COMMISSION INTERNATIONALE DES GRANDS BARRAGES

------- VINGT TROISIEME CONGRES

DES GRANDS BARRAGES Brazilia, Mai 2009

-------

RAPPORT CFBR / Q. 88

TEHRI PUMPED STORAGE PLANT PROJECT: THE CHALLENGE OF HIGH HEAD VARIATIONS

Boris SMONDACK

Project Engineer, COYNE ET BELLIER, France

Daniel FAYOLLE Partner, POWER CONSULTING ASSOCIATES, France

Sabha Kant SHUKLA

Director Technical, TEHRI HYDRO DEVELOPMENT CORPORATION, India

G.M. PRASAD, General Manager Civil and HM Design, TEHRI HYDRO DEVELOPMENT

CORPORATION, India

FRANCE

1 INTRODUCTION The 4x250 MW Tehri Pump Storage Plant (Tehri PSP), located in the North

of India (State of Uttarakhand), is an integral part of Tehri Hydro Power Complex (HPC).

The construction of Tehri PSP is intended to be awarded on an EPC type-base in 2008, for a total implementation duration period of 54 months.

For this project, the main challenging characteristic is the exceptionally wide head range of operation, from approximately 130 to 230 m, in both pump and turbine mode, which will constitute the widest head variation worldwide for a pump storage plant.

The article aims at presenting the context, the main features of this outstanding project as well as the technological solutions that have been retained to cope with the large head variations.

2 BACKGROUND

2.1 PUMPED STORAGE PLANTS AND INDIAN MARKET The development of Pumped Storage Plants in providing peaking power

and maintaining stability in power system is gaining importance in India. The relatively low cost of off-peak energy and surplus power which may come from run off river hydro, thermal, nuclear stations etc. is utilized to pump water from lower (tail) reservoir to an upper (head) reservoir for a Pumped Storage Scheme. The water from the upper reservoir is used for generation of power to meet the demand during peak hours. In addition, pumped storage plants increase capacity utilization and reduce operational problems of thermal power stations, thereby improving the overall economy of power system operation. The energy output from the pumped storage plant being less than the energy input does not obscure the fact that when compared to the substantial savings in fuel, when these stations are operated in an integrated manner, the loss to the system is small.

The Pumped Storage Plants do not depend on hydro potential and can be

developed at sites with very little run-off and also at developed / existing sites with storage / pondage reservoirs (head and or tail). The supply of peak power from pumped storage plants is different from the supply of peak power from conventional hydro plants as its feasibility depends upon reliable availability of surplus off peak power capacity in the system on the basis of existing capacity or on the basis of projected thermal / nuclear plants.

The hydro power potential in India has been estimated as 150 000 MW

(corresponding to 84 044 MW at 60% load factor ) with 845 identified schemes whereas 56 sites of Pumped Storage Plants with total installed capacity of 94,000 MW with individual capacities varying from 600 MW to 2 800 MW have been identified. The Indian Power System requires a hydro thermal mix of 40:60 for flexibility & efficiency in system operation in view of typical load pattern. The present ratio of hydro thermal mix is 26:74, which may further be skewed in view of capacity addition in thermal sources. So far, only about 23% of conventional hydro potential and 3.4% of PSP has been developed in India. The peak shortage of Power is of the order of 13.8%. Thus, there is an immense need for development of hydropower for alleviating peak power shortages.

The regions with identified potential for Pumped Storage Scheme include 13 065 MW in Northern region, 38 220 MW in Western region, 16 650 MW in Southern region, 9 085 MW in Eastern region and 16 900 MW in North Eastern region. The Western region because of topographical features with steep gradients of rivers originating from Western Ghats has the largest potential (about 40% of the total).

It is observed from the frequency duration curve for the months from April

2005 to December 2005 for the Northern region that for around 20 to 30% of time,

grid frequency is more than 50 Hz and for around 5 to 15% of time, grid frequency is lower than 49 Hz. This frequency duration curve would smoothen and would result in optimum balance in grid with the operation of Pumped Storage Plants. The power could be drawn for pumping during off peak hours, when high frequency conditions exist in the grid & the Unscheduled Interchange (UI) charges are low and returned to the beneficiary States during peak hours when UI charges are quite high.

