Problems with oil level alarms of magnetic flap level gauges in Jinxi Power Plant and corresponding suggestions

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This post was last edited by admin2 on 2019-7-1 at 14:40. Authors: Zhu Li, He Wang, Hu Baoxian, Li Ke (Jinping Hydropower Plant). When reproducing this article, please acknowledge the copyright of its authors. Abstract: The Jinxi Power Plant uses two methods for measuring the oil level in the upper guide, lower guide, thrust, and water guide oil grooves; one method involves using level transmitters to transmit the measured analog oil level signals to the monitoring system； Another type is the magnetic flap level gauge, which allows for an intuitive observation of the oil level at the site through the changes in the magnetic flaps. This gauge is equipped with four magnetic switches, corresponding to alarm contacts for low oil level, low level, high level, and excessive high level. When the oil level approaches these magnetic switches, they activate, and the monitoring system receives digital alarm signals. The hydro-turbine generator is the main equipment in a hydropower station, and the oil system is an essential component of such generators. In Jinxi Power Plant, the turbine oil is used to lubricate and cool elements such as the generator’s thrust bearings, upper guide bearings, lower guide bearings, and water guide bearings. The liquid level transmitter provides analog signals, while magnetic switches provide digital signals, which are used to monitor the oil level. If the oil level is too low or too high due to water ingress, it can lead to serious damage to the bearings. Therefore, it is crucial to monitor the oil level; appropriate alarm thresholds must be set so that operators can detect any issues promptly and take appropriate action. 1 Introduction to the Power Plant: The Jinxi Hydropower Plant is located in Yan Yuan County and Mu Li County of the Liangshan Yi Autonomous Prefecture in Sichuan Province. It constitutes a key reservoir-based power station complex in the lower reaches of the Yalong River (from Kara to Jiangkou), approximately 358 kilometers downstream from Jiangkou. The main task of a power plant is to generate electricity. The total installed capacity of the power plant is 3.6 million kilowatts (6 units × 600,000 kilowatts), with an average annual electricity generation over many years amounting to 16.62 billion kWh. The first unit is scheduled to begin generating electricity in August 2012. The normal storage level of the reservoir is 1,880 meters, while the dead storage level is 1,800 meters. The total storage capacity is 7.76 billion cubic meters, with a regulating storage capacity of 4.91 billion cubic meters; it is an annual-regulating reservoir. The hub structure consists of permanent structures such as water retaining structures, spillways, energy dissipators, and power generation facilities. Among them, the concrete double-curvature arch dam has a height of 305 meters, making it the tallest double-curvature arch dam in the world. The crest elevation of the dam is 1,885 meters, while the elevation of its foundation is 1,580 meters. 2 Existing problems: (1) At Jinxi Power Plant, no fixed values have been set for the dynamic oil level alarm thresholds of too low, low, high, and too high levels in the various oil tanks. The design only specifies the range of oil levels when filling the tanks, that is, the static oil level values. It is currently not possible to generate real-time alarms for the oil level during unit operation; only the real-time oil level value can be displayed on the monitoring system. The static oil level values are as follows, taking the upper guide as an example: Normal oil level: 485 mm from the bottom of the tank (elevation 1645.295 m). Low oil level alarm value: -20 mm (elevation 1645.275 m); low oil level shutdown value: -30 mm (elevation 1645.265 m). High oil level alarm value: +20 mm (elevation 1645.315); excessive high oil level alarm value: +30 mm (elevation 1645.325). (2) The installation position of the on-site magnetic switch is determined based on the set value of the static oil level. When the oil level fluctuates during unit operation, the actual oil level exceeds this static set value, resulting in frequent alarms from the oil level magnetic switch. (3) For the analog oil level signal from the oil level sensor, no dynamic oil level alarm threshold is set in the monitoring system, which prevents accurate alerting regarding the oil level status. 3. Analysis of the set values for bearing oil levels in various components of the units: The data obtained from the monitoring system includes the oil level information for the water guide, upper guide, thrust, and lower guide bearings of Units #1–#6. There are data corresponding to three different unit conditions: the oil level before the unit is started after maintenance, the oil level during operation, and the oil level after the unit stops operating. The calculations and analyses are as follows: (1) Static oil level after unit maintenance: Table 1 shows the static oil level after unit maintenance (all oil level values are in mm). http://www.huoyumi.com/d/file/news/industry/2017-07-26/b2db99851eb51cd8508af26e385b1da7.gif The maximum value minus the minimum value for each bearing’s oil level gives the results shown in Table 2. http://www.huoyumi.com/d/file/news/industry/2017-07-26/19c20bc43554bcedc93931f1b78ab8c0.gif Summary: After oil was injected into the bearings of various