Modern excavation techniques (in particularthe wide use of TBMs for tunnel excavation).XLPE HV cables (instead of oil-filled cables) toconnect the step-up transformers to theswitchyard.Modern telecommunication systems (opticalfibres). Figure 6 shows a typical layout of a modern

underground powerhouse. As far as theequipment layout is concerned, the undergroundsolution often has the step-up transformers in aseparate cavern from the powerhouse. In ourrecent projects, transformers are located inindividual cells equipped with anti-blast andfireproof doors. The air insulated switchgear (AIS)or gas insulated switchgear (GIS) switchyard isthen connected via XLPE HV cables runningalong the access tunnel or along a dedicatedvertical pit. Alternatively, if the run of the busducts is not too long, the step-up transformersmay be located outside near the switchyard.

In some cases, as an alternative to the AISswitchgear, where there is a shortage of spaceand/or environmental constraints, a GIS solutionhas been adopted.

Hydraulic transient analysisNumerical modelling for transient analysis hasallowed for design optimisation in some of thecompany’s recent large projects. The relativelylow water head (DH=210m), smallish head lossesand high flow in the power tunnels (Q =2x475m3/sec) require a large surge shaft crosssection to ensure, on the one hand, acceptablewater level surges in case of load rejection and, onthe other, quick dampening of water level and

34

Project development

INTERNATIONAL WATER POWER & DAM CONSTRUCTION January 2013

Powering on with hydro designs

Based in Rome, Italian consulting engineersStudio Pietrangeli was founded in the1960s by Giorgio Pietrangeli. Since then

the company’s engineers have developed theirskills and experience working mostly in Africa,Europe and South America. Powerhouse designhas played an integral part of many of thecompany’s projects. Several of these schemes,given their innovative solutions and importance,have been written about and/or cited inprestigious magazines, such as NationalGeographic, and used to illustrate banknotes andbond issues. The company has also recentlycompleted the design of its 150th large dam and50th hydropower project.

Powerhouse location: underground oroutdoors?The most important choice to be made during thedesign of a powerhouse is its location: cavern oroutdoors? A number of factors influence thischoice. Most importantly is the hydraulic scheme,ground morphology, geological factors and last,but not least, environmental aspects.

For medium to high head plants, thepossibility of an open-air penstock with thepowerhouse on flat ground on the river banks isan important, decisive factor for the outdooralternative. On the other hand, a homogeneousand impervious rock mass, capable ofwithstanding high water pressures, favors anunderground powerhouse.

For plants with a low to medium head and ahigh flow, it may well be convenient to build thepowerhouse just downstream from the dam oreven to incorporate it in the dam itself. In the caseof pumped storage plants, the cavern solution isoften obligatory given the high submergencerequired by the pump also at the minimum levelof the lower reservoir. All these considerations aretaken into account in choosing the most suitablesolution, together with an economic comparisonof different, technically feasible alternatives.

Recent trends Although the cavern solution requires moreattention to be paid to rock characteristics,drainage problems, ventilation requirements, fire-fighting systems etc, its adoption has recentlybeen encouraged by the use of:

power and frequency oscillations in the shaftduring transients.

A straightforward application of Thoma’stheoretical stability criteria (shown below) wouldlead to extremely high values for the surge shaftdiameter.

: Where As= area of surge shaft; Lt = tunnel length;At = area of power tunnel; J = head losses; H =waterhead; q = flow rate

Therefore, knowing that the control systemparameters and grid regulating energy also have astrong influence on system stability, acomprehensive model, including the hydraulicsystem, control system and electric grid, was setup which allowed a substantial reduction of thesurge shaft diameter. The size of the shafts are, inany event, remarkable: 18m diameter, 120m high.

Fast track implementationThe “fast-track” implementation methods have beenadopted by the company for turnkey construction oflarge hydro plants, especially in Africa.

As >Ltq2

2gJAtH

With over 60 years of experience, Studio Pietrangeli prides itself on merging technical qualificationswith culture and creativity to find solutions to challenging engineering problems. Here the companydiscusses the importance of powerhouse design in hydropower project development.

Above: Figure 1 – Sierra Leone's banknotewith Bumbuna dam on the reverse side.Top right: Figure 2 – Grand EthiopianRenaissance Dam Bond, issued in 2012.Right: Figure 3 – Recent outdoor powerhouse with high head (Gibe II, 420MW)

034_036wp0113CS 14/1/13 10:27 Page 34

Project development

35January 2013 WWW.WATERPOWERMAGAZINE.COM

The basic concept of this method is toimplement as many critical operating phases aspossible simultaneously thereby drasticallyreducing, in some cases up to 30% of totalconstruction time. Thus the fast-track plant startsproducing benefits and income long beforetraditionally built plants, generating a muchquicker economic return against a minimum (orno) increase in upfront costs.