The development of Pumped Storage Plants in the country has not been

encouraged and up to the end of Fifth five year plan i.e. till 31/03/1980 no pumped storage plant was installed. It was only in the Sixth Plan that the first pumped storage hydro electric scheme Nagarjunasagar (7x100 MW) in the State of Andhra Pradesh was installed. First six units of 100 MW each of the Project were commissioned in the Sixth Five year Plan (ending) March, 1985. The maximum pumped storage development had been in Southern region followed by Eastern Region and Western Region.

The availability of sites of conventional hydropower project is on decline; as

such the exploitation of pumped storage potential could go a long way in meeting the ever increasing demand of peak power.

2.2 THE CONTEXT OF TEHRI PSP One of the few sites in Northern region of India suitable for development of

PSP is the Tehri Pumped Storage Plant. The main power house cavern of Tehri PSP is located adjacent to the existing cavern of Tehri Hydro Power Plant, which is under operation and all the four units, each of 250 MW, are in commercial generation mode since July, 2007. Location of Tehri Hydro Complex, in the Ganga Valley in the Himalayas, which has a very large hydroelectric power potential, is illustrated in Fig. 1

The river Ganga (Ganges) has two major tributaries: Bhagirathi and Alaknanda, which join at Deoprayag to form river Ganga. The huge water and power potential of this river remains largely, untapped. Though, few hydro–plants have been commissioned in Ganga Valley, a number of schemes have been identified to tap the vast potential. These schemes are under various stages of investigation and execution. The Tehri Pumped Storage Plant is one of such schemes, the works of which shall be awarded in 2008 on an EPC basis and shall be completed in 54 months time.

Fig.1 Location of Tehri PSP Project

Localisation du projet Tehri PSP

3 PROJECT CHARACTERISTICS

Tehri Pumped Storage Plant is part of the Stage-II development of Tehri

Hydro Power Complex. The Tehri Hydro Power Complex Comprises of Tehri Dam & Hydro Power Plant (4x250 MW), Tehri Pumped Storage Plant (4x250 MW) and Koteshwar Hydro-electric Project (4x100 MW). The Stage-I of the Complex consists of a 260.5 m high earth and rock fill dam, chute and shaft spillways, water conductor system and underground transformer hall and underground power house cavern housing 4 conventional units, each of 250 MW. As mentioned above all the four units are in commercial operation since July 2007. Stage II of the complex consists of a Pumped Storage Plant of installed capacity of 1 000 MW (4 reversible turbine units, each of 250 MW and housed in an

TEHRI PSP

underground cavern) and Koteshwar Hydroelectric Project of installed capacity of 400 MW (4 conventional units, each of 100 MW). The Koteshwar Dam, a 97.5 m high concrete dam is being constructed about 20 km downstream of Tehri Dam on the same river, Bhagirathi. The Koteshwar Project, under advanced stage of construction, shall form the downstream reservoir for the Tehri Pumped Storage Plant.

Many of the structures of Tehri PSP were so designed that they had to be

completed with Tehri Stage I works. These structures included Intakes, headrace tunnels, and maintenance gate shafts. The underground transformer hall of size 161 m (L) x 18.5 m (W) x 34.5 m (H) for Tehri Stage I shall also accommodate the four generator transformers for PSP. The main cable tunnel and interface facility for power evacuation have already been completed. Major civil works to be taken up in PSP involve construction of upstream and downstream surge shafts, machine hall (underground), penstocks, bus duct galleries, tailrace tunnels and outlet structure. The butterfly valve chamber & penstock assembly chamber constructed for Stage I works shall be extended for Stage II works. Each of the two concrete lined headrace tunnels of 8.5 meter diameter, having lengths of 997 m and 1 033 m has an upstream surge shaft at its end. The headrace tunnel bifurcates into two steel lined penstocks in the upstream surge shaft to feed two turbines. The water from turbine units will be discharged into two tailrace tunnels of 9 meter diameter, which would carry water of all the four units into the downstream reservoir. The construction of the underground structures of Tehri PSP shall pose a challenging task as it would be executed within a surcharged rock mass formed because of Tehri reservoir. The layout of Tehri HPP & Tehri PSP is shown in Fig. 2. The cross-section through waterway of PSP is shown in Fig. 3.

