Diagnosis and Treatment of Oil Film Whirl Fault in the Bearing of Large Synchronous Condenser

The oil film vortex of a synchronous condenser rotor can cause an increase in bearing vibration. When the vibration amplitude exceeds the threshold value, it can cause the condenser to trip. The oil film vortex is related to many factors and often occurs during the start-up and acceleration process of the unit, as well as during the loading process of the unit. This article elaborates on the low-frequency oscillation accident of the bearing system caused by the oil film vortex fault of the rotor of a large synchronous condenser and its handling methods. This article presents a sudden low-frequency vibration of a large synchronous condenser. Based on the data collected on site and experimental test results, this condenser analyzes and diagnoses the low-frequency vibration incident, and ultimately determines it as an oil film vortex fault in the bearing system. The article combines the operation and maintenance plan of the synchronous condenser and conducts on-site dynamic balance tests on the synchronous condenser. After the dynamic balance test, the low-frequency vibration of the bearing system disappears and the fault is eliminated.


Introduction
The support in radial direction of large synchronous condensers (hereinafter referred to as condenser) is usually sliding bearings, which bear the centrifugal force generated by the weight and unbalanced mass of the rotor.Bearings commonly include cylindrical bearings, elliptical bearings, three oil wedge bearings, and tilting pad bearings.Based on certain boundary conditions, sliding bearings may experience self-excited vibration faults of the rotating bearing caused by oil film action.The selfexcited vibration of the condenser bearing often occurs during the start-up and acceleration process.When the speed reaches a certain value, the rotor suddenly experiences oil film whirl motion, which increases the vibration and affects the bearing system.The self-excited vibration of the bearing system is not only related to the speed, but also to many other factors [1][2][3].Therefore, during the process of load up, oil film whirl faults may also occur [4] on the condenser.
This article introduces the oil film whirl fault of a certain condenser, which occurred during the start-up and operation of the condenser.The vibration of bearing NO.1 and NO.2 showed a significant "sudden increase" phenomenon, and was forced to trip due to the 1Y vibration value exceeding the trip value.After implementing dynamic balancing test, the vibration of the NO.1 and NO.2 bearings decreased significantly, reaching the excellent level according to standard.There was no sudden increasing vibration during the start-up and operation stage any longer, and the dynamic balancing test achieved the expected effect.

Overview of Bearing System
The bearing system of the condenser consists of two supporting bearings, which are the condenser rotor and the exciter rotor, as shown in figure 1. Vibration sensors installed perpendicular to each other (X is 45R, Y is 45L) are arranged on the bearings to measure the relative vibration at the bearing neck, as shown in figure 2.

Fault Diagnosis
After maintenance on July 7, 2020, one condenser was started.During the process, the vibration of No.2 bearing reached 490um, and vibration of No.1 bearing reached 130um.It exceeded the limit and condenser could not pass through the critical speed zone.Excessive vibration seriously threatened the safety of the condenser, as shown in table 1.Based on the above vibration characteristics, it is believed that sudden vibration is a true signal.Common faults that cause sudden changes in condenser vibration include detachment of rotating components, misalignment of the wheels, steam induced vibration, and oil film instability.The balance state of a bearing system that experiences rotational component detachment and wheel misalignment faults is usually irreversible.Vibration induced by steam often occurs on the highpressure rotor and high-medium pressure rotor of high parameter condenser.Therefore, it can be ruled out these three types of faults as follow： detachment of rotating parts, misalignment of wheels, and steam induced vibration.and preliminarily judged that the condenser was experiencing an oil film instability fault.
Oil film instability fault is a common self-excited vibration.It is a phenomenon that the lubricating oil flow with high-speed, which was driven by the bearing diameter, excites the bearing diameter in turn, and causes strong vibration.Most of these types of incidents occur during the start-up and overspeed tests of the condenser, while some cases occur during the loadup operation.
Oil film instability faults have the following characteristics: (1) Before the instability of the oil film, the vibration is mainly composed of power frequency components.Once oil film instability occurs, low-frequency components will significantly appear and cannot be eliminated by increasing the speed.
(2) Vibration has suddenness and usually has a large amplitude, which increases sharply in a short period of time (a few seconds).
(3) Speed lag phenomenon.The speed while oil film instability occurs during the acceleration and deceleration processes is inconsistent.The speed while oil film instability disappears during deceleration is lower than the speed while oil film instability occurs during acceleration.

