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PROJECT LOCATION

State Sikkim
District West
Stream Rathang Chu River
Location of  (nearest village) Yuksum
Latitude 270 13” N
Longitude 880 12” E

HYDROLOGY

Catchment area at Dam Site 372 sq. km
Average Annual precipitation in snow fed catchment 1200 mm
Average Annual precipitation in rain fed catchment 2500 mm
Average Annual Inflow        2578 mm
Flood Discharge for River Diversion (~Q25) Non-Monsoon  125 Cumec
Standard Project Flood Discharge (SPF) (~Q 500) 1415 Cumec
Probable Maximum Flood Discharge (PMF) 1885 Cumec

DAM AND APPURTENANT STRUCTURES

Type Concrete Gravity
Length of dam at  top 98.5 m      
Spillway Width 36 m
Spillway Crest Level: 1130.00
River Bed Level (Lowest) 1125.00 m
Minimum Draw Down Level 1143.00 m
Full Reservoir Level 1165.00 m
Top level of Dam 1169.00 m
Pondage above MDDL (Diurnal storage) 0.46  MCM
Submergence area at FRL      (approx) 4.02 ha
Stretch of Reservoir 0.46 km

Spillway Gates

Type Radial
No. & Size 3.0 nos. of 6.0m(w) x 6.5m (H)
RIVER DIVERSION AT DAM

Upstream Cofferdam

Crest Elevation EL 1142  m
Length 35.8 m

Downstream Coffer dam

Crest elevation El. 1125.00 m
Length 63.1 m

Diversion Tunnel

Shape & Size D-Shape, 3.6 m
Length 131.0 m
Invert level at inlet EL 1137.0 m
Invert level at outlet EL 1122.4 m

Intake

Location

On Left bank, 5.9 m upstream of Dam axis

Size of opening for trash rack

16 m x 6.02 m

Size of Intake opening

4.2 m(W) x 4.4 m(H)

Invert level of Intake at Entry

EL.1133.80  m

Design Discharge 46.13 Cumec
Velocity of flow through Trash racks 0.75 m/sec
Feeder Tunnel from Intake One no. of 4.4 m Horse Shoe Shaped
Invert Level of Feeder Tunnel EL.1134.40 m
No. Type & Dimension of Gates One, Vertical lift gate,4.2 mx4.4 m
Trash Rack Units 8 panels of  4.0 m x 3.01m

HEADRACE TUNNEL-HRT

Shape & Size Horse Shoe, 4.4 m
Length 2141 m  
Flow Velocity at Design Discharge 2.87  m/sec
Design Discharge 46.13 cumec
Lining Thickness (PCC) 200 mm to 300 mm

Adit To HRT

Location  Just U/s of Surge shaft
Slope & Size D-shaped, 4.5 m
Length   122 m
Level El. 1120 m

SURGE SHAFT

Vertical Shaft Restricted Orifice Type
Internal Diameter 10 m
Height of shaft 63.9 m
Concrete lining thickness Varying from 300 mm to 600 mm
Lining Type RCC Lining
Vertical Lift Gate size 3.4 m (W) x3.4 m (H)
Top of Surge shaft El. 1188 m
Bottom of Surge shaft El. 1124.1 m
Max. Surge level El. 1185 m
Min. Surge level El. 1127.80 m
Orifice dia. 2.25 m

Adit to HRT & Surge Shaft Bottom

Shape & Size
D-Shaped, 4.5m
Entry Sill Level EL1120.70 m
Length 77 m

PRESSURE SHAFT PENSTOCK

Total Length upto Bifurcation 760 m
Number of anchor blocks 3
Diameter after Bifurcation 2.4 m
Branch penstock length 23.1 m  & 32.6 m
Steel liner thickness 16  mm to 36 mm

Grade of Steel lining

ASTM 537 Class-II

POWERHOUSE

Type

Surface Powerhouse

Size 48  m (L) x18 m (W)x44 m(H)
Number of units Two (2)
Rated Discharge per unit 23.06 Cumec
Turbine Speed 500  rpm
Min. Tail Water Level EL912.60 m
Normal Tail Water Level EL 915.00 m
Max. Tail Water Level EL 924.00 m
Gross Head (monsoon period) EL 250 m
Net Rated Head 233.98 m
Installed Capacity 2x 49.5 MW
Annual Plant Load Factor (90 % year) 0.473
Inlet Valve Type Spherical Valve
Number 2
Inlet Valve diameter 1.8 m
Turbine Axis Elevation EL 904.60 m
Generator type Suspended
Nominal Speed 500 rpm
Voltage / Frequency 11 kV /50 Hz
Power Factor 0.9
Draft Tube Gates, No & Size Two,  4.71 m (W) x 3.764 (H)

TRANSFORMER

Type and capacity Single phase,11KV/220KV, 21.0  MVA, OFWF
Location  Outdoor on left bank of the river
Number 7

TRAILRACE TUNNEL

Type Open Channel
Length (Including tail pool) 78.5 m
Bed Slope (Tail Pool)     1:4
Nominal Discharge   50.76 Cumec
River Bed Elevation EL.686.00 m

SWITCHYARD

Type

Twin box, cut and cover tunnel

Location of Switchyard Downstream of  PH on right bank at El. 940.00 m
Bus bar Voltage  220 KV         

TRANSMISSION LINE

Type Switch yard  to pooling station
Single Circuit 220 KV

ESTIMATED COST

Civil Works (Including gates & hoists)

Rs.  226.61 Crore

E&M Works (Including cost of Transmission line to pooling station) Rs. 167.83 Crore
Total Basic Cost Rs. 394.44 Crore
Escalation during construction Rs. 34.76 Crore
Interest during Construction Rs. 59.93  Crore
Total (Generation Works) Rs. 489.13 Crore
Cost per MW installed Rs. 4.91 Crore

POWER BENEFITS

Design Energy Generation (90% Dependable Year with 95% machine availability) 410.24 GWh
Annual Energy Generation in  (90% Dependable Year)   424.09 GWh
Annual Energy Generation including additional energy  utilizing high inflow  (90% Dependable Year)
444.63 GWh
Design Energy Generation (90% Dependable Year with 95% machine availability) including additional energy utilizing high inflow 429.75 GWh
Annual Energy Generation including additional energy  utilizing high inflow  (50% Dependable Year) 478.77 GWh

FINANCIAL ASPECTS

Levelized Tariff  for Design Energy at 90%Dependable Year       

Rs 2.15 /kWh

FIRR at Rs 2.22 /kWh tariff (35 years 8.96 %
FIRR at Rs 2.5 /kWh tariff (35 years) 11.09 %

CONSTRUCTION  PERIOD

Construction Period in months (excluding pre-construction works) 30 months