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The vibration of steam turbine generator sets is a relatively complex issue. There are many causes of vibration, but as long as we can identify the particularities of the contradictions, that is, the different characteristics that appear during vibration, and analyze and judge them, it becomes possible to determine the underlying causes of vibration and address them. 01 Excitation current test: The purpose of this test is to determine whether the vibration is caused by electrical factors, and what specific electrical factors are responsible for it. If vibration occurs in the unit when the excitation current is applied, the vibration disappears once the excitation current is disconnected. It can be confirmed then that the vibration is caused by electrical factors, and at this point the excitation current test can be continued. The excitation current tests yielded the following two results: 1) As the excitation current increases, the vibration levels also increase, which indicates that the vibration is caused by an imbalance in the magnetic field. The reasons for magnetic field imbalance include: short circuits in the generator rotor coils; uneven air gaps between the generator rotor and stator, etc ； 2) As the magnetic field current increases, the vibration does not increase immediately; instead, it increases in a stepwise manner over a certain period of time as the magnetic field current rises, which is particularly evident when the excitation current increases. This indicates that vibration is related to the mass imbalance of the rotor in its thermal state. 02 Rotational speed test: The purpose of this test is to determine the relationship between vibration and rotor mass imbalance, and it also helps to find out how close the rotor’s critical rotational speed is to its operating rotational speed. Tests are generally carried out during startup (or shutdown) processes. Record the vibration value once for every increase of 100–200 r/min in rotational speed. The maximum rotation speed for the test should preferably be set at 105% of the operating speed, in order to observe the trend in vibration changes. This test can be conducted with the turbine and generator disconnected, or with them connected. Through this test, it is also necessary to check whether the critical speed and the operating speed are too close to each other. In general design, the difference between the two should be around 30%; however, due to the removal of some components during operation or machining on the rotor, it is possible to achieve perfect balance with high precision. In such cases, the operating speed becomes too close to the critical speed, which inevitably leads to significant vibrations during the operation of the unit. 03 Load test: The purpose of this test is to determine the relationship between vibration and the unit’s center, thermal expansion, as well as rotor mass imbalance, and to check whether the components that transmit torque (idler gears, reduction gears) have any defects. The test can be conducted at increased load or reduced load, and it can generally be divided into five levels: zero load, 1/4 load, 1/2 load, 3/4 load, and full load. Vibration is measured twice at each level, once immediately after the load is changed. Measure again after the load has been stable for 30 minutes. When performing a load test, it is necessary to measure the thermal expansion of the unit simultaneously with measuring the vibration. Generally, load tests can yield the following three results: 1) Vibration increases as the load increases (by a small amount). This indicates that the cause of the vibration is the imbalance of the rotating mass; in such cases, analysis can be carried out by referring to the \"speed test\". 2) The vibration increases as the load increases, and it is independent of thermal expansion (i.e., the vibration values measured twice for each level of load change little). This indicates that vibration is related to rotational torque. Possible reasons include: the unit failed to find the center properly when using the backup wheel for alignment ； The movable or semi-movable backrest wheels are defective in themselves, such as poor tooth engagement or uneven wear ； In this type of vibration, sudden changes in the vibration levels generally occur when the units are connected in parallel or disconnected. 3) For some time after the load changes, the vibration increases as time goes by (that is, there is a significant difference between the vibration values measured after the system has stabilized at each load level and those measured initially). This indicates that the vibration is related to the thermal condition of the turbine; possible causes include a faulty slip pin system and uneven settlement of the foundation ； The main steam pipeline was improperly arranged, exerting force on the cylinder during thermal expansion ； Other abnormal thermal deformations cause changes in the unit’s centerline, etc. 04 Bearing oil film test: The purpose of this test is to determine whether the vibration is caused by an unstable oil film, a damaged oil film, or improper bearing shell tension. The tests were conducted by changing the oil temperature while maintaining the lubricating oil pressure and volume of the bearing; the range of variation in oil temperature was generally ±5°C from the normal temperature. Vibration was measured once for every 1°C change in oil temperature, with an additional measurement taken after 30 minutes of stabilization at each extreme oil temperature value. There are two possible outcomes for the oil temperature test: 1) Vibration increases as the oil temperature rises. This indicates that the vibration is mostly caused by an excessive clearance in the bearing shells. This situation is quite common, as during operation wear of the bearing surfaces often leads to multiple repairs and scraping, which increases the inner diameter of the bearing shells and results in an unstable oil film ； 2) Vibration decreases as the oil temperature rises. At this time, the vibration is mostly caused by an excessively small clearance in the bearing shells. It should also be noted that since the lubricant temperature only indirectly affects the formation of the oil film by changing its viscosity, it is necessary to determine whether the vibration is caused by an unstable or damaged oil film through observation of the vibration phenomenon itself. The characteristics of vibration caused by an unstable or disrupted oil film are mainly that the vibration occurs suddenly and intensely, and it is generally difficult to discern the patterns of its occurrence and disappearance. The vibration waveform is distorted, and the vibration frequency does not match the rotational speed ； The engine makes abnormal noises when vibrating, as if it’s shaking. Insufficient bearing tension can also cause vibration; in such cases, the vibration levels are unstable as well, and a \"thudding\" sound can be heard at the location of vibration. 05 Other: In addition to using the aforementioned tests to identify the cause of vibration, it is also possible to analyze the cause of vibration through vacuum tests or tests on the external characteristics of the unit. The purpose of the vacuum test is to determine whether the vibration is caused by a change in the vertical position of the unit’s center due to variations in vacuum. The principle underlying the vacuum test is that as the vacuum changes, the force exerted by atmospheric pressure on the exhaust cylinder changes, causing the rear bearing seat, which is connected to the exhaust cylinder, to move up and down ； When the vacuum changes, the exhaust temperature changes as well, which leads to variations in the thermal expansion of the exhaust cylinder. This in turn causes vertical displacement of the rear bearing housing, and all of these factors can affect the vertical alignment of the unit. If not addressed properly, this can result in vibration. The external characteristic test of the unit essentially involves measuring the vibration distribution of the unit under conditions of high vibration levels, and using this vibration distribution to identify the areas that are abnormal. For example, defects such as loose fastening screws, poor contact between the bearing housing and the base plate, looseness of the frame of the machine base and the bearing housing relative to the foundation, local looseness of the unit’s foundation, and resonance in certain pipes can be detected through external characteristic tests. 06 Summary: Vibration abnormalities in steam turbine generator sets are one of the most common faults during operation. Their causes are diverse and highly complex, and they are directly related to manufacturing, installation, maintenance, and operational standards. Vibrations exceeding the allowable range are often a sign of equipment damage. Excessive vibration will cause the stress on the rotating components of the turbine, such as blades and impellers, to exceed allowable levels, leading to damage ； In severe vibration conditions, it may cause the emergency shutdown device to malfunction, leading to plant shutdowns, as well as damage due to resonance in the bearing housings, the foundation, and even the building structure itself. Therefore, it is necessary to keep the vibration level of the unit within the specified allowable range. It is worth noting that as the turbine power increases, even with a considerable stiffness of the bearing housing, larger vibrations of the rotor do not manifest themselves in the bearing housing. It is reasonable to directly measure the rotor’s vibration values as the standard for vibration assessment. During operation, once abnormal vibrations are detected, in addition to strengthening the monitoring of relevant parameters and listening carefully to the sounds inside the turbine, it is also necessary to reduce the load or even shut down the turbine for inspection, depending on the specific circumstances. When necessary, various tests are conducted to analyze the causes of abnormal unit vibration, and corresponding treatment methods and elimination measures are taken.