As safety is absolute priority, hydro-dams are designed to last over a century. Planning and construction of water reservoir and hydropower projects have fundamental requirement of carrying out detailed geological, geophysical, hydrological and environmental feasibility studies, ensuring safety and security of the hydro-dams and power stations. Best practices for these investigations are adapted, and necessary measures are put in place. In fact, practices related to dam safety are well-defined and accepted the world over. The conditions at the hydro-dams are carefully and regularly monitored for ensuring structural soundness throughout the operating time.
Hydropower encompasses the most robust generating and allied facilities, and is therefore considered one of the safest sources of power. Yet there are potential risks of unexpected severe incidents. To prevent or minimize possible accidents, more attention is being paid today to technical parameters of the electromechanical equipment, aiming at enhanced security and safety of the hydropower stations. State-of-the-art technology is therefore employed to study the behavior of installed equipment regularly.
Even under normal operating conditions, all turbo-generator components like turbine, turbine shaft, bearings, and generator are subject to ageing and damages, whereas abnormal ageing and failure is crucial. Therefore there has been growing awareness globally to install the monitoring, analyzing, protection and diagnosing systems at medium and large hydropower plants, in particular, for the purpose of early fault detection before serious damage occurs.
Besides poor construction and obsolesce, there are two main factors causing major accidents; unpredictable natural events (such as heavy rains causing flooding, earthquakes and landslides), and failure of electro-mechanical equipment (turbine, generator, main circuits equipment) including incidental fire, etc. due to design & manufacturing defects or human error in operating critical equipment. The accidents could result in damage to civil facilities infrastructure, immediate loss of power generation, disruption in transmission and dispatch network, destruction of turbine and/or generator, damage to other infrastructure of the powerhouse, and alike.
Accidents in hydropower stations do occur, though very seldom, causing loss of human life, property and assets in term of major damages to the installations. During the past more than five decades (1963 to 2009) there have been 23 severe accidents in conventional hydropower plants across the world, including the USA, Germany and Italy, and more recently in India and Vietnam. The 1,269-MW (3x423-MW) capacity hydropower plant in Bieudron, Switzerland was damaged in December 2000 due to rupture of penstock feeding the Pelton turbines. The power plant located in the Swiss Alps has the highest head of 1,869 meters and was commissioned in 1998. It was made operational again after complete redesign of the penstock, besides taking other measures.
In India, Srisailam 770-MW (7x110-MW) hydropower plant was damaged in 1998 due to poor management of reservoir operating the floodwater, which overflowed into the semi-underground powerhouse. The floodwater caused complete submergence of powerhouse. Srisailam power station, with addition of 6x150-MW reversible turbines in later years, is now India’s third largest hydropower plant. Likewise, in August 2009, a catastrophic accident occurred in Sayno-Shushenskaya power station, of cumulative 6,400-MW capacity. in Siberia, Russia, claiming as many as 75 deaths.
This was due to failure of one of the ten turbines (each of 640-MW capacity, Francis type) as excessive vibrations had resulted in cracks in turbine blades. The entire powerhouse and its machinery was rehabilitated and replaced completely by November 2014. The recent global accident occurred in September 2016 when 100-MW Song Bung-2 hydropower plant in Vietnam was badly damaged, which was constructed in 2012. These accidents had caused long downtime generation loss, considerable restoration cost and, in some cases fatalities too, besides other socio-economic impacts.
Fortunately, no severe accident involving major causalities has taken place in hydropower plants in Pakistan operated by the Water and Power Development Authority (WAPDA). In August 1990, an accident being the first its kind in the history had occurred in one of the units of the 243-MW Warsak power station. Severe erosion on lower stems of the guide-vanes resulted into failure of lower stem of one of the guide-vanes. The guide-vane interfered with the turbine runner blades while the Unit was in operation at 38-MW. Spinning action of the guide-vanes resulted into a cascade failure of all of the runner blades, stay-vanes and the remaining gates. Pieces from stay-vanes blown ruptured the spiral casing at one point which resulted into flooding of powerhouse.
Failure of steel liner in the draft tube of Unit-11 (capacity 432-MW) of Tarbela power station was observed in 2014. Uplift pressure developed under the steel-line pulled out the steel liner along with anchors. The water discharge from turbine and vortex in the draft tube ruptured the steel liner into pieces and dumped the whole scrap into the draft tube outlets. Similarly, a catastrophic failure of penstock had occurred in 1982 in Jabban hydropower station in Khyber Pakhtunkhwa, the oldest power station installed in 1938.
While the power station was in operation at 18-MW load, water from the forebay was diverted into the drain nullah due to malfunctioning of the outlet gates, resulting in trapping of air into the corresponding penstock. The penstock burst and caused serious damage to powerhouse equipment and building as it was completely flooded. Loss of human lives was also reported. Also, in 2006, fire broke out in Jabban power station that damaged the powerhouse totally. The power station has recently been rehabilitated, modernized and machinery completely replaced, with an installed capacity of 22-MW.
There are state-of-the-art online (as well as offline) systems tailored to individual power stations backed up with latest hardware and software. Various modules and variants in different combinations depending on characteristics of a power station or specific needs are available. The applications include data management, recording of turbine operation periods monitoring under varying conditions (of magnetic field, cavitation, vibration, rotor-winding temperature, rotor-pole temperature and machine gap), thermal diagnosis, cooling air analysis (hotspot detection, ozone diagnosis etc.) and structure-borne sound/vibrations diagnosis.
These modern systems linked with the existing SCADA (supervisory control and data acquisition) system and MIS (management information system) of power station evaluate all measurements and characteristic values, offering probable root-cause analysis of the machine behavior. This enables the management to undertake measures and actions for predictive maintenance, which averts possible incident, and also increases life-cycle of hydro-generator, besides improving plant availability and reliability. All new hydropower plants being constructed in Pakistan, whether in private sector or public sector, have installed such systems.
In pursuit of higher safety of hydropower plants, WAPDA is continually refurbishing and modernizing some of its old large and medium hydropower stations. Ageing units of Tarbela power station are being rehabilitated, modernized and upgraded, under a phased program. Turbine governors and controllers have been replaced. Periodic tests and checks of generator are being carried out. Excitation systems of most of the old units have been replaced with latest static excitation systems. Based on insulation tests and partial discharge analysis, stator windings are being changed if necessary. Penstock and turbine protection system is planned for replacement with latest software and hardware.
Likewise, Chashma hydropower plant of 184-MW, which was designed with little regard to its safety, will shortly install a condition monitoring system to determine and relay the current condition and status of installed machines and systems, and to provide advanced detection or prediction of a fault. On the other hand, dozens of WAPDA engineers are being trained every year in the European countries and China, as well as within the country, in the fields of the O&M and monitoring of hydropower plants.
However, the measures so far adopted by WAPDA are not considered adequate and timely, given the importance of safety in hydropower plants. It is therefore imperative that WAPDA adopts an integrated and comprehensive approach towards safety of all its ageing small, medium and large hydropower stations, on priority.
The writer is former chairman of the State Engineering Corporation