The Project would generate an average annual energy of 1 377 GWh . With

the construction of Tehri PSP, Tehri HPC shall function as a major peaking station having an installed capacity of 2 400 MW.

Fig.2 General Layout of Tehri HPP and PSP

Disposition générale des ouvrages de Tehri HPP et Tehri PSP

Fig.3 Tehri PSP – Longitudinal Section through waterways

Tehri PSP – Section longitudinale le long du chemin d’eau

4 PLANT OPERATION: THE CHALLENGE OF HIGH HEAD VARIATION

4.1 OPERATION CHARACTERISTICS The reservoir of Tehri Dam will operate as the upper reservoir and

Koteshwar reservoir as the lower reservoir. The availability of water for Tehri PSP will be therefore governed by the mode of operation of the Tehri Power Complex.

Tehri Dam reservoir has a live storage capacity of 2 600 millions m3, which allows for inter-seasonal regulations. The Tehri Dam which has created the upper reservoir will allow the monsoon water to be stored up to El. 830 m (Full Reservoir Level). The water shall be released as per peak demand of the grid besides fulfilling the irrigation requirements and shall be brought down to Mean Drawdown Level (MDDL) at El. 740 m before the monsoon of the next season. As shown in Fig. 4, yearly decrease of the reservoir water level starts after the end of the monsoon period, that is to say at the end of June. The reservoir reaches its minimum water level at the end of June. During the monsoon period, between July and September, the reservoir level rapidly increases to reach its maximum elevation at the end of September. Within a year, the gross head may vary between around 227 and 127.5 m.

The daily operation of Tehri PSP has negligible influence on the upstream and upstream reservoirs level. Within a daily cycle of pumping-turbining, the gross head can be assumed as constant. During monsoon period, production at Tehri HPP will be maximum. Pumping mode of Tehri PSP is assumed not to be used.

Fig.4 Yearly Mean Gross Head Variation between Tehri and Koteshwar Reservoir

Variations annuelles de la chute brute entre le réservoir de Tehri et de Koteshwar

E s tim a te d G ro s s h e a d e v o lu tio n th ro u g h o n e a v e ra g e d y e a r

1 0 0

1 5 0

2 0 0

2 5 0

June

21-

30

July

01-1

0

July

11-2

1

July

21-3

1

Augus

t 01-

10

Augus

t 11-

21

Augus

t 21-

31

Sept 0

1-10

Sept 1

1-21

Sept 2

1-30

Oct 01

-10

Oct 11

-21

Oct 21

-31

Nov 0

1-10

Nov 1

1-21

Nov 2

1-30

Dec 0

1-10

Dec 1

1-21

Dec 2

1-31

Jan

01-1

0

Jan

11-2

1

Jan

21-3

0

Feb 0

1-10

Feb 1

1-21

Feb 2

1-30

Mar

01-

10

Mar

11-

21

Mar

21-

30

Apr 0

1-10

Apr 1

1-21

Apr 2

1-30

May

01-

10

May

11-

21

May

21-

30

June

01-

10

June

11-

21

M o n th s

Gro

ss H

ead

[m

]

M O N S O O N P E R IO D

Such outstanding feature implies that a conventional synchronous single

speed pump-turbine solution will lead to important drawbacks for the project: (1) as often in pump-turbine performance, the best efficiency point when turbining is out of the operating head range, (2) the efficiency would deteriorate sharply as soon as the head is far away from the rated head particularly in case of low delivery head in pumping (around -20 %), (3) the required submergence to avoid any cavitation risk (estimated at -150 m) is extremely high as illustrated in Fig. 5, (4) it would imply difficulties in designing and manufacturing reliable shaft seal and a risk of counter thrust and (5) imply greater diameters of the upstream and downstream guard valves.

