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Causes of vibration of hydro-generator units

Release date: 2019-10-05 09:36:40
The vibration of hydro-generator sets is different from that of general power machinery. The phenomenon of unit vibration is relatively obvious, but the source of the vibration is often hidden. In addition to the vibration caused by the rotating or fixed part of the machine itself, the electromagnetic force of the generator must also be considered. And the influence of the flow pressure on the flow part of the turbine to the vibration of the system and its components. There are various reasons for the vibration of hydro-generator units. Often, several sources of vibration exist at the same time. It is generally believed that the source of interference that causes the unit to generate vibration mainly comes from three aspects: hydraulic, mechanical and electrical. The three interact and interact with each other. , Often intertwined, forming coupled vibrations. The general vibration of hydro-generator units will not endanger the unit, but when the unit vibration exceeds the allowable value, especially for long-term vibration and resonance, it is the performance of the power supply quality, unit life, ancillary equipment and instruments, unit foundation and surrounding buildings. Materials, even the safe and economic operation of the entire hydropower station, will bring serious harm.
Its harmfulness can be roughly divided into the following categories:
1) Causes the formation and expansion of the fatigue damage zone between the metal parts and welds of the unit, which causes it to crack, even break and be scrapped.
2) Loosening the fastening parts of the unit will not only cause the fasteners themselves to break, but also exacerbate the vibration of the connected parts, prompting them to accelerate damage.
3) The degree of mutual wear of the rotating parts of the acceleration unit. If the large shaft oscillates violently, the temperature of the shaft and the bearing bush can be increased and the bearing bush can be burned. Excessive vibration of the generator rotor can increase the degree of wear of the slip ring and the brush, and increase the temperature. The sparks kept increasing.
4) The eddy current pulsating pressure formed in the draft pipe can cause the passing water system to oscillate, and the unit will oscillate, causing cracks in the draft pipe wall. In severe cases, the overall draft facility may be damaged.
5) Consequences caused by resonance of the water turbine unit are more serious. For example, the resonance between the unit equipment and the workshop can cause the entire equipment and the workshop to be damaged to varying degrees.
Hydraulic vibration The vibration caused by the disturbance of the dynamic water pressure of the hydraulic part of the turbine is called hydraulic vibration. The main hydraulic factors that generate vibration are: low-frequency vortex bands in the draft tube, Karman vortex trains, hydrodynamic instability caused by leaf vortices, instability in the excessive process, hydraulic imbalance, cavity cavitation, gap jets (axial flow Water turbine) and so on.
First, the low-frequency vortex zone in the draft tube
The low-frequency vortex zone in the draft tube is an unstable flow phenomenon in the draft tube when the mixed-flow turbine and the axial-flow fixed-paddle turbine are under partial load. When the turbine is operated under non-design conditions, the hydraulic pressure pulsations are often caused in the draft tube due to the effects of the rotating water flow at the outlet of the runner and the vortex and cavitation. Especially after a large vortex band appears in the draft tube, the vortex band rotates in the tube at a nearly fixed frequency, causing low-frequency pressure pulsations in the water flow. As soon as the water flow in the pipe occurs, the pressure pulsation will cause the vibration of the tail pipe wall, runner, water guiding mechanism, volute, and pressure pipe.
Prevention and treatment:
1) Optimize hydraulic design. Adopting a negative-inclined rotor airfoil, reasonable design of the blade outlet ring distribution law, appropriate crowns and drain cones, for high-head mixed-flow runners, a runner with auxiliary blades can be considered.
2) Avoid running in vibration conditions.
3) Change the flow and rotation of the water flow; for example, lengthen the drain cone; lengthen the tailpipe cone; increase the cone angle of the tailpipe; increase the water barrier and partition; control the eccentricity of the vortex zone.
4) During the operation of the unit, appropriate gas supplementation is performed in the vortex zone. Natural gas supplementation is generally used, and gas supplementation can be forced when necessary.
Carmen Vortex
When water flows through non-streamline obstacles, vortices appear on both sides of the exit side, forming vortexes with opposite rotation directions and regular staggered arrangements, which then interfere with each other and attract each other to form a non-linear vortex, commonly known as Carmen vortex . This vortex train rotates clockwise or counterclockwise alternately, and in the process of its continuous formation and disappearance, it will cause alternating vibration force in the direction perpendicular to the mainstream. When the frequency of the Carmen vortex is close to the natural frequency of the blade, the dynamic stress of the blade increases sharply, sometimes it makes a sound, and even makes the blade root crack.
