The operations and maintenance (O&M) of hydropower plants is critical for meeting grid requirements, ensuring proper functionality and minimising downtime. Moreover, plants must be available for operation round the year. The lack of effective O&M can result in the loss of electricity production and revenues, high replacement fuel costs, high outage rates, performance losses, and large premature refurbishment costs. The consequent lack of reliable power can have a serious impact on the cost and development of power plants. In addition, breakdowns in hydropower plants lead to increased use of thermal resources, with detrimental environmental consequences. Poor O&M practices can also affect employee and public safety.
A look at some of the best practices and solutions for hydropower O&M…
Operations analysis of all hydropower plants on a daily basis is one of the best O&M practices. An analysis of the operations of the generating unit with reference to the inflow, water utilisation, reservoir level, spillage, operational parameters, etc. must be regularly carried out. In winters, an analysis of the injection/drawal of MVAR as per the generator capability curve and the grid requirement by the generating unit/plant must be carried out. For instance, NHPC Limited has carried out restricted governor-mode operation and free governor-mode operation at six of its power stations. In addition, a mock black-start exercise is done once a year as per the schedule given by the regional load despatch centres. During 2020-21, NHPC conducted the mock black-start exercise at four of its power stations – Chamera I, Chamer II, Parbati III and Bairasiul.
Critical spares and inventory management is another effective O&M strategy for hydro 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.
Another best practice that must be followed is tripping and breakdown analysis. Power stations must be regularly monitored for forced outages/breakdowns, and tripping/breakdown analysis of each event must be undertaken to identify the root cause of the fault for early restoration. Corrective action and restorative works must then be intimated to the power station to prevent and minimise the reoccurrence of tripping/ breakdowns. Such an analysis helped NHPC in reducing forced outages to 0.51 per cent in 2019-20 from 5.25 per cent in 2014-15.
Apart from the powerhouse, maintenance of the dam is another key O&M area that helps to keep the plant operational during the monsoon. The maintenance of the skin plate assembly, rubber seals and embedded parts such as the sill beam and wall plates of the spillway gate, as well as the various components of the rope drum hoist and supporting structures must also be undertaken. Further, landslides and flooding may cause damage to reservoir structures, including the roads and rims.
Other O&M-related issues and challenges faced by hydropower plants are ensuring the availability of critical spares, procurement of spares from original equipment manufacturers (OEMs), response time of OEMs for the restoration of plants during forced outages, and monitoring/maintaining a minimum inventory level. Thus, it is important to identify critical spares for each power station along with the minimum/maximum inventory and re-order level.
The other O&M best practices include regular technical inspections, annual preventive maintenance, ancillary services to the grid, technical inspection and dam safety inspection, and regular training programmes.
Silt/Sediment management is another effective O&M practice. 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. At NHPC’s power stations, energy loss due to high slit/trash and reservoir flushing increased to 538 MUs in 2019-20 from 375 MUs in 2013-14. 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.
New technologies and the way forward
New technologies can significantly help hydropower plant owners in O&M operations. For instance, NHPC has successfully tested automatic generator control (AGC) in the Teesta V and Dulhasti power stations in coordination with the National Load Despatch Centre (as per the CERC’s order, AGC has to be implemented at 11 power stations).
Further, digital systems such as supervisory control and data acquisition (SCADA), digital governors and excitation systems have helped in the better implementation of grid requirements. SCADA has been installed by NHPC in 15 out of 20 power stations, and it is being implemented in a phased manner at five older power stations. NHPC is looking to adopt more digital solutions in the near future for the remote operation of its power plants. It has taken the initiative to remotely operate the 45 MW Nimoo Bazgo power station as well as the Teesta Low Dam Project (TLDP) II and TLDP IV power stations.
To conclude, following best practices in O&M will help maximise the lifetime and reliability, and improve the performance of hydropower plants. Further, the adoption of best O&M practices can optimise the performance aspect of hydropower plants for a longer duration and improve asset management for competitive performance in the cost-driven market with a stringent regulatory framework.
(Based on a presentation by NHPC Limited at a recent Power Line conference)