(Head- Discharge) characteristics curve

100

125

150

175

200

225

250

275

300

70 80 90 100 110 120 130 140 150 160 170

Q[m3/s]

Hea

d [

m]

H minimum

H Maximum

(Submergence - Discharge) characteristics curve

0

50

100

150

200

70 80 90 100 110 120 130 140 150 160 170

Q [m3/s]

- su

bm

erg

ence

val

ue

[m] H minimum

H Maximum

Fig.5 Conventional Head-Discharge and Submergence-Discharge characteristics

curves in pumping mode Courbes caractéristiques usuelles de charge-débit et submergence-débit en

mode pompe

4.2 MINIMIZING CONVERSION LOSS

It should be reminded that a Pumped Storage Plant is basically an energy consumer and not an energy producer. Its asset is to value such energy by releasing it at the most appropriate period of time. The consumed energy to perform these “pump-turbine” cycles is therefore of prime interest, as this feature directly impact the profitability of the project. Basically, the losses involved in every cycle - head losses in the hydraulic circuit, turbine conversion losses and electrical losses - should be minimized. Any effort to reduce any of those strongly impacts the performance of the scheme as they are counted twice (during pumping and during turbining).

The Conversion loss, which is to be understood as the total energy loss rate within the year, that is to say for the entire range of head variations, was defined as the key objective performance parameter for Tehri PSP. The challenging target

of THDC to keep the Conversion Loss below 20% directly impacted the technical requirements and the contractual aspects of the projects.

In particular, as described here above, the conventional pump-turbine solution is not able to face such objective. Alternate solution had therefore to be found.

5 TECHNOLOGICAL ALTERNATIVES: VARIABLE SPEED MACHINES Hydropower engineers and turbines specialists have known for a long time

that the efficiency of a hydraulic turbine, working at different heads or loads, can be significantly improved if the turbine speed is adapted to the head or load. In addition, as already mentioned, if the turbine is to be operated as a pump, the optimum efficiency speed for the machine operating as a pump is different to that of the machine operating as a turbine. This is because the specific speed of a pump is greater than that of a geometrically similar machine working as a turbine. Therefore theoretically two different speeds would normally be necessary, for optimal efficiency, if a machine is to operate as turbine and pump, one speed for turbine operation and one for pump operation. Furthermore when the head variation is large it is difficult to design a single machine with good efficiency over the whole head range. Compromise solutions have therefore been generally adopted.

5.1 TWO-SPEED MACHINES The above has been true for a long time. As it is necessary for the machine

to be connected to the grid with a fixed frequency it was difficult to modify the speed of the electrical machine. The first historical step towards the resolution of this problem was to use two speed synchronous generator motor. This technique has allowed obtaining a better overall efficiency operation both as pump and as turbine especially when the head variation was appreciable.

Basically the two speed synchronous motor-generator is a synchronous machine with two windings arrangements in the stator and with a possibility of poles commutation on the rotor in order that the stator and rotor number of poles match. Clearly this entails some complications and the resulting rotor is also heavier. Furthermore the possible combinations of dual speeds are somewhat limited. Nonetheless this technique has been applied with some success on some pumped storage schemes although this came at a cost. The principal problems were the significant increase in cost of the machine added to an appreciable increase in weight of the machine rotor. Furthermore this increase in weight had a knock-on effect on other aspects of the plant. The thrust bearings had to be increased in capacity with consequential increase in thermal losses of this bearing. Similarly the power house crane capacity had to be higher because the

increase in weight for a dual-speed rotor is quite appreciable. Lastly, the machine itself is slightly less efficient than a conventional single speed synchronous motor generator.

5.2 THE ADVENT OF POWER ELECTRONICS The advent of power electronics, with the invention of the thyristor also

called the silicon controlled rectifier, marked a new era for electrical engineering in general. It allowed designers for the first time to design continuously variable, variable-speed alternating current machines. Up to then only direct current machines could have continuous variable speed features. However the first power electronics components were of limited power capabilities and only applications to very small machines could be contemplated in those early years and more specifically the small induction electric motors.

It was not until the 1980’s that we began to see high capacity power electronics components appearing on the market, allowing the design of medium voltage variable speed machines and a whole range of variable speed technologies applicable to both squirrel cage and rotor-wound induction motors and also to synchronous machines. In these early years high power electronics components applications were mainly limited to variable speed of medium voltage induction electric motors of ratings not too far above the MW. Applications were in the field of metallurgy and steel rolling mills and also in the field of railway traction etc. This is also when variable speed application to hydropower became a possibility.

