With increasing renewable energy integration into the grid, hydropower is set to play a major role in addressing the intermittency and balancing requirements. Hydropower projects are capital-intensive assets. If these assets are maintained well, significant benefits can be derived from the plant for a longer period of time as compared to other energy sources. The operations and maintenance (O&M) needs of hydropower plants are primarily focused on mitigating the challenges posed by sedimentation.
O&M challenges
Sediment erosion from the river can affect various parts of a hydropower plant. For instance, the trailing edges of runner blades are particularly prone to damage due to their thinness. The severe loss of material along the length of the blades can result in a weight loss of up to 3 per cent of the original runner weight after every season. Typically, a properly repaired and coated runner can last three seasons, whereas an uncoated runner does not last more than one season.
Sediment erosion is also a problem for the draft tube section, which is closer to the runner, as it is exposed to the highest velocity of water. Similarly, the labyrinth seals working with coarse sand may have erosion as well as abrasion effects.
In the process, mechanical shaft seals can get damaged if they are exposed to sand-laden water. In a mechanical shaft seal, the damage can be more due to abrasion than erosion. Apart from the selection of erosion- and abrasion- resistant material for seals, the insertion of pressurised clean water in the shaft seal prevents wear.
Silt also impacts the auxiliaries in a hydropower plant. Cooling water piping in stator coolers, guide bearings, generator transformers, etc. can get eroded and choked due to the presence of silt. Clogging of these pipes affects cooling and causes an increase in temperature, leading to a forced outage of the machine.
Apart from the powerhouse, maintenance of the dam is required to keep the plant operational during the monsoons. 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 be undertaken. Further, landslides and flooding sometimes cause damage to reservoir structures, including the roads and rims.
O&M best practices
Operations analysis is one of the best practices for O&M. 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 also be carried out. For instance, NHPC has carried out restricted governor mode operation and free governor mode operation at six of its power stations, and a mock black- start exercise is done once a year as per the schedule given by the regional load despatch centres.
Tripping and breakdown analysis is another best practice that must be followed. Power stations must be regularly monitored for forced outages/breakdowns, and tripping/breakdown analysis of each event done 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 has helped NHPC in reducing forced outages to 0.3 per cent in 2018-19 (up till December 2018) from 3.27 per cent in 2015-16.
Critical spares and inventory management is another effective O&M strategy. At times, the performance of power stations is affected by the non-availability of critical high-value spares. These high-value spares need more lead time due to 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.
Meanwhile, three basic procedures can be followed for silt management: flushing, drawdown sluicing, and desilting of chambers. The internationally accepted practice of drawdown flushing during monsoons is recommended for small reservoirs, preferably once in a month during the monsoons. To control the level of sediment in the reservoirs, drawdown sluicing is done for passing a large quantum of incoming sediment through the spillway, which reduces trap efficiency. (However, as per the verdict of the Court of Arbitration for projects falling under the purview of the Indus Water Treaty [Western Rivers], flushing is not permitted in projects commissioned post the verdict. In such projects, sediment management would need to be done through sluicing only.) Another way of sediment management is desiliting of chambers, which entails the removal of coarser sediment particles, thereby preventing these from entering the turbines.
Other O&M best practices include regular technical inspections, annual preventive maintenance, and regular technical programmes for manpower.
New technologies
The digitalisation of hydropower plants, control systems and the surrounding networks is an emerging industry trend that promises to optimise performance and asset management. With the growing requirement for ancillary services and the introduction of regulatory frameworks, more analytical tools are needed to optimise operational performance. Digital systems such as supervisory control and data acquisition (SCADA), digital governors and excitation systems have helped in better implementation of grid requirements. At NHPC, SCADA has been installed in 15 out of 20 power stations, and it is being implemented in a phased manner at five older power stations under renovation and modernisation. Most of the company’s power stations are now equipped with digital governors and excitation systems.
Another solution is the computational fluid dynamics (CFD) method. CFD is an efficient and inexpensive tool to make internal flow predictions for good accuracy, and any sort of flow problems can be detected and further improvements made on the geometry of turbine components. It is used for checking the efficacy of alternative turbine designs for optimisation before the final experimental testing of selected designs. Another emerging trend is allowing for flexibility in turbine design. Today, turbines are available in varying configurations such as horizontal and vertical, to ensure the efficiency and reliability of operation under the most extreme conditions. This enables longevity, durability, reliability and robustness, and allows a wide range of operations.
Another development in the O&M space is the use of a thermal spray coating process for extending equipment life. 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 currently, it has various advantages over HVOF such as no oxygen usage, leading to lower costs, better adhesion, hardness and toughness.
Predictive maintenance of hydropower systems is another solution that can yield immediate savings in maintenance costs. Some of the diagnostics for condition monitoring, which is a major component of predictive maintenance, being implemented by NHPC include air gap monitoring, stator frame and rotor vibration, generator temperature, thrust and guide bearing temperatures, and ultrasonic flow measurements.
Conclusion
Inflow forecasting is essential for preventing unplanned outages. The adoption of O&M best practices can optimise the performance aspects of hydropower stations during longer durations. Moving ahead, hydropower stations must adopt emerging technologies and incorporate innovative approaches for improving their operational performance.
Based on a presentation by NHPC Limited at a recent Power Line conference