Analysis
Through the analysis of the DCS data and vibration monitoring data of the condenser it was found that the vibration characteristics of the condenser conform to the oil film instability fault characteristics, and the first critical speed of the excitation rotor is greater than 1/2 of the working speed.The oil film oscillation fault was ruled out, and comprehensive analysis suggests that the condenser was experiencing an oil film whirl fault.
There are two main reasons for oil film whirl faults.firstly, excessive disturbance of the bearing diameter, and secondly, poor stability of the bearing pads.
The excessive disturbance of the bearing neck mentioned above does not refer to the disturbance generated in transient moment, but rather to the stable disturbance.Furthermore, it refers to the relative vibration between the bearing neck and the bearing shell, abbreviated as bearing vibration.According to the vibration situation of various operating conditions of the condenser, the 2Y vibration power frequency amplitude is greater than 90μm.This is an excessive bearing diameter vibration.
From many condensers, it has been observed that excessive bearing vibration is one of the most important reasons for self-excited vibration of bearing shells.Some condenser testes result indicate when the bearing vibration exceeds half of the normal top clearance of the bearing pad, it is easy to cause self-excited vibration of the bearing pad on general cylindrical, elliptical, and three oil wedge bearing pads.
There are many factors that affect the stability of bearing shells, including their design, manufacturing, maintenance, and operation.
(1) Excessive clearance between bearing shells In the stability calculation of bearing pads, whether it is cylindrical pads, elliptical pads, or three oil wedge pads, the stability will increase with the increase of the radial clearance of the bearing pads.However, based on operating experience, this is not the case.Excessive top clearance of these three types of bearings will significantly reduce their stability, especially when the bearing vibration is high, which is more likely to cause instability of the bearings.
The excessive top clearance of these three types of bearings will significantly reduce the oil film force on the upper bearing, that is, reduce the preload of the bearing, reduce the eccentricity of the bearing, and decrease stability.
(2) Bearing shell form At present, cylindrical tiles, elliptical tiles, three oil wedge tiles, and tilting tiles are used on site.The first two types of bearing tiles have a long history of use on site and have accumulated rich experience in use.In terms of stability, elliptical tiles are good.Therefore, when self-excited vibration occurs on site, the first step is to change the cylindrical shape to elliptical tiles.Practice has proven that the effect is good.
(3) Lubricating oil viscosity The factors that affect the viscosity of lubricating oil include oil quality, oil grade, and oil temperature.As the viscosity of the oil increases, the stability of the bearing will decrease.The main factors affecting oil quality are water content and deterioration in the oil, all of which can reduce the viscosity of the oil.Currently, there are two types of lubricating oils used in China: No. 32 and No. 46, with the former having a lower viscosity than the latter.There are also examples in China where oil film oscillation occurs due to the misuse of oil.
(4) Specific pressure Increasing the specific pressure can improve the stability of the bearing, but it is not a simple proportional relationship.At present, the specific pressure of the bearings of large generators is generally 1.2-1.6Mpa, while the specific pressure of the bearings of large synchronous condenser has been increased to 1.7-1.9Mpa.Excessive specific pressure will increase the temperature of the bearing shell and accelerate wear.
(5) Length to diameter ratio Reducing the length to diameter ratio can improve the stability of bearing shells.Reducing the length of the bearing pad under a certain bearing neck diameter can, on the one hand, increase the specific pressure to maintain high stability of the bearing pad, On the other hand, reduce the oil film force on the lower bearing to increase the eccentricity of the bearing, and improve stability.
(6) Changes in bearing coordinates The elevation of each bearing seat in the bearing system will be changes during the cold and operating states of the condenser.Although the load distribution of each bearing pad is reasonable in the cold state, the load of some bearing pads in the bearing system may be too low during operation, resulting in a small specific pressure and instability.Due to the dual support mode of the condenser with 2 bearings, this factor is not considered.
Based on the above, it can be concluded that the main measures to improve the stability of bearing pads on site are as follows: ① Reduce the top clearance of the bearing shells; ② Change the form of the bearing shells; ③ Using lubricating oil with low viscosity coefficient; ④ Increase the bearing specific pressure; ⑤ Reduce length to diameter ratio; ⑥ Changes in bearing seat elevation [5].
For oil film instability faults, it is recommended to take the following vibration reduction measures: (1) When the condenser has the condition for conducting variable oil temperature tests, the lubricating oil temperature test shall be carried out within prescribed range, and determine the degree of impact of oil temperature changes on vibration, and provide guiding measures for subsequent control for operators.
(2) During shutdown, on-site dynamic balancing measures can be taken to reduce rotor vibration levels and minimize bearing diameter disturbances.
(3) By means of maintenance, the stability of the No. 2 bearing can be improved, such as controlling the top clearance of the bearing at the lower limit and improving the bearing's self positioning ability.
According to the on-site meeting, the condenser was temporarily shut down for lubrication oil temperature testing and on-site dynamic balancing work.The lubrication oil temperature test showed that changes in lubrication oil temperature had no effect on improving bearing vibration.There are multiple optional methods to eliminate bearing oil film whirl faults.According to the condenser maintenance plan, on-site dynamic balancing measures should be considered.