We can say that the first application of power electronics and variable speed to hydropower was the that of the replacement of the traditional starting device of pump turbines of hydroelectric pumped storage plants (which was mainly the pony motor until then), by the static variable frequency converter (SVFC). This electronic device is basically a device for supplying power to the synchronous generator motor at a frequency from 0 to 50 Hz. This is necessary as the synchronous machine is not a self starting machine when connected directly to the grid at 50 Hz. It has to be first brought to synchronous speed before being “locked” onto the grid and then continue to operate as a motor at synchronous frequency. The only power these devices had to supply was for overcoming frictional losses of the dewatered machine in order to bring the machine to synchronous speed. Thus the rating required of these devices is limited and this was also the limit of technological possibilities anyway.

5.3 TECHNOLOGICAL POSSIBILITIES FOR LARGE HYDROPOWER MACHINES The application of variable speed power electronics to large hydropower

synchronous machines has been limited and is still limited by available technology on the market. Hydropower machines are essentially large machines

of tens or a few hundreds of MW and the maximum available ratings of variable speed converters is still modest. Therefore direct application of variable speed to synchronous machines has been somewhat limited.

To circumvent the power limitations of the variable speed converters designers turned their attention to the wound rotor induction machine. By injecting a slip frequency current to the rotor of these machines the machine’s speed could be controlled and made to vary from very low sub-synchronous speed to speeds above synchronous speed. These machines are also known as doubly fed asynchronous machines (DFAM). The slip power required for effecting these variations in speed was much lower than the stator power. Thus a limited rating frequency converter was enough for controlling the speed of a much larger machine. We understand that machines of the order of 400 MVA are planned to be commissioned in the near future.

The first pumped storage hydropower plants to which this technique was applied were commissioned in Japan in the 1990’s. And in those days the technology used was the cycloconverter. In effect the cycloconverter is a power electronics technique for obtaining a low frequency three phase output from a 50 Hz grid supply. Thus the low frequency current obtained was fed to the rotor of the doubly fed asynchronous machine while its stator was fed by the 50 Hz grid.

Since that date a number of pumped storage variable speed units have been commissioned. Most of these machines are in Japan. However one of the latest plants to be commissioned was Goldisthal in Germany where the cycloconverter variable speed technology was also applied.

The cycloconverter is a fairly complicated technique requiring three three-winding transformers and a complicated structure with a large number of power semiconductors. Besides these converters do not allow the starting of the motor up to synchronous speed from the electronic device itself and thus an additional separate SVFC is also required for starting the machine as a motor in the pumping mode.

The latest technological evolution of variable speed for pumped storage schemes is the multi-level Voltage Source Inverter (VSI). These variable speed devices also work by injection of a slip frequency current to the rotor of an asynchronous machine. They have a more simple architecture; do not require the expensive three-winding transformers needed for 12-pulse mode, as the cyclo-converters, and also they are made with far less electronic power devices. In addition these devices allow starting of the machine in motor mode for pumping operations. They are in essence simpler, less bulky, of lower ratings than cycloconverters and offer superior technical features.

5.4 THE CASE OF TEHRI PUMPED STORAGE PLANT In the case of Tehri Pumped Storage Plant, the Owner chose to have

asynchronous variable speed generator motors fed by VSI Inverters Excitation systems. The four machines will have nominal ratings of 250 MW when operating as turbines at a specified head.

Studies have been made by the Owner and his Consultant and the choice had been between dual-speed synchronous speed machines and continuously variable asynchronous machines for some time.

Fig. 6 clearly illustrates the advantage of variable speed versus one single speed or dual-speed. In a conventional fixed speed installation, for a given head, there is only one operation point in pumping mode. Such operation point is, for most lift heads, far away from the best efficiency point. Furthermore, the power input per machine cannot be controlled by the operator. In contrast, with variable speed machine, any point of the doted line within the light blue area is accessible. Consequently the pump can be operated from low power (point n°8) to high power (point n°10) depending on the available power in the network. If the pump is not used for power compensation in the network, it can be operated at best efficiency (point n°9).

