TIS ABAY II HYDROELECTRIC PROJECT

Abebe Tesfaye


GENERAL

Tis Abay II Hydroelectric Project, which has entered construction phase, is expected to be commissioned in April 2001. From the highly potential resources of Abay River, Tis Abay II Hydroelectric Project is the one whose construction is going to be completed, and start generating electricity in very near future.BlueNileIII.jpg (293447 bytes)

The commissioning of Tis Abay II Hydroelectric Project will boost the current installed hydropower capacity of the country from 375 MW to 450 MW (20% increase on the existing installed hydropower capacity).

 

1. CONTEXT OF THE PROJECT

Tis Abay II hydroelectric power plant is located on the Abay River, some 32 km down stream of Lake Tana at site where the riverbed suddenly drops by approximately 45 meters; thus creating the well-known Tis Issat Water Falls.

This head naturally created by the falls has already been used to generate electricity since 1964 and named, Tis Abay I hydroelectric plant with installed capacity of 11.4 MW.

The construction of a low height weir at Chara Chara in 1995-1997, across the Abay River at the out let of Lake Tana, enables to regulate the out flow of water from the Lake Tana as long as the lake level is below the crest of the spillway (1787.00 mast).

The above mentioned main important features, regulated flow from Lake Tana and a natural head at 35 km down stream of the lake, had created ideal conditions for an economical and environment friendly hydropower to be designed and implemented.

With aim of such project development, the Engineer (Joint Venture Coyne et Bellier, Howard

Humphrevs & Rust Kennedy & Donkin) had commenced its Feasibility Studies of Tis Abay II Hydro Power Project under the Ministry of Water Resources (MWR).

In 1996 the Consultant has finalized its study and submitted the results of the study, which was more detailed in discussing the important features of the project and its impact on the environment.

The results of the study has confirmed that, unlike other Hydropower projects with a big dam, Tis Abay II Hydro Electric Project has negligible impact on the environment and it is economically very attractive for investment.

The study finally leads to a scheme which consists of building a new hydropower plant named Tis Abay II Hydro Electric plant capable of turbining a discharge of 150 m3/s, with an approximate gross head of 53 m, with 73 MW installed capacity producing a firm energy of 359 GWH yearly.

 

2. NATURAL FEATURES OF THE PROJECT AREA

2.1 Hydrology

The Chara Chara weir regulates the out flow of the Lake Tana, which controls a catchment area of 15320 km2 including approximately 3000 km2 of Lake Tana. The intermediate catchment between Chara Chara and Tis Abay has an area of 1,100 km two third of which is the catchment of Andassa River, main tributary of the Abay River on the right bank.

The controlled active storage of Lake Tana, between elevation 1784.00 and elevation 1787.00 masl, is 9.1 Billion m3, which represents approximately 2.4 times the average annual out flow of the lake.

On average, the contribution of the intermediate catchment, (between Lake Tana and Tis Abay roughly 35 km apart) to the discharge of Abay River is 12 m3/s.

 

2.2 Topography

The topography of the project area is characterized by the plateau shape of the right bank of the Abay River and the narrow gorge, which has been formed by the River (see. Fig.1).

2.3 Geology

The project entirely lies in a series of Lava flow of the Aden formation, which is also most probably the cause of the formation of Lake Tana.

The plateau of the right bank, on which the entire project is situated, is formed successively from top to bottom by:

• a variable thickness of overburden material made essentially of black cotton soil, weathered product of basaltic materials. This layer is generally 2 to 3 m thick along the waterways.

• a layer of strong basalt, approximately 10 to 20 meters thick.

• a layer of poor volcanic material, 20 to 25 m thick

• a layer of strong basalt again at the bottom of the series.

