Pakistan is being deprived of millions of units (kWh) low-cost energy on an annual basis that could be contributed to national grid system. The hydropower experts believe that ten old power generating units of Tarbela hydropower station, if overhauled and modernised, can be safely operated at much higher capacity, adding at least 200 megawatt (MW) during high-demand summer season, to available and reliable capacity of these units.
Tarbela Dam, completed in 1976, is not only the largest earth-and-rock filled dam of the times it is also the 5th largest structure by volume in the world. Globally, it is ranked the 8th largest dam and has installed world’s largest manifold for penstock for the powerhouse. It has been a challenge in dams design and construction, given the geological conditions of the site. Among various other distinctions that the multipurpose Tarbela project enjoys, its hydropower station is the largest power station in the country with an installed capacity of 4,888 MW, playing critical role in energy security and sustainability. The international experts consider Tarbela power station to have been satisfactorily operated and well-maintained by Pakistani engineers, despite various technical and financial constraints.
There are five tunnels for irrigation supply releases and for power generation, which, in fact, is the secondary function of the Tarbela project. According to the original design, three of the right bank tunnels (Tunnel No 1, 2 and 3) are being used for power generation whereas the remaining tunnels (Tunnel No.4 on the right bank and No.5 on the left bank) were, till recently, being used exclusively for irrigation releases. Provision of an additional tunnel was also made on the right bank for subsequent development, which was constructed in 2002-03 known as Gandaf Tunnel.
Sometime in the 1980s, Water & Power Development Authority (WAPDA) carried out a power optimisation study at Tarbela project, and it was conceived that Tunnel No 4 could also be utilised for power generation without affecting the irrigation releases. Thus, the 4th Extension of Tarbela project, of total 1,410 MW installed capacity (three units of 470 MW each), was proposed and implemented after modification of Tunnel No 3 and No 4 intakes. Tarbela 4th Extension project was commissioned in October 2018 that has enhanced Tarbela powerhouse capacity to 4,888 MW, ranking it as the 15th largest hydropower station in the world. Now WAPDA has launched the 5th Extension of Tarbela powerhouse of the same size, 1,410 MW, on existing Tunnel No 5, which, on completion, will bring total installed capacity of Tarbela power station to 6,298 MW.
In the first phase of Tarbela powerhouse project, ten generating units of 175 MW capacity each (Units 1 to 4 on Tunnel No 1, and Units 5 to 10 on Tunnel No 2) were installed during the 1977-1985 period. Four additional units, each of 432 MW capacity were commissioned on Tunnel No 3 under the 3rd Extension of Tarbela hydropower project in February 1993. The first ten generating units i.e. Units 1 to 10 of cumulative installed capacity 1,750 MW are ageing and obsolescing, and require major overhauling, refurbishment, and modernisation of electro-mechanical equipment.
During the past two decades or so there have been major advancements in materials and developments in design & engineering in the field of hydropower the world over. This has resulted in improved output, efficiency and reliability of hydropower plant, besides its increased service life. There is great potential for not only regaining the original installed capacity of the Tarbela powerhouse old Units 1 to 10, but also increasing their efficiency and capability to generate more than the rated capacity under overload conditions, if state-of-the-art technology is employed by replacing outmoded machinery and components. These Units have the flexibility to be operated on higher than the rated capacity, at least to ten percent, as allowed by the equipment manufacturer, and as demonstrated in the past by the Tarbela management.
Units I to 4, commissioned in 1977, of rated capacity of 175 MW each were being operated up to 210 MW at high reservoir level till the year 1995. However, with the passage of time the capability and efficiency of these Units deteriorated. At one time, Unit 4 was giving not more than 87 MW. Under WAPDA’s short-term Repair and Maintenance Programme financed by the US Agency for International Development (USAID), Generators of Units 1, 3, and 4 were rehabilitated during 2011-14 (Unit 2 was rehabilitated earlier, in 1997). Generator windings of these Units were replaced with the latest Class-F insulated windings, possessing a higher degree of thermal stability compared to the old Class-B insulation. Likewise, the Excitation System of these Units have recently been replaced with latest static excitation system. Resultantly, these Units have regained their original installed capacity of 175-MW. Currently, in the month of May, Units 1 to 4 are operating at 162 MW each due to non-availability of required waterhead.
In fact, these Units could be loaded safely up to 200 MW each. The limitation of these Units is now that of the Turbines, which have not been overhauled and rehabilitated since commissioning more than four decades, having already completed their useful life. Turbine Runners of these Units have worn out due to ageing, having developed serious problem of vibrations experienced beyond 175 MW and Power Swing phenomenon (power fluctuation), resulting in overload instabilities.
Runners of these four Turbines need to be replaced with the new ones, designed using modern technology of Computational Fluid Dynamics (CFD), having X-Blade hydrodynamic contours, instead of the conventional existing design. The X-Blade Runner technology has been developed in the West to provide improved stability to the hydraulic turbines over large ranges of water inflow and head, as well as strong cavitation behavior and higher efficiency levels. The technology is being successfully used the world over also to upgrade existing turbines in hydropower plants.
Likewise, Units 5 to 8 were installed and commissioned in 1983-84. With the passage of time, capacity and efficiency of the Units have got badly affected. None of these Units can be loaded beyond 175 MW due to problem of abnormal vibrations in Turbine. Also, Generators of these Units are already due for replacement with Class-F insulated Stator Windings. Generator Exciters and Turbine Governors of these Units have recently been replaced with digitally controlled system. Simply replacing Turbine Runners with X-Blade CFD design and new Stator Windings of Generators will enhance capability of these Units to 200-MW each, achieving maximum efficiency.
Similar is the situation with Units 9 & 10, which were installed and commissioned in 1985. Initially, these two Units were also operated at 201-MW during high reservoir level. However, with the passage of time Turbine Runner developed vibrations at high water-head, and thus performance and maximum capability of both Units suffered. Recently, Generator Stator Windings of these Units were replaced with new one having Class-F insulation. Also, the Exciters and Turbine Governors were modified and replaced with Digital Governors and Static Exciters. The output of these two Units could also be enhanced to 200 MW each by simply replacing the old Turbine Runners with new modern ones, and refurbishment of its existing Guide-vanes.
Thus, all ten Units could be operated, in future, smoothly beyond their rated capacity, additionally to cumulative 250 MW at high waterhead, or to at least 200 MW if requisite waterhead is not available for some period. However, this additional power generation of about 246 GWh net energy would be possible only for two months, and also subject to irrigation releases by the Indus River System Authority (IRSA). Tarbela Reservoir, during a normal year of water inflows, achieves waterhead during end July, and water level is retained latest by end September.
It is therefore imperative for WAPDA to prepare a detailed project study for the rehabilitation, refurbishment and upgradation of the old ten power generating units, for implementation on fast track. Timely and effective execution of the phased programme will result in contributing significant additional power generation during summer, minimising prolonged and forced power outages, and also improving availability of these Units. With the completion of Tarbela 4th Extension project (Units 15 to 17) that basically serves the purpose of power substitution, there has come a greater flexibility for carrying out major overhauling of Units 1 to 10 on the suggested lines, one after the other during water lean-flow periods, without disturbing peak power supplies in winter and baseload power supplies in summer.
The writer is retired chairman of State Engineering Corporation
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