Fig.6 Tehri PSP – Illustrative Head-Discharge capacity Curve in Pumping Mode

Tehri PSP - Courbe caractéristique hauteur-débit en mode Pompe

1 Minimum pump Lift 2 Maximum Pump lift 3 Possible Pumping Operation range 4 Optimum operation curve (maximize

efficiency) 5 Maximum speed (256 rpm) 6 Minimum speed (214 rpm) 7 Increase of Speed and Power Input 8 Lowest operation point for a given head

(low Power Input) 9 Optimum operation point for a given

head (maximize efficiency) 10 Highest operation point for a given

head (high Power Input)

1 Hauteur de refoulement minimum 2 Hauteur de refoulement maximum 3 Zone d’opération en mode pompe 4 Courbe de rendement optimal 5 Vitesse Maximum (256 t/min) 6 Vitesse Minimum (214 t/min) 7 Sens de l’augmentation de la vitesse et

de la puissance consommée 8 Point bas d’opération pour une hauteur

donnée (basse puissance consommée) 9 Point optimum d’opération pour une

hauteur donnée (maximum de rendement)

10 Point haut d’opération pour une hauteur donnée (haute puissance consommée)

In the case of Tehri PSP the plant is intended to be operated so that the machines are mostly run at their optimal efficiency point whatever the head available. This means that the machine’s speed is intended to be mostly adjusted in such a way that they always run at best efficiency point of the pump turbine on the best efficiency locus line (illustrated by a red line in figure 6).

Given (1) the extremely large head variation for operation of this plant, (2)

the expected efficiency gains, minimizing therefore the total cycle conversion losses (3) the flexibility of operation in pumping mode and (4) the lower required submergence, the choice was finally made to use continuously variable speed asynchronous machines with VSI Excitation. In essence Tehri PSP is intended to be a modern, state-of-art technology plant offering very interesting features on the plant efficiency standpoint.

6 CONCLUSION Tehri PSP is a challenging project requiring state-of-the art technology in

civil works and electro-mechanical equipments. Despite the outstanding large head variations, the optimization of conversion efficiency has been a constant objective during design stage of the project. Adopting the latest available technology with variable speed machines will not only fulfil such challenging target, but it will also provide a much more flexible mode of operation. This technology should meet with a rapid expansion, in particular for Pump Storage Plants, for which conversion efficiency is of prime importance.

REFERENCES [1] COYNE ET BELLIER & EDF, Tehri Pumped Storage Plant Project,

Detailed Project Report, May 2002.

SUMMARY The development of Pumped Storage Plants in providing peaking power

and maintaining stability in power system is gaining importance in India. One of the few sites in Northern region of India suitable for development of PSP is the Tehri Pumped Storage Plant (Tehri PSP). The 4x250 MW Project, located in the North of India (State of Uttarakhand), is an integral part of Tehri Hydro Power Complex (HPC) comprising of (1) a 260-m-high earth-and-rock-fill dam (Tehri dam), already completed, (2) an underground 1000-MW hydropower plant (Tehri HPP), completed, (3) a combined 400 MW hydro-power plant and dam (Koteshwar), currently under construction.

Major civil works to be taken up in PSP involve the challenging construction of upstream and downstream surge shafts, underground machine hall, penstocks, bus duct galleries, tailrace tunnels and outlet structure.

The reservoir of Tehri Dam will operate as upper reservoir and Koteshwar reservoir as lower reservoir. The availability of water for Tehri PSP will be governed by the mode of operation of the Tehri Power Complex. During non-peak hours, water from lower reservoir would be pumped back to upper reservoir by utilizing the surplus available power from the grid.

For this project, the main challenging characteristic is the exceptionally wide head range of operation, from approximately 130 to 230 m, in both pump and turbine mode, which will constitute the widest head variation worldwide for a Pump Storage Plant. Such a feature implies that standard solution with single-speed reversible units would lead to major drawbacks: high loss in pump-turbine efficiency for large portions of the head range, cavitation risk, prohibitive setting level, rigid mode of operation. The first alternative solution would consist in installing “two-speed” synchronous generator-motors coupled to conventional pump-turbines. As a result, two distinct synchronous generator-motors windings would be available, each with its own rated speed but in the same frame. Depending on the available head, the most suitable speed would be then selected for allowing the pump-turbine to work with the best efficiency. The second alternative consists in installing “variable-speed” asynchronous generator-motors coupled to conventional pump-turbines. Such arrangement allows the unit to work within a variable speed range. Frequency is then adjusted in order to enable the pump-turbine to operate at the highest efficiency possible for each available head.

