Operations and maintenance (O&M) of hydroelectric plants (HEPs) is crucial for efficient operations, maintaining long life of the plant and reducing failure rates. The lack of effective O&M processes can result in major losses in terms of power generation and revenues and at the same time affect public safety. Effective O&M strategies also improve asset management and maintain competitive performance in the cost-driven market with a stringent regulatory framework.
Generally, O&M of HEPs is planned based on performance parameters such as availability factor, efficiency, force outage rate and accident frequency rate. Moreover, with the emergence of new-age digital technologies, online monitoring of HEPs as well as real-time monitoring of silt, vibration, discharge level and energy generation is gaining prominence.
Best O&M practices
At NHPC Limited, periodic dam safety inspections, both pre- and post-monsoon, are undertaken as per the Central Water Commission guidelines. Meanwhile, the Dam Safety Act, 2021, formulated in December 2021, provides for the surveillance, inspection and O&M of specified dams for the prevention of dam failure-related disasters. Overall, the act provides an institutional mechanism to ensure the safe functioning of dams. As part of O&M works, the preparation of comprehensive reports along with an action plan for repair and rehabilitation is undertaken. It is followed by monthly monitoring of identified repair works, comprehensive dam safety review by an independent panel of experts and submission of report for compliance. The guidelines for inspection of dams include an overview of the dam, a dam safety programme, inspection of embankment/ concrete/masonry dams, spillway outlets and mechanical equipment as well as general areas, visual inspection, use of remotely operated vehicles and unmanned aerial vehicles, documentation, and a comprehensive dam safety review by a panel of experts.
On the water intake front, water storage (reservoir) and a water conductor system, comprising a head race tunnel, a surge shaft, emergency valves and pressure shafts, penstock and main inlet valves, form the vital components of an HEP. Managing the sudden changes in water flow could lead to equipment damage and needs to be restored by timely O&M of the plant. It is necessary to regularly test the operation of conduit isolation system/equipment including intake gates, butterfly valves, the excess flow device and surge equipment of the HEP. Periodic physical inspection of the water conductor system from inside as well as outside to know its condition, silt deposition and rusting/erosion of the conduit system is essential to find out various changes occurring due to plant ageing and stresses developed due to water hammer. By maintaining records of physical inspection of HEPs, their performance can be compared overtime. Any abnormality in performance could be further investigated by carrying out hydraulic testing, measuring thickness by ultrasonic testing and conducting tests for measuring and computing stresses at strategic locations such as intake point and bends.
With regard to turbine and auxiliary equipment, regular inspection of runners is necessary to maintain the HEP. Due to cavitation, there may be damages to the turbine wheel leading to deterioration in plant performance and operational efficiency. Sometimes, it might be necessary to undertake in-situ repair of turbine buckets to recoup/fill up erosions/ white pitting by using various cold compounds. On the stator and rotor winding front, regular recording of insulation resistance values of stator and rotor winding is required at periodic intervals. Tan Delta and DLA tests of stator winding indicates the condition of stator winding insulation. Likewise, the impedance test (voltage drop test across each pole) indicates the condition of rotor winding. Maintaining a proper cooling system is necessary to limit the rise in stator winding temperatures and consequently increase the life of stator winding.
Another crucial aspect of HEP O&M is silt/sediment management. The Himalayan region carries huge amounts of sediment during the monsoon season. Over 80 per cent of the average annual sediment comes in the monsoon season. An effective sediment management system is needed to safeguard the life of the reservoir. Sedimentation or silt erosion affects reservoir capacity and underwater parts. To resolve this issue, various mitigation techniques can be adopted. These include reservoir flushing during the high inflow season, maintaining a near-minimum drawdown level at the reservoir during the monsoon and the provision of desilting chambers. The coating of underwater parts (runner and guide vanes) is another mitigation technique. This step significantly increases erosion resistance and enhances corrosion protection. High velocity oxygen fuel (HVOF) coating of underwater parts such as runners, guide vanes and cheek plates can be done at silt-affected power stations for mitigating abrasion/erosion problems. The high velocity air fuel (HVAF) coating process, which is similar to HVOF, uses compressed air in place of oxygen fuel. While HVAF is at the testing stage in India, it has various advantages over HVOF such as no oxygen usage, leading to lower costs, and better adhesion, hardness and toughness. Silt also impacts the auxiliaries in a hydropower plant.
Critical spares and inventory management is another effective O&M strategy for hydropower stations. The performance of power stations may be affected by the non-availability of critical high-value spares. These high-value spares need more lead time on account of their design, engineering and manufacturing cycles. The remote location of hydropower plants further exacerbates the problem. Hence, it becomes imperative that critical spares for each power plant are identified and a minimum/maximum critical-level inventory is maintained at all times. This also entails proper monitoring of the procurement/contract process – the initiation of purchase, award and delivery – to ensure the availability of critical spares for carrying out annual/ capital maintenance as per schedule and in exigency situations.
Digital solutions
Digitalisation of HEPs is useful in rationalising O&M, besides helping in cutting costs and increasing the effectiveness of workforce management. Digitalisation is also useful in the integration of HEP operations with other variable renewable energy sources since digital solutions help in closely monitoring HEP operations. Digitalisation also optimises reservoir management and extends the operating range of the existing hydro units. Various new and emerging digital solutions such as machine learning (ML), artificial intelligence, internet of things (IoT) and internet of services are increasingly being adopted in the hydropower segment. These are useful in asset management, plant and fleet enhancement, outage management and condition monitoring equipment, among other things.
Online monitoring of power plants using various technologies such as supervisory control and data acquisition (SCADA) and IoT-based systems can be used for continuous monitoring and remote operation. Monitoring of silt, vibration, discharge, level, energy generation and tandem operation is important. It helps assess the current status of machines and based on the current situation the O&M can be planned. An IoT-based system can be utilised to develop a low-cost architecture for power plant monitoring and ML techniques can be used on the historical data for automatic decision-making.
SCADA systems are computer based and have real-time control. These systems are used for monitoring and controlling water and power operations at a variety of power facilities. These systems operate continuously and, in many ways, are self-diagnosing, but some maintenance and testing of these devices are necessary to ensure system integrity and identify potential failures. Other than this, circuits that are infrequently used may require periodic functional testing to ensure they are operational when the need arises. Notably, NHPC Limited is taking steps for automation and remote operation of its hydroelectric projects through SCADA. This would enable the centralised operation of machines and digitalise various operational parameters.
Conclusion
Good O&M practices are critical for maintaining power plant longevity. As HEPs ramp up operations to meet the fluctuating power demand and to complement variable renewable energy sources, effective O&M strategies are becoming all the more important. Moving ahead, HEP operators are expected to increasingly adopt emerging technologies and incorporate innovative approaches for improving plant performance.