Moreover the extensive use of the cutting-edge technologies for the investigations, such assatellite imagery, laser scanning, tomographicalgeophysics, etc. remarkably shortens thetraditionally lengthy periods required savingseveral months of time during feasibility design.

Over the past few years Studio Pietrangeli hasdesigned numerous large hydropower plantstotaling over 20000MW. The most remarkableprojects currently under construction or recentlycommissioned include Bumbuna (50MW), Beles(460MW), Gibe II (420MW), Gibe III (1870MW) andthe Grand Ethiopian Renaissance dam (6000MW).

Above: Figure 5 – Semi-outdoor powerhouselocated at the dam toe (Bumbuna, IP=50MW).Below, left: Figure 6 – Typical layout of amodern underground powerhouse (Beles ,IP=460MW); Below: Figure 7 – Underground powerhouseduring construction (Beles, IP=460 MW)

Figure 4: Large semi-outdoorpowerhouse located downstreamof the dam (Gibe III, 1870MW)

034_036wp0113CS 14/1/13 10:28 Page 35

All the engineering services for these plants were,or are being performed entirely by StudioPietrangeli.

Bumbuna (Sierra Leone, 50MW First Phase)Bumbuna is the first large hydropower project inSierra Leone. It includes a 88m high rockfill dam, asemi-outdoor powerhouse and a 161kVtransmission line, 200km long, from the powerstation to Freetown.

The powerhouse is at the downstream toe ofthe dam and houses two 25MW Francis turbinescoupled with 33MVA synchronous generators.The machine hall is 41m high x 17m wide and24m deep. To allow for future expansion, a secondblind off-take from the power tunnel was providedto serve the future penstocks.

Beles (Ethiopia, 460MW)Beles exploits the water resources of Lake Tana.The plant’s 20km, 7.2m diameter, undergroundwaterway was excavated entirely by TBM. Theunderground powerhouse includes a mainmachine hall equipped with four 115MW high-head Francis turbines and 130MVA, 15kVgenerating sets, together with a 400/15kV

transformer cavern (figures 6 and 7). Thepowerhouse cavern (L=90m, W=18.5m, H=40m)hosts the erection bay and control building. Asuspended floor design was adopted for themachine hall to provide maximum working area atturbine floor level. The powerhouse is linked tothe outdoor switchyard via 400kV XLPE cableswhich run through a vertical shaft.

Gibe III (Ethiopia – 1870MW)Gibe III is the third plant of the Gibe-Omocascade comprising Gilgel Gibe (IP=200MW) andGibe II (IP=420MW), both operating, and Gibe IVand V (planned). It includes a 246m high RCCdam which, when completed, will be the world’shighest of its kind.

The powerhouse is on the left bank of theriver, about 500m downstream of the dam axis.The building, 235 x 38m wide and 50m high,houses ten Francis turbines (H=214m,Q=95m3/sec, IP=187MW each) at 18m intervalswith the erection bay and control building at thedownstream end. The outdoor transformer yardlies on a berm excavated at the powerhouse roofand will host five terns of single-phase step-uptransformers (15/15/400/√3 kV, 147MVA) eachconnected to two generators by means of bus-bars passing through dedicated shafts.

Grand Ethiopian Renaissance dam(Ethiopia – 6000MW) Once completed, with 6000MW of installed power,Grand Ethiopian Renaissance dam will be thelargest hydroelectric power plant in Africa.

36

Project development

INTERNATIONAL WATER POWER & DAM CONSTRUCTION January 2013

Located on the Ethiopian Blue Nile, about 40kmeast of the Sudanese border, the 155m high,11Mm3 RCC gravity dam will create a reservoir ofabout 63Bm3, one of largest on the continent. Theoutdoor powerhouse is situated at the dam toeand is divided into two main buildings on theright and left banks of the river, in order tooptimise river diversion works. The twopowerhouses will host respectively 10 and 6vertical shaft Francis turbines (375MW each).

ReferenceM. Cadeddu et al., Frequency stability ofan islanded grid supplied byhydropower plants, Hydropower andDams, issue No. 6, 2010.

Author informationThe authors are Antonio Pietrangeli,Partner at Studio Pietrangeli,antonio.pietrangeli@pietrangeli.it

Antonio Brasca, Dam & hydropowerexpert at Studio Pietrangeli,antonio.brasca@pietrangeli.it

Stefano Galantino, Electro-mechanicalexpert at Studio Pietrangeli,stefano.galantino@pietrangeli.it

Giuseppe Pittalis, Senior dam &hydropower engineer at StudioPietrangeli, gibe3@pietrangeli.it

Above: Figure 8 – Large semi-outdoor powerhouse under construction (Gibe III, 1870MW.)Left: Figure 9 – Design of the huge Grand Ethiopian Renaissance HPP ( IP=6000MW).

034_036wp0113CS 14/1/13 10:28 Page 36