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Largest hydropower machines in Asia.

The vertical Francis turbines developed by Sulzer - Escher Wyss in Zurich using model tests have a runner diameter of 7,900 mm and a maximum runner throat diameter of 7,150 mm. They will be the largest units in Asia (440 MW at 117.35 m net head). The runners, made from stainless steel (13% Cr, 41% Ni), are to be manufactured by DBS-Escher Wyss in Canada and each shipped to Pakistan in one piece. The turbines have to operate over a very large head range (49.4 to 135.6 m net head range). At the rated net head of 117.35 m, each turbines produces 440 MW at an efficiency of at least 91%. The best efficiency is obtained at 82.3 m net head (>94.9%). The following table summarises the most important characteristics of the turbines.

Characteristics of

Turbines 11 to 14
 Runner diameter 7900mm
 Runner weight 190 t
 Runner throat diameter 7150 mm1
 Rated speed 90.9 min1
 Runway speed 199 min
 Maximum discharge 424.8m3/s
 Maximum rated capacity 440 MW
 Maximum gross head 137.2 m
 Minimum gross head 61 m
 Maximum net head 135.6 m
 Minimum net head 49.4 m
 Shaft diameter 2.2 m

At heads higher than the rated values, the values of pressure and flow permissible for tunnel 3 limit the output per unit to 440 MW. The servomotors of the units are designed so that the flow is the maximum 425 m3/s per unit at the rated head. If all four units are not operating at the same flow, then under operating head upto 135.6 m (maximum water level in reservoir), the flow per unit could be increased above 425 m3/s, but not for all four units. Digital governers of type DTL 725 from Sulzer - Eacher Wyss control the speed by adjusting the guidevanes, subject to the pressure constraints applying for the penstock.

The generators have a unit rated output of 480 MVA and a rated voltage of 18 kV. The turbine output will be limited by the generator to 432 MW (power factor 0.9). At rated conditions, the generator efficiency is 98.72%. The rated speed is 90.9 min1 and the runaway speed 199 min1. The generators are of standard design, but with very large dimensions. They are being manufactured in Germany by a consortium made up of Siemens and ABB (led by Siemens), and will be assembled on site. The stator bore diameter is 14 m, the rotors weigh 1475 t and the stators 993 t.

The special layout of the hydropower station extension requires the installation of butterfly valves of 7.35 m nominal internal diameter upstream of each unit, as well as four relief valves with their guard valves to limit the overpressure in the penstock.

Electricity for the

Whole Country

Thirteen single-phase transformers installed outdoors, each of 160 MVA rating, are to be supplied by Ansaldo (Italy). Hence three transformers per unit plus one spare are available. The present switchyard, which is already one of the largest in Pakistan, is being extended by three new bays. The new switchyard will have SF6 type breakers. AEG (Germany) is to supply and install this equipment.

Moreover, a new 500 kV line is to be constructed, since the existing 500 kV lines only have the capacity to transmit the power from two of the new units. The line is to go from Tarbela to Lahore and from there via Multan to Jamshoro. This section is under construction and is scheduled for completion in 1992. In the future, it is planned also to build lines from Lahore via Faisalabad to Jamshoro and from there to Karachi. Hence Tarbela is to be connected to Karachi, the largest city in Pakistan, via a 500 kV line.

At present, a new load dispatch control centre is under construction in Islamabad, the capital. This will control distribution for the whole country. The centre is to be responsible for frequently control; however, until it is commissioned, Tarbela is responsible for said frequency control.

Plant Monitoring System

with Many Functions

On completion the new plant will be controlled at first from the control room of the existing power plant. One year after commissioning of the last unit, it is intended to introduce the SCADA remote control and data compilation system. This will be connected to the new national load distribution centre. The plant monitored system (PMS) to be supplied by Sulzer - Escher Wyss, Zurich, monitors the penstock and tunnel protection, controls the units and maximises the efficiency of each unit. The transient parameters are measured, processed and monitored; in addition, trend analyses are carried out.

Project Fact Sheet

Project: Tarbela Dam 3rd Ext. Project. Owner: WAPDA, Pakistan Engineer: Main-Nespak Consulting Engineers, Tarbela Pakistan Location: Tarbela Dam - 100 KM Northeast

of Islamabad on the Indus River Rate Head: 117 m Operating Range: 72 m to 136 m Rated Output: 440 MW/Unit Speed: 87.5 RPM

Project Description

The addition of turbine units 11 to 14 at the Tarbela Dam is the third major extension to this project, and will increase the installed capacity at the power station to 3510 MW. The DBS Escher Wyss contract is to design, supply, erect and commission the 4 large Francis turbines and 4 governor systems. The turbine runner is made from type 13.4 stainless steel and will be shop fabricated and shipped in one piece weighing approximately 200 tonnes. The runner throat diameter is 7510 mm.

The notice to proceed was effective in March 1988, and turbine Unit 11 was scheduled to be in service in July 1991 and the contract, will conclude with Unit 14 in service May 1992. The successful performance of Units 9 to 10 (178 MW each) supplied DBS in 1984 was an important factor in obtaining this contract from WAPDA.
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Title Annotation:Tarbela dam, Pakistan
Publication:Economic Review
Date:Apr 1, 1992
Previous Article:Thermal power project Muzaffargarh.
Next Article:Cogeneration techniques: economically viable alternative.

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