Innovation and Application of Reliability-Centered Maintenance Technology for Pumped Storage Power Plant

With the increasing capacity and scale of pumped storage power plant, the maintenance management and resource input of system equipment are continuously increasing. Exploring and establishing a system engineering method that balances safe production and maintenance resource input has become an important issue that enterprises urgently need to solve. Based on the reliability-centered maintenance (RCM) technology, this paper combines the characteristics of pumped storage unit operation and maintenance management and years of maintenance practice experience to improve and innovate the RCM technology method, forming a “reliability-centered pumped storage unit maintenance strategy optimization technology method” applicable to pumped storage power generation units. This technology method has been applied on the system equipment of pumped storage power generation unit. It shows the application of this technology method has made significant contribution to optimizing equipment maintenance strategies, reducing maintenance workload and the overhaul time.


Introduction
Pumped storage is a green, low-carbon, and clean flexible power source in the current power systems.It has mature technology, good economic performance, and the most favorable conditions for largescale development.The installed capacity and scale of pumped storage units are increasing [1,2].With the demand for high-quality development of pumped storage enterprises, it is important to reduce the operation and maintenance costs, decrease the maintenance workload, and shorten the overhaul time of pumped storage units [3].In previous studies, some pumped storage companies have conducted analyses on relay faults and reliability [4], rotor inter-turn short circuit fault detection and diagnosis under various operating conditions [5], as well as hydrogenerator rotor fault analysis, spindle swing fault analysis and vibration monitoring and analysis [6][7][8].These studies have provided guidance for optimizing the maintenance strategies of pumped storage units, but have not formed a systematic and standardized maintenance optimization technical method applicable to these units.
This paper starts from the production and operation practices of pumped storage units.Based on the reliability-centered maintenance (RCM) technical method [9], the RCM method is improved and innovated to be suitable for the characteristics of pumped storage unit operation and maintenance

Introduction to Reliability-centered Maintenance (RCM)
Reliability-centered maintenance (RCM) originated in the late 1960s in the US aviation industry.It is an analysis method widely used internationally to determine preventive maintenance requirements and optimize maintenance strategies.Based on six failure rate curves and seven questions, the RCM analysis method involves multi-disciplinary teamwork, standardized logical decision-making processes, and considerations for safety environment impact, production impact, and maintenance economic impact.It formulates "reliability-centered maintenance strategies" that meet the needs of enterprise users.RCM has been widely applied in the optimization of maintenance and reliability assurance in aviation, military, and power fields, obtaining significant benefits by reducing maintenance costs and improving equipment reliability.

Research and Establishment of RCM Technical Method for Pumped Storage Unit
Compared with conventional thermal power units, pumped storage units have fewer systems, equipment, work tasks, higher task maturity, and have undergone multiple A and C overhauls.With abundant operation and maintenance experience, combined with the needs and current situation of maintenance optimization in pumped storage power plants, this study researched and established the "reliabilitycentered maintenance optimization method for pumped storage units" (referred to as "pumped storage reliability maintenance technical method" for short) based on fully learning from the traditional RCM concepts and analysis processes to optimize the existing preventive maintenance strategies of the units.The method is introduced as follows.

Technical Route of Pumped Storage Reliability Maintenance
The pumped storage reliability maintenance technology method starts from the existing preventive maintenance strategies of the unit, first identifies the type of maintenance strategy, then lists the failure modes prevented by the maintenance strategy, including time-related and random failure modes, further analyzes the failure effects and consequences corresponding to the failure modes, determines whether the failure consequences are worth managing and the soundness level of management, judges the applicability of maintenance strategies, and finally combines operation and maintenance big data, new monitoring and detection tools, while ensuring the basic premise of safe production of units, taps the implementation cycle margin of preventive maintenance strategies.

