given plant capacity, powerhouse costs are lower for high head plants.

This is because the units run at higher speeds and high-

s eed machines are smaller than low-s eed machines.

Because smaller water volumes are required at high headplants, reservoir drawdowns are usually smaller at both

.


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Figure. Reservoir storage required vs. head for 1000 MW plant with 14

hours of storage


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. ,

penstocks, and discharge tunnels) can represent one-quarter or

more of a um ed-stora e ro ect’s costs

so sites should be sought which will require minimum penstockand dischar e tunnel len ths.

This is particularly important at the lower head sites, because of

the lar er enstock and tunnel diameters involved.

The economic limits to length of water conduits is a function of

head and can be ex ressed in terms of the len th between the

two pools along the water passage to head (L/H) ratios.


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e ess e va ue o s ra o, e more

economic is the pumped-storage project.

Recent experience suggests that maximum

acceptable L/H ratios range

from 10 to 12 for high-head (370-460 m.) projects

down to 4 to 5 for low-head (150-180 m.) sites.


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.

either with a dam across a natural valley or with an enclosure

, .

To minimize costs, sites should be sought where minimum

excava on an em an men vo umes are requ re , an

sites having natural depressions are particularly desirable

Large drawdown may cause slope instability,

so sites with large, relatively shallow reservoirs are usually

preferred to narrow, steep reservoirs.


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. ower eservo rs: ro ec cos s can o en e re uce

by using existing reservoirs as lower reservoirs.

However, care should be taken to insure that sufficient storage

is available to handle fluctuations due to pumped-storage

operation in addition to fluctuations resulting from existing

reservoir operations. Because of the limited head range for efficient pump-turbine

operation and submergence requirements, caution should be

exercised when considering the use of existing multiple-

ur ose reservoirs with lar e fluctuation ran es.


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,

Then,

Where = the overall efficienc of eneration includin

turbine, generator and transformer efficiency).

And

p .

Then,

=


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then

Average values of ηt, ηp, and k are respectively 0.88,

0.85 and 0.02 to 0.03. With these values the overall

efficiency comes out to be 72%.


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Example

A closed cycle pumping-storage plant with a gross head of

350 m, has a head race tunnel 4 m diameter and 700 m.

reservoir. The flow velocity is 6.5 m/s and the friction factor

= . . e overa e c enc es o e pump ng an

generation are 85% and 88%, respectively, estimate the

plant efficiency.


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o u onFriction

head

loss

=

.

m

Therefore, hf = kH

.

K = 0.0194 ≈ 0.02


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,

=

71.86%

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