On Site Dynamic Balancing Measures
Based on the vibration data of 1 and 2 watts, using the temporary stop opportunity, add 190g ∠ 90 °to the rectifier ring of the exciter rotor.After the first dynamic balance measure, the Bode plots of the vibration acceleration of the 1 and 2 watt axes are shown in figure 5.The vibration amplitude and phase in different directions are shown in table 4.After the first increase in load, the vibration of bearings 1 and 2 slightly decreased in the critical speed and not ideal at a constant speed of 3000 rpm.Based on the vibration data before and after the trial weighting, the second weighting decision was made to remove the trial weighting mass block and add a new weight of 185 g ∠ 0° at the rectifier ring of the magnetic machine rotor (in reality, the mass block was removed by 185g/180°.After the first increase in load, the bode plots of the vibration acceleration of the 1 and 2 watt axes are shown in figure 6.The vibration amplitude and phase in different directions are shown in table 5.  expected effect.During the start-up and acceleration stage, there was no abnormal sudden increase in vibration of the first and second bearings, providing a safety guarantee for the operation of the condenser.

Conclusion
The oil film vortex of a synchronous condenser rotor can cause an increase in shaft vibration.When the vibration amplitude exceeds the threshold value, it can cause the condenser to trip.The oil film vortex is related to many factors and often occurs during the start-up and acceleration process of the unit, as well as during the loading process of the unit.This article elaborates on the low-frequency oscillation accident of the shaft system caused by the oil film vortex fault of the rotor of a large synchronous condenser and its handling methods.One large synchronous condenser mentioned in this article experienced a sudden increase in vibration during the start-up operation.After ruling out the oil film oscillation fault, it was diagnosed as an oil film whirl fault.Due to No.2 bearing has an excessive vibration in power frequency, combined with the condenser operation and maintenance schedule, dynamic balancing measures are prioritized to reduce bearing vibration levels.After implementing dynamic balancing measures, the vibration of bearings 1 and 2 decreased to an excellent level, and the sudden vibration disappeared during the acceleration process, effectively solving the oil film whirl fault of the condenser.

Figure 1 .
Figure 1.Diagram of the condenser bearing system.

Figure 2 .
Figure 2. Diagram of vibration measurement point layout.

Figure 3 .
Figure 3. the trend of impulse vibration.

Figure 5 .
Figure 5. Bode plots of vibration of bearings no.1 and no.2 after the first Field Balancing.

Figure 6 .
Figure 6.Bode plots of axial vibration acceleration of 1 and 2 watts after the second dynamic balance.

Table 1 .
Vibration list (startup stage).When the speed increased to 2969 rpm, there was a sudden increase in vibration of the No.1 and No.2 bearings.The trend and the bode plot of impulse vibration is shown in figure3and figure4

Table 4 .
List of vibration of the generator after the first Field Balancing (3000 RPM).

Table 5 .
List of 3000 rpm vibration of the condenser after the second dynamic balance.After implementing the second dynamic balancing measure, the vibration of the first and second bearings of the condenser decreased significantly during the loaded operation stage, reaching the excellent level specified in the vibration standard, and the dynamic balancing work achieved the