Steps of Pumped Storage Reliability Maintenance Technical Analysis
The steps of pumped storage reliability maintenance technical analysis include: analysis group establishment, analysis boundary definition, listing and classification of maintenance strategy inventory, failure mode analysis, analysis of failure impact and consequence, analysis of the applicability of maintenance strategies, optimization evaluation of reliability analysis of maintenance strategies, Introduction of steps for implementing reliability maintenance technology for pumped storage units: 1) Group establishment and boundary definition Reliability maintenance analysis for pumped storage units should be carried out in the form of a group, including training in technical methods and concepts to fully absorb the operational and on-site maintenance experience of the units.The analysis boundary of the "reliability-centered optimization of drafting and drawing unit maintenance strategy" system is defined to ensure that there is no omission in the analysis.
2) Writing a maintenance strategy checklist and task classification Write a current list of effective operation tasks for the drafting and drawing units, form a summary table including project name, project cycle, and operation standards as the target object of this maintenance optimization analysis, and classify them by type, including C (condition monitoring), R (regular refurbishment), D (regular replacement), and periodic testing (T).
3) Fault mode analysis After identifying the maintenance tasks corresponding to the work tasks, it is necessary to analyze the time correlation of the fault modes, such as wear, aging, and dirt as time-dependent fault modes, and human factors, design, and manufacturing issues as random faults.During the identification process, if there are no corresponding aging or random fault modes for the maintenance strategy, it is considered excessive maintenance, and the maintenance strategy can be deleted after evaluation.
4) Fault impact and consequence analysis Fault impact analysis can be divided into three dimensions: component impact, system impact, and unit impact.It is necessary to evaluate the potential signs, fault phenomena, and the impact on safety, environment, and production losses.The failure consequences are divided into two levels: explicit and implicit (implicit: H; explicit: E), and the second level is further divided into safety (S), environment (E), production (O), and nonproduction (N) consequences.
For failure modes that do not cause serious equipment failure and have preventative maintenance investment larger than the maintenance investment for fault repair with production or non-production consequences, corrective maintenance can be performed.
For failure modes with severe consequences, a qualitative judgment is made on whether the current work task manages and reduces the safety/environmental consequences to an acceptable level.A qualitative judgment is also made on whether the current task maintenance investment is worth it for production and non-production consequences, and a final analysis conclusion is provided.If the maintenance task is worth implementing, a job task applicability analysis is conducted.
5) Applicability analysis of maintenance strategy Check whether the maintenance tasks correspond to the failure modes according to the reasonable principle: a. Tasks corresponding to time-dependent fault modes: condition monitoring, regular refurbishment, regular replacement.
b. Tasks corresponding to random fault modes: condition monitoring.c.Tasks corresponding to implicit functions: regular fault inspection.
If the maintenance tasks do not meet the applicability principle, modifications and improvements to the maintenance tasks are required.
6) Optimization of reliability analysis for maintenance strategy Combine current operation and maintenance data statistics, introduce new monitoring and inspection tools and methods, and incorporate feedback from external significant events to analyze and optimize the cycles and task content of the job tasks that have passed the applicability analysis.
The results of the assessment and optimization of job tasks include four types: interval extension, interval reduction, task deletion, and task addition.The argumentative analysis process needs sufficient operational and maintenance data to prove the feasibility of optimization.
7) Summary and continuous optimization of maintenance strategy Summarize the optimized job tasks and compare them with the original maintenance tasks, record the analysis process information of the job task optimization.
With the increase in the number of operational energy storage units and the frequency of A/C maintenance cycles, engineering construction data, daily operation and fault data, and preventive maintenance data of energy storage units are continuously accumulated.In addition, internal and external experience feedback information is collected.A dedicated operation and maintenance database platform can be established, and continuous optimization of A maintenance task cycle and content can be achieved through statistical analysis of big data.

Application of RCM Technology for Pumped Storage Unit
In response to the optimization needs of energy storage unit maintenance, a reliability maintenance analysis and optimization of the water pump and water turbine system of an energy storage power plant was conducted as follows:

Introduction and Boundary Definition of Water Pump and Water Turbine System
Functions of the water pump and water turbine: The water pump and water turbine can be used for both forward operation of the generating unit and reverse operation of pumping water.During the period of surplus grid power, it can use the electric energy to pump water from the downstream to the upstream reservoir.During power shortage, it can use the stored water to generate electricity, transmit active power to the grid, and is the core power equipment of the energy storage unit.
Equipment of the Water Pump and Water Turbine: The main components of the water pump and water turbine include the volute, seat ring, top cover, guide vane operating mechanism, impeller, water turbine shaft, water guide bearing, main shaft seal, etc., as shown in figure 2. Boundary analysis of the water pump and water turbine: 1) the volute, seat ring, top cover, guide vane operating mechanism, impeller, water turbine shaft, pressure water return system.
2) to the steel pipe connection joint between the volute and the downstream of the inlet valve.
3) to the connection flange between the water turbine shaft and the intermediate shaft.
4) to the connecting oil pipe of the guide vane actuator.

Case Study of Reliability Maintenance Analysis of the Water Pump and Water Turbine System
The current preventive maintenance tasks for the water pump and water turbine system mainly include the preventive maintenance outline for A-class maintenance (with a cycle of 10 years) and C-class maintenance (with an interval of 1 year).After conducting a reliability maintenance analysis the Aclass projects, the analysis results included two types: a. the A-class maintenance project is converted to condition-based maintenance project; and b. the A-class maintenance project cycle is extended.

