The dynamic loads absorbed by pressure conduits of hydroelectric stations (HESs) and pumped-storage stations (PSSs) are composed of water:hammer, occurring during transient processes and caused by a change in the operating regime of the unit, and of fluctuations occurring both during steady and transient processes. Whereas the phenomenon of water hammer has been studied rather thoroughly and many scientific investigations and works have been devoted to this problem, the problem of fluctuations is still in the initial stage of study. Yet fluctuations can have a substantial effect on the reliability and life of pressure conduits and equipment and in connection with this their investigation and development of methods of taking them into account during design and operation is an urgent problem.Various types of pulsation loads are possible in pressure conduits. Sometimes selfexcited pressure fluctuations occur, which are created in a hydroelastic system having a device acting on the flow rate and characterized by the fact that with increase of head (pressure) the discharge being passed decreases. This system is hydraulically unstable and strong and even dangerous fluctuations with a pronounced resonance frequency can occur in it. Such fluctuations induced by vibrations of a spherical valve were noted, for example, at the Bersimis HES [6]. Stretches of unstable flow, for example, in forks, which occur in the presence of an unbalanced relationship of the discharges of the branches, when fluctuating stagnant zones are created, can be exciters of pressure fluctuations. But these are all special cases. The most common are forced pressure oscillations induced by disturbances created by the operation of the units. It is necessary to note that the nature of these disturbances has still notbeen completely revealed and on this account various hypotheses are expressed. It is considered that the cause can be vane disturbances (impacts) [I, 2], rotational separation of the flow in which the axial symmetry of flow past the circular rows of the blades of the unit is disturbed [3], and vortex filaments behind the runner and their movement (precession) in the draft tube [2, 4, 8, 9, i0]. Obviously, all the factors mentioned can act to a greater or less degree depending on the geometry and operating regime of the units.For calculations of pressure fluctuations in pressure cond@its the resultant action should be expressed in the change in the discharge being passed by the unit at a given head or in the change in the head being developed at a given discharge. So long as there is insufficient data on the actual characteristics of the disturbances it is ncessary to base oneself on some kind of working hypothesis. In the majority of works it is assumed that the disturbance can be represented in the form of the fluctuation of the discharge capacity or reduced discharge AQ'_.I for a given opening of the unit. Then the change in the discharge in dimensionless parameters Aq will be represented by the equa~lon Aq = Aq~ ~l + ~ + +~,where Aq = AQo...
The problem of reconstruction and enhancement of the technical level of equipment at active HPP requires a complex approach, and, in the general case, calls for studies in the following directions:-replacement of basic water-power equipment with improvement of power-producing qualities of the generating sets;-reconstruction and modernization of electrotechnical and auxiliary equipment with enhancement of technical characteristics (reliability, response time, maneuverability, lowering of production costs, etc.); and, -use of modern control systems for HPP based on computer technologies and systems for the protection of generating sets using microprocessor equipment.The first of the directions indicated defines to the greatest degree the effectiveness of the reconstruction of equipment at active HPP. Considering the above, a methodology is proposed below for replacement of water-power equipment. Criteria for Selection of New Equipment (goal of reconstruction)Replacement of hydraulic-turbine equipment should certainly be accompanied by an improvement in the energy efficiency of the generating sets, to wit an improvement in the capacity or an increase in the generation of electric power from the volume of water passing through the turbines ( first basic reconstruction criterion). Replacement of obsolete and physically worn turbines by analogous machinery fashioned from a higher quality material, but with the same power indicators (replacement of "metal by metal") is considered unacceptable.Simultaneous improvement in the above-cited power indicators has not always been successfully provided. In that case, selection will depend on whether a new turbine with a forced operating regime is adopted, and as a result, an additional capacity with reduced power generation is obtained from the volume of water passing through the turbine, or maximum gain in power generation is achieved from the same volume of water, but at a lower additional capacity (or in general, its absence).Being disposed to power effects as regards power generation and capacity, it is possible to convert to a cost expression for reconstruction effectiveness in terms of an "ith" variant:where s N is the cost of 1 kW of displaced capacity at the replaced power plant, s P