Research On The Water Body Resonance In Penstock System Of Three Gorges Hydropower Station

A strong abnormal vibration happened during the over-speed test on the Three Gorges left bank hydropower station. When vibration occurs, the opening of the guide is about 4% and the rotational speed is about 73 rpm. The frequency of vibration is 1.359 Hz. Although there is high frequency in the vibration of structure, it is affected by the 1.359 Hz.The abnormal vibration under extremely small opening condition, which is a very serious threat for unit to operate safely, is never reported until now and can not be explained using the usual stability theory of hydro-turbine. Vendors have given out the explanations as above, but it is not tenable and can not be accepted widely.According to experimental and theoretical analysis of the results, the abnormal vibration of Three Gorges hydropower station may be caused by water body resonance. In-depth studies for this view have been done in the following two aspects in this paper.First, we gave the controlling equation for the fluctuation pressure. With this equation we derived the formula of the inherent frequency of water in open and closed pipes when considering the pipe as a deformable material or as a rigid wall. Using this theoretical formula, we estimated the inherent frequency of penstock system in Three Gorges hydropower station. The result is close to 1.359 Hz and this proved that the abnormal vibration is possibly caused by water body resonance.Then we modeled the penstock system according to prototype parameters and simulated the water body resonance by the means of a dynamic response simulating software. A pressure disturbance was introduced by a plate's lateral vibration in the end of the penstock. The water body resonance happened when the frequency of forced vibration was close to the main frequency, which was obtained by analysis of the transient impulse response, and the strong responses occurred along the whole penstock. Three kinds of boundary condition were discussed: (1) Considering the surface of the pipe as a rigid wall. (2) All the nodes in the outer layer of the concrete outside the pipe are restrained when taking the material elastic deformation into account. (3) Only the below half of the nodes in the outer layer of the concrete outside the pipe are restrained. The result showed that the frequency that caused the water body resonance is close to 1.359Hz when the boundary condition is near the actual condition.It is reasonable and acceptable for the view that the abnormal vibration is caused by water body resonance after theoretical analysis and numerical simulation in this paper. The work in this paper is useful for large turbine group to prevent abnormal vibration and improve operational stability.