components of the unit, there were some fluctuations in the oil level. The main reasons for this were the static self-balancing effect within the oil tank, as well as variations in the readings from the sensors; hence, there were certain fluctuations in the data. The variation ranges of the bearing oil levels in the 6 machines do not follow the same pattern. (2) Dynamic oil level during unit operation Table 3 shows the dynamic oil level during unit operation: http://www.huoyumi.com/d/file/news/industry/2017-07-26/efd8ee8dea2097ad5c44e64eb5b54477.gif. The result is obtained by subtracting the minimum value from the maximum value of the oil level in each bearing, as shown in Table 4. http://www.huoyumi.com/d/file/news/industry/2017-07-26/6de506ccd5cbbc393a3523bb39694215.jpg Summary: Fluctuations in oil level during the operation of the units are a normal phenomenon. Due to the differences in the operating conditions of each unit, as well as fluctuations in the readings from the sensors, there is no uniform pattern regarding the range of fluctuations in the oil level of the bearings in the 6 units. (3) Static oil level after the unit is shut down: Table 5 shows the static oil level after the unit is shut down. http://www.huoyumi.com/d/file/news/industry/2017-07-26/89f8809be5841924734c3f1c81161663.gif The result is obtained by subtracting the minimum oil level from the maximum oil level for each bearing, as shown in Table 6. Table 6: Fluctuation values of oil level after the unit is shut down. http://www.huoyumi.com/d/file/news/industry/2017-07-26/8e7b61eec96089563f8b3ba42c7a7eda.gif Summary: After the unit is shut down, there are fluctuations in the oil level in various shaft oil tanks. The main reasons for these fluctuations are the static self-balancing within the oil tanks, as well as variations in the readings provided by the sensors; hence, there are certain ranges of data fluctuations. The variation ranges of the bearing oil levels in the 6 machines do not follow the same pattern. 4 Data analysis results and handling recommendations (1) Based on the above calculations and analyses, no clear pattern can be observed in the data. The main reasons for this are factors such as the installation location of the bearing oil level sensors on each machine, the measurement errors of the sensors themselves, and variations in the flow rate of oil within the oil tanks; these factors affect the oil level readings of the various bearings, resulting in no consistent pattern in the values. It is recommended to check the zero points of the oil level sensors in the oil tanks, and to replace those sensors that do not have sufficient precision. (2) In accordance with the specified value requirements provided by Chengdu Survey and Design Institute, it is recommended to set different alarm values for each machine. Setting principle: For the oil levels in the upper guide, lower guide, and thrust oil grooves, the high oil level alarm value is set at +20 mm above the maximum oil level during unit operation, while the extremely high oil level alarm value is set at +30 mm ； Low oil level alarm value at the minimum oil level: -20mm; shutdown value at low oil level: -30mm ； Oil level in water-guided oil groove: High oil level alarm value +45mm, Excessively high oil level alarm value +90mm ； Low oil level alarm value at the minimum oil level: -130 mm; shutdown value at low oil level: -155 mm ； Avoid the alarm threshold. On this basis, the actual oil level can only decrease, and the set value is safe for the operation of the oil level. The dynamic oil level alarm values for the oil levels in each oil tank of units #1-#6, as determined through analysis, are as follows: the alarm setpoints for too low, low, high, and too high oil levels during unit operation are given below. It is possible to use the data in Table 9 to set the values for each of the 6 units individually; alternatively, Table 10 provides a set of values that can be used to set the parameters for all 6 units simultaneously. Table 7: Full image – http://www.huoyumi.com/d/file/news/industry/2017-07-26/f28f2598f7b36a255d5efb8750bf9ac7.jpg. (3) During unit maintenance, the oil level alarm set points are marked on the magnetic flap level gauge located at the oil tank; the static oil level range and the dynamic oil level alarm set points are also determined, and the installation locations of the oil level sensors are optimized and standardized. (4) The monitoring team adjusted the installation position of the oil level magnetic switch again, based on the dynamic alarm values for the oil level in the oil tank as indicated by the magnetic flip-level gauge. (5) For monitoring, alarm values of too low, low, high, and too high are set for the oil level signal (analog signal), with alarms triggered by this analog signal. (6) In the unit LCU program, a special filtering process is applied to the oil level values collected in real time. Since instantaneous large fluctuations during unit operation can cause false alarms in monitoring, the oil level value is filtered separately. 5 Conclusion The oil system is an important factor in ensuring the safe and stable operation of the unit. The above analysis identified the problems associated with the use of magnetic flap level gauges for oil level monitoring at Jinxi Power Plant, and corresponding solutions were proposed. These solutions can be utilized during maintenance to address the issues, thereby enabling proper monitoring and alerting of the oil level in the oil tanks.