The operations and maintenance (O&M) of hydropower plants is critical for meeting grid requirements, ensuring proper functionality and minimising downtime. Moreover, plants are required to be kept available for operation round the year, at least 90 per cent of the time, to get the benefits of capacity charges as per the Central Electricity Regulatory Commission’s norms.
A look at the O&M best practices for various components of hydropower plants…
Turbines should be operated at rated conditions, taking into consideration parameters such as rated head, and rated output. Also, operating turbines outside the prescribed range, as defined by the selection of the runner model, should be avoided because when turbines are operated in restricted zone, there is excessive vibration, rope formation, cavitation and massive pulsating sound in the draught tube. Turbine parameters, such as bearing temperature and oil level, pressure and flow in various systems, machine vibration, discharge measurement, guide vane opening and power output, head race and tail race water levels, should be monitored regularly. Corrective measures should be undertaken where necessary after due analysis of the data and trends. Ideally, the specific maintenance schedule mentioned by the original equipment manufacturer (OEM) should be followed and the inventory of mandatory spare parts should be maintained, especially parts that experience water wear in projects in high-silt areas.
For better availability and reliability of the machine, the OEM-optimised governing system settings should be maintained. Periodic monitoring of oil levels in the system and regular filtration of oil should be conducted to ensure the quality of oil in the governor oil pumping unit. Also, periodic checks for safety valves, over-speed devices and pressure relief valves (PRVs) are recommended.
In order to ensure the efficient functioning of generators at hydropower plants, the generator bearing oil level, shaft vibration and winding temperature should be periodically monitored and analysed. The carbon brush gear assembly should be cleaned and its polarity should be swapped regularly. Additionally, the health of the generator protection equipment like firefighting systems, temperature sensors and oil level sensors should be ensured. OEM specifications should be followed when checking the tightness of stator core bolts, pole rim bolts, foundation bolts, pole keys and other rotating parts, and for levelling, centring and unit axis alignment.
Generator step-up transformers also require performance monitoring and trend analysis. Key tests to be conducted include breakdown voltage and water content in oil; dissolved gas analysis to examine the rising trend of fault gases; Tan Delta Test of high voltage bushing and transformers, oil temperature indicator and winding temperature indicator readings; and thermal scans to ensure that temperature levels are consistent and normal. Routine checks are required for the oil level in conservator and bushing, and Buchholz relays.
Improving plant safety
The regular checking of components is the best way to ensure plant safety. For example, conducting periodic checks of the functioning of the over-speed protection device, emergency shutdown action, the firefighting systems, safety valves and PRVs, and protection relay settings to reduce emergency situations and their ramifications. For efficient functioning, the regular audit of start permissive, fasteners and locking of nozzles and deflector assembly, shaft seal function and runner aeration system and inspection of rotor assembly rim bolts, locking and pole keys on over-speed tripping is suggested. Another healthy practice is to ensure the reliability of the station drainage and dewatering system.
People involved in the operation of the machines are as integral a part of efficient O&M practices as the machines themselves. Staff should be trained with simulators for the proper operation of a machine. For the optimal utilisation of data captured by supervisory control and data acquisition systems, further training is required.
Issues and challenges
Since generation companies feel a compulsion to keep machines available for generation to take advantage of capacity charges, there is a significant reduction in the time available for maintenance. Avoiding generation losses by reducing the scheduled downtime also results in risks from machine operation. During maintenance, routine or otherwise, the availability of qualified workmen, machinery and spare parts is limited at sites. Another challenge is that high silt and quartz levels during the monsoon in rivers originating in the Himalayan range cause erosion and increase the un-
planned downtime of the machine. There is new technology such as thermal spray or plasma coating to increase resistance to silt erosion and the installation of improved ventilation systems in generators to counter these challenges.
Overall, following best practices in O&M will help maximise the lifetime and reliability, and improve the performance of hydropower plants.
(With inputs from Sanjeev Kumar, Deputy General Manager Hydro Business, BHEL)