Given that one the main objective of the scheme is to minimize the mean loss for a pump-turbine cycle, and given its high operation flexibility, the variable speed solution has been selected for Tehri PSP. This technology should meet with a rapid expansion, in particular for Pump Storage Plants, for which conversion efficiency is of prime importance.

RESUME

L’utilisation de stations de pompage-turbinage, qui permettent de fournir de la puissance pendant les périodes de pointe et contribuent à la stabilité du réseau électrique, se développe en Inde. L'usine de turbinage-pompage de Tehri (Tehri

PSP) est un des emplacements de la région nord de l'Inde approprié au développement de ce type d’aménagement. Ce projet de 4x250 MW, situé dans l’état de l'Uttarakhand, fait partie du complexe de « Tehri Hydro Power » (Tehri HPC) comprenant (1) un barrage en enrochements de 260 m de hauts (barrage de Tehri), construit, (2) une usine souterraine hydroélectrique de 1000-MW (Tehri HPP), réalisée, (3) une usine d'hydroélectrique de 400 MW et un barrage (Koteshwar), actuellement en cours de construction.

Les travaux de génie civil comprennent la construction de cheminées d’équilibre amont et aval, d’une salle souterraine des machines, de conduites forcées, de galeries de transmissions électriques, de tunnels hydrauliques aval et d’une structure de sortie.

Le réservoir du barrage de Tehri fonctionnera en tant que réservoir supérieur et la retenue de Koteshwar en tant que réservoir inférieur. La disponibilité en eau pour Tehri PSP sera régie par le mode de fonctionnement du complexe de Tehri. Pendant les heures creuses, l'eau du réservoir inférieur sera pompé vers le réservoir supérieur en utilisant le surplus de puissance disponible dans le réseau électrique.

Un des caractéristiques exceptionnelle de ce projet est la variation importante de chute, de 130 à 230 m environ, en pompage et en turbinage, ce qui constituera la variation la plus importante pour une usine de turbinage-pompage. Un tel dispositif implique que les solutions usuelles avec unités réversibles à simple vitesse mèneraient à des inconvénients majeurs : chute du rendement de la turbopompe pour une grande partie de la tranche de fonctionnement, risque de cavitation, niveau de réglage vertical de l’axe des turbines prohibitif, mode de fonctionnement rigide. Une première alternative consiste à installer des générateur-moteurs synchrones « à deux vitesses » couplés aux turbopompes conventionnelles. En conséquence, deux enroulements synchrones distincts, mais dans la même armature, sont disponibles, chacun avec sa propre vitesse spécifique. Selon la chute disponible, la vitesse la plus appropriée peut alors être choisie pour permettre à la turbopompe de fonctionner avec le meilleur rendement. La deuxième alternative consiste à installer des générateur-moteurs asynchrones à « vitesse variable » couplés aux turbopompes conventionnelles. Un tel arrangement permet à l'unité de fonctionner dans une marge variable de vitesse et donc de débit. La fréquence peut alors être ajustée afin de permettre à la turbopompe de fonctionner au rendement le plus élevé possible pour chaque hauteur de chute disponible.

Étant donné qu’un des objectifs principal de l’aménagement est de réduire au maximum les pertes énergétiques des cycles de turbinage-pompage, et étant donné la meilleure flexibilité d'opération, la solution à vitesse variable a été finalement retenue pour Tehri PSP. Cette technologie devrait rencontrer une expansion rapide dans un avenir proche, en particulier pour les stations de turbinage-pompage, pour lesquelles les questions de rendement sont primordiales.

KEYWORDS / MOTS CLES Tehri Dam Barrage de Tehri Cavitation Cavitation Earthfill Dam Barrage en remblai Power Station Centrale Power Supply Production d’énergie