 

 

3 THE MAIN COMPONENTS OF TIS ABAY II HYDRO ELECTRIC PLANT (TIS ABAY II HEP) SCHEME

 

3.1 Chara Chara spillway at Lake Tana (see Figure 1)

The water required to turbine both hydropower plants (Tis Abay I&II HEP) and also to the fall at Tis Abay location is being regulated and released using the seven gates at Chara Chara (two of which were constructed in 1995 — 1996 together with 635 m long spillway).

The released water will start its natural journey having its famous name as Abay River or the Blue Nile, after being joined by Andasa River. It wilt travel approximately 32 km to face a sudden 45 m fall of its bed at Tis Issat Fall.

Just before the fall on the plateau, there is a weir to divert some of the released water at Chara-Chara to be used for power generation at Tis Abay I HEP & TIS Abay II HEP with a maximum discharge of 30 m3/s & 150 m3/s, respectively.

 

3.2 Head Pond at Tis Issat Falls, Main Intake

 

The diverted water from the fall using small weir constructed on the top of the fall, will be momentarily accumulated in the pond to be led into main channel through four main intake gates. The poundage capacity has been increased by about 80,000 m3 of excavation and by rehabilitating the small weir along the edge of Tis Issat Fall, which was constructed together with Tis Abay I HEP.

There are four main intake Gates, two of them are newly constructed by demolishing and extending the left wing of the old gates which was serving Tis Abay I hydro electric plant for more than 30 years.

The four gates are capable to allow inflow of 180 m3/s, which is the maximum discharge, required to run both plants Tis Abay I & Tis Abay II.

 

 

3.3 Canal and Intermediate Intake

 

The old headrace channel for Tis Abay I HEP, made of masonry wall, has been enlarged by demolishing right-side of its slanted wall and half of its bed, increasing the bed by 8.2 m.

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The new section is made of half concrete slab with integrated vertical wall and half masonry bed with its slant masonry wall.

The water from the head pond will be let in through the four main intake gates and it will bifurcate just before Tis Abay I penstock intake, after being conveyed by the composite channel section for 332 m.

At the point of bifurcation, there is an intermediate gate made of metal plates to control the flow to Tis Abay II Hydro plant.

The newly constructed rectangular concrete canal, with 16-17 m bed width and 8.5 m height internal dimensions, will convey the water remained from Tis Abay I plant to Tis Abay II Plant's penstock intake covering a distance of 1,500 m.

 

 

 

 

3.4 Penstock & Power House

The twin surface penstocks (Figure Manufactured by local manufacturer, Mesfin Industrial Engineering, with an internal diameter of 4.60 in each, serve to bring down water from the forebay to center of Francis turbines, which have operating level differences of 59 m.

After turbining both units, the water wilt eventually merge with those separated from it at Tis Issat Fall to play the role for site sighting seeing at Tis Issat and to turbine Tis Abay I Hydroelectric plant and continue its rest of journey with its name as Abay River or Blue Nile.

 

3.5 The Switch Yard at Tis Abay, Double circuit 132 KV Transmission line & the Bahir Dar Substation

The power generated from Tis Abay II Hydro plant will be transmitted to Bahir Dar substation via Tis Abay switchyard (roughly 3,200 m2) using a double circuit 132 KV power transmission tine.

By being connected to Bahir Dar substation, the power contributed from Tis Abay II Hydropower plant will play its part by boosting the interconnected grid power capacity of the Country. Tis Abay II HP project data summary is given in Table 1.

 

4 IMPLEMENTATION STAGE OF THE PROJECT & CURRENT STATUS

After being transferred in 1997 from Ministry of Water Resources for implementation, Tis Abay II Hydro Electric Project started its physical growth to generate power by engaging the following Contractors and Engineer under EEPCO's responsibility:

  • The Engineer is a Joint Venture of Coyne et Bellier, Howard Humphreys & Rust Kennedy & Donkin;
  • Civil Works Contractor including Gates & Penstock is Joint Venture of Energoprojekt —Niskogradnja from Belgrade and Sur Construction from Ethiopia;
  • Supply Erection Commission of powerhouse Electro Mechanical Equipment Contractor is China Wanbao Engineering Corporation from China;
  • Supply — Erect Commission of switchyard & substation equipment Contractor is Alstom —AEG from Germany; and
  • Spie from France is supplier of 132 KV Transmission Line material, which EEPCO took over to build the line.