Prevention and treatment:
1) Reduce the thickness of the runner blade or fixed guide vane water edge, increase the Carmen vortex frequency and avoid resonance; 2) add support between the blades and change the natural frequency of the blades;
3) In the design stage of the hydraulic turbine, the frequency of the Carmen vortex, the natural frequencies of the blades and guide vanes are predicted, and the frequency of the Carmen vortex and the flow around components are staggered.
3. Hydraulic instability caused by leaf vortex
When the mixed flow turbine deviates from the optimal working conditions, the angle of attack of the blade inlet increases. If the angle of water flow is too large, it will cause the blade head to flow out and form a leaf channel vortex, which may cause medium-frequency or high-frequency hydraulic pressure pulsation. . The incoming flow is a positive angle of attack above the design water head, and the outflow occurs at the back of the upper crown blade inlet; the inflow is a negative angle of attack below the design head, and the outflow occurs at the front of the upper crown blade inlet.
The leaf channel vortex originates from the deflow at the upper crown inlet after deviating from the optimal working conditions, and is divided into a high head blade vortex and a low head blade vortex. It is observed from the turbine model test that it flows out from the blades of the runner. With the change of the turbine operating conditions, when a visible vortex begins to appear between 2 to 3 blades at the same time, it is considered that a blade vortex has occurred under this condition.
Prevention and treatment:
Fourth, the instability in the transition process
When the working conditions of the turbine change, a hydraulic transition process will occur, and the unit will often experience various vibrations. The hydraulic transition process includes small fluctuations and large fluctuations. During the small fluctuations, the turbine governor can quickly achieve stability. During the large fluctuations, such as unit startup, shutdown, sudden load increase and decrease, and dumping, During the process of load, synchronous phase adjustment, accident flight, etc., the state of the water flow through the turbine is bound to be more disordered and complicated, which will not only cause hydraulic instability, but also may generate a variety of high-frequency dynamic stress on the runner and other components.
Five, hydraulic imbalance
The water current with potential energy and kinetic energy forms a circulation through the action of the volute, and then uniformly acts on the runner through the uniformly distributed fixed and movable guide vanes, and stimulates the runner to rotate. Due to processing and installation errors, when the shape and size of the guide vane and flow channel are greatly different, an unbalanced lateral force is generated when the water flow acting on the runner loses axisymmetricity, causing the runner to vibrate. Strong vibration during low load operation.
Six, mechanical factors
The mechanical vibration of the unit caused by the structure of both the turbine and the generator is various, mainly including: unbalanced mass of the rotating shaft), improper or misaligned axis of the unit, defective guide bearings or improper clearance adjustment, support structure or shaft system of the unit Insufficient rigidity and improper shaft seal adjustment.
1. Unbalanced mass of rotating part
If there is a mass imbalance in the rotating parts (mainly the rotor) of the unit during manufacturing, installation and maintenance, a strong centrifugal force will be generated during operation, causing the rotor to rotate in an arc, increasing bearing wear, reducing mechanical efficiency, and forming rotor and bearing vibrations. And even cause destructive accidents. Due to the imbalance of the mass of the rotor, the center of gravity of the rotor and the shaft center produce an eccentricity. When the main shaft rotates, due to the effect of centrifugal inertial force of the unbalanced mass, the main shaft will bend and deform. The greater the shaft deformation, the more severe the vibration.
2. The axis of the unit is incorrect or misaligned
Because the geometric center of the rotor and runner deviates from the center of rotation, lateral and longitudinal vibrations will occur during operation, and the direct formation of a swing poses a threat to thrust bearings and guide bearings. It can also increase the centrifugal inertia force, both of which increase amplitude. For newly put into production units, generally, the axis will not cause severe vibration due to the incorrect axis, but for units that have been running for a period of time, the axis will be changed for some reason, such as improper cooperation between the thrust head and the shaft, uneven compression of the snap ring, thrust Deformation or damage of the pad between the head and the mirror plate will cause the unit to vibrate.

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