Research On The Abnormal Vibration Of Unit 6 Of The Three Gorges Left Bank Station Under Small Guide Opening Conditions

Francis turbines are used in Three Gorges hydraulic-generator unit and the X-type blades are utilized. There are many advantages to utilize this kind of blade, such as good flexibilities of wide range varying of head and load, high average efficiency, good performance for preventing flow separations and cavitations at leading edges under high head or small opening conditions. However, there are many factors to affect the stabilities of Francis turbine operation; some of them have not been recognized sufficiently. In addition, the Three Gorges units are huge and accordingly, the natural frequency of water is low and water resonance vibration is easily happen. On 6 Aug 2003, a strong abnormal vibration happened during the over-speed test on the unit 6 of the Three Gorges left bank hydropower station. When vibration occurs, the opening of the guide is about 4% and the rotational speed is about 73rpm. The frequency of vibration is 1.359Hz and its max peak to peak value reaches 500kPa in the spiral, which is larger than the half of the total test head (about 68m). Although there is high frequency in the vibration of structure, it is affected by the 1.359Hz. Besides, the vibration of water before the guide open is very strong; however, the vibration after the guide open is small.Considering this kind of vibration, ALSTOM Co. evaluated the time history of flow utilizing Gibson method; and they believed that the vibration reason is the negative flow. A model test was carried out by Harbin Institute of Large Electrical Machinery in China and the full performance curves of model were obtained. According to the model test, the water hammer is believed to be the reason leading to abnormal vibration.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 actual stability theory of hydro turbine. Vendors have given out the explanations as above, but it is not tenable and can not be accepted widely. In-depth studies for the vibration have been done in this paper. Considering the fact that the model test can not reflect dynamic characteristic of prototype and it is unallowed to measure the interior flow of prototype, the dynamic numerical method is utilized to research the vibration mechanism based on the theoretical analysis. The results are as follow.Firstly, an effective method and strategy to research the flow in hydraulic machinery dynamically is put forward and developed from many aspects such as mesh, turbulent model, difference scheme, time step control and output control etc., of which DES (Detached-Eddy Simulation) is chosen as the turbulent model and MARS (Monotone Advection and Reconstruction Scheme) which is a TVD (Total Variation Diminishing) type is chosen as the difference scheme. These choices have been validated by the classical cylinder flow problem. In addition, the author of this thesis writes a C++ code to decompose the time-consuming and hard-disk-consuming solution process into some sub processes. These sub processes can be executed automatically and steadily.Secondly, the flow characteristic of hydraulic machinery in small flow is discussed according to the basic principle in this thesis. The results are: (1) Rotating stall happens at some flows for an axial pump. When this occurs, the main frequency of velocity and pressure is below the frequency of rotation of the impeller. However, the amplitudes are far greater than those in design conditions. (2) Stall happens at some flows for a centrifugal pump; however, the stall cells do not always propagate back along the annular cascades. In this thesis, a stall happens in a centrifugal pump with 8 blades, but the stall cells are stable with the impeller speed and do not propagate back along the annular cascades. However, when the number of blades is changed to 7 and the flow, the topology of mesh remain unchanged, slow propagation happens. Combining the rotating stall theory, the author deems that the developments of the four stall cells are same and the positions of them are symmetry for the centrifugal pump with 8 blades, so there is a lack of power of rotation; for the pump with 7 blades, the number of stall cells is still four, but the developments are not same and the positions are not symmetry, so there is the power of rotation.In addition, the propagation rule is researched for 7-blades pump. The propagation way is found to be jumping, that is, the stall cell propagates to the third blade but not the adjacent one.(3) The flow separates from the suction surface of the blade at small flow for hydro-turbine. The developments of the separations are same for the researched model. Neither stalls nor rotating stalls happens. Combining the results of axial pump and centrifugal pump and the mechanism of rotating stall, the author infers that one of conditions leading to rotating stall for hydraulic machinery is that there are unstable separations exist. However, it is contracting channel for hydro-turbine and accelerating pressure gradient exists in the channel, the separations is stable in general and accordingly, it is difficult for rotating stall in hydro-turbine to occur.Thirdly, through the analysis of the vibration detail messages and the full performance curves of model, the author deems that the viewpoints of the vendors are not acceptable and put forward the view point of water resonance. Finally, the dynamic numerical simulations on the abnormal vibration condition of the Three Gorges units validate the existence of the excitation source. The frequency of the excitation source has a good agreement with the experiment data. The computational results can be summarized as:(1) A local high pressure region exists on the front surface of the guide near the lower end of the exits of guides. The flow is non uniform along the direction of the guide height. A back flow exists near the lower end of the exits of guides. This makes most of the water flow out to runner from the top half of the exits of guides. The characteristic of flow shows that the runner's state of the bottom half and the top half is non uniform, that is, the bottom half is reverse pump and the top half is brake operation of hydro turbine.(2) An obvious velocity and pressure fluctuation of which frequency is 1.359Hz exists in and originates from the region near the exits of guides. There are two points should be proved for the explanation of water resonance: one is that there is an excitation source of which frequency is equal to 1.359Hz; the other is that the natural frequency of the vibrating body is 1.359Hz.The existing of the frequency of 1.359Hz proves the first point, but then it indicates logically that the second one also is true. In addition, the vibration reason about negative flow and water hammer is deduced to be not correct according to the settings of dynamic numerical computation and above results.(3) The statistical analysis for the fluid cells near the exits of guides shows that the amplitude values of which corresponding frequency is 1.359Hz reach maximum in the center section of the guides. Combining the results of (1), it can be deduced that the runner states of the top half and the bottom half are unstable which leads to the frequency of 1.359Hz.From the view of the position and the mechanism about the frequency of 1.359Hz, it is never reported previously, so the results of this thesis development the contents of stability theory of Francis turbine.