At the moment the project is progressing well to be completed in April 2001.

 

Table 1: Tis Abay II Hydroelectric Power Project Data Summary

1. MAIN FEATURES OF THE PROJECT

1.1 HYDROLOGY

1.1.1 Catchment Area at Tis Abay 16300 km2

1.1.2 Average Discharge at Chara Chara 118 m3/s

1.1.3 Firm Release at Chara Chara 110 m3/s

1.1.4 Flood Peak of Abay at the Power House 1720 m3/s

1.2 DIMENSION & VOLUME OF THE WORKS

1.2.1 Canal

1.2.1.1 Length of Enlarged Part 335 m

1.2.1.2 Length of New Part 1,500 m

1.2.1.3 Concrete Volume 40,000 m3

1.2.1.4 Embankment Volume 200,000 m3

1.3 PENSTOCK AND FOREBAY

1.3.1 Number 2

1.3.2 Length 135 & 145 m

1.3.3 Diameter 4.6m

1.3.4 Thickness 0.012 m

1.3.5 Concrete 13,000 m3

1.4 POWER HOUSE

1.4.1 Width 29 m

1.4.2 Length 54 m

1.4.3 Excavation Volume 150,000 m3

1.4.4 Concrete Volume 17,000 m3

1.5 POWER GENERATION

1.5.1 Number of Units 2

1.5.2 Turbine Type Francis

1.5.3 Turbine Rated Output (Each) 36.8 MW

1.5.4 Turbine Rated Head 52.3 MW

1.5.5 Turbine Rated Discharge (Each) 75 m3/s

1.5.6 Turbine Rated speed 214.3 rpm

1.5.7 Generator Rated Capacity (Each) 40 MVA

1.5.8 Generator Rated Power Factor 0.9

1.5.9 Generator Voltage 13.8 KV

1.5.10 Generator Transformer Rated Voltage 13.8/132 KV

1.5.11 Firm Energy (Total) 400 GWH/Year

(Depending on Allocation of Water to the Fall)

1.6 SUBSTATIONS

1.6.1 Tis Abay 11132 KV Switchyard, Concrete 500 m3

1.6.2 Extension to Bahir Dar Substation, Concrete 1,700 m3

1.7 TRANSMISSION LINE

1.7.1 Length 32km

1.7.2 Voltage (Double Circuit) 132 KV

1.8 Lake Tana Control at Chara Chara

1.8.1 New Radial Gates ( Each 4.8 in x 6.0 m) 5

1.8.2 Concrete Volume 2,000 m3

1.8.3 Excavation for New Inlet and Out Let Channels 100 000 m3

 

 

REFERENCES

  1. Feasibility study report compteted by JV, Coyne et Bellier, Howard Hamphreys and Kennedy & Donkin.
  2. Final Design Report.
  3. Contract Documents of the Project.

ACKNOWLEDGEMENT

To Mr. Philippe de Felix, Project Manager- IV Coyne et Bellier, Howard Humphreys and Kennedy and Donkin, for their checking of the manuscript which highlights the important technical points from the referenced documents.

ABOUT THE AUTHOR

Abebe Tesfaye, B.Sc. Civil Engineering is the Project Coordinator for Tis Abbay and Gojeb hydropower projects. His major interest is on hydropower engineering.

The article appears in the publication of the EACE (Ethiopian Association of Civil Engineers) who owns the copyright. All due acknowledgements and copyright belong to EACE (POBox 20930, Code 1000, Addis Ababa)

EA CE Bulletin Vol 2, No 1, 2000

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