1) Case Study of A-class Maintenance Project Converted to Condition-based Maintenance Project
According to the "reliability-centered maintenance optimization method for energy storage unit maintenance" task types, parts involved, failure modes, time relevance, failure impact and consequences, applicability of operation tasks, and reliability analysis were performed for the A-class projects of the water pump and water turbine system, such as the "inspection of turbine shaft and flange/10 years" and "bolt pre-tension check/10 years".The analysis process is shown in table 1.
The "shaft cracking" corresponding to the "inspection of turbine shaft and flange" which is an Aclass project of the water pump and water turbine system, belongs to a random failure.Regular A-class inspections every 10 years cannot effectively manage it.It can be managed through C-class state maintenance to timely detect potential failures and improve equipment availability.The "bolt and joint surface corrosion and rust" fault mode corresponding to the "bolt pre-tension check" A-class project can result in time-related fault modes.This fault phenomenon is common; therefore, it is converted to Cclass state maintenance management or installation of online monitoring devices to detect potential faults timely and improve equipment availability. 2

) Evaluation Case of Extended Cycle Margin of A-class Maintenance Project
The "clearance check of the upper and lower leakage prevention rings/10 years" of the water pump and water turbine system is related to the fault mode of "wear of the leakage prevention ring" and is timerelated.The leakage prevention rings will be continuously worn due to hydraulic erosion and cavitation during the operation of the generating unit, resulting in an increase in the clearance of the leakage prevention ring with the increase of the operating time of the unit.Through the analysis and calculation of the wear data of multiple units in the Huizhou pumped storage power plant for many years, the wear rate of the leakage prevention ring of the 8 units in the energy storage unit was statistically calculated, as shown in figure 3.

Table 1. Conversion of A-class projects of water pump and water turbine system to condition-based maintenance (partial).
No.
A  The wear rate of the sealing ring of the Huizhou Pumped Storage unit is between 0.016-0.033mm/1,000hours, and the average wear rate is 0.024mm/1,000 hours.The average wear rate is similar to that of the sealing ring of the Guangxu A plant unit (0.023mm/1,000 hours).In order to ensure the safety margin of the evaluation, the wear rate is selected 0.028mm/1,000 hours.
Referring to the replacement cycle of the labyrinth seal of the Guangxu A plant unit, the fixed sealing ring of Guangxu unit#7 was replaced during the second overhaul, and the unit has been in operation for 67,033.58hours.The average clearance of the upper sealing ring is 3.44mm (design clearance is 1.20-1.35mm),which is 2.55 times the design clearance.Considering the analysis and evaluation of the sealing ring clearance of the Huixu unit by the equipment manufacturer and the replacement cycle of the labyrinth seal of the Guangxu A plant unit, it is recommended that the standard for inspection and replacement of the sealing ring of the Huixu unit should be when the clearance reaches 2-2.5 times the design clearance, corresponding to the replacement cycle of the sealing ring of the Huixu unit#5 when the cumulative operating hours of the unit reach between 51,784 hours and 78,570 hours, equivalent to a service life of 20 years.The detailed analysis is shown in table 2.

Reliability Maintenance Analysis Benefits of the Water Pump and Water Turbine System
After reliability maintenance analysis of the water pump and water turbine system, a portion of A maintenance projects were converted to C maintenance projects, while another portion of A maintenance projects underwent extended implementation based on a 10-year basis.Finally, through comprehensive planning, the A maintenance cycle was extended by 3 years.Within a 40-year operational lifespan of the pumped storage unit, the maintenance cost of a single unit for A maintenance and overhaul is calculated to be 15 million RMB.With each extension of the A maintenance cycle by 3 years, one A maintenance is saved throughout the entire lifespan, resulting in a cost saving of 15 million RMB for a single unit over 40 years, averaging 375,000 RMB in savings per year.Furthermore, in order to ensure the detection of potential random failures that may occur during the extension process of important 10-year A maintenance projects, targeted measures for failure symptom monitoring have also been implemented to guarantee equipment reliability.

Summary and Outlook
By using the RCM (Reliability-Centered Maintenance) technique-based "reliability-centered pumped storage unit maintenance strategy optimization technique" in the pumped storage unit's water pump turbine system A and C maintenance projects, this study has introduced innovation and application of the RCM technique in the hydropower industry.This not only effectively reduces the maintenance costs of large-scale high-value equipment but also lowers the risks introduced by maintenance of human factors, enhances the reliability of the system equipment, and improves the theoretical level of maintenance optimization for maintenance personnel.
To fully leverage the value of the "reliability-centered pumped storage unit maintenance strategy optimization technique" in pumped storage unit maintenance optimization, it is suggested to plan and establish a data management platform for pumped storage units, standardize the management of maintenance data during the period of daily and overhaul, continuously accumulate maintenance data, and provide data support for ongoing maintenance strategy optimization.Simultaneously, reliability analysis of unit equipment should be conducted on demand, exploring equipment failure mechanisms, identifying potential failure risks, and formulating targeted maintenance strategies for important equipment to achieve safe, reliable, and economically efficient operation of the unit.

Figure 1 .
Figure 1.Reliability Maintenance Implementation Process for Pumped Storage Unit.

Figure 2 .
Figure 2. The Water Pump and Water Turbine picture.

Figure 3 .
Figure 3. Statistics on Wear Rate of the Sealing Rings from Unit 1to Unit 8.

Table 2 .
An extension of the A-class project interval for the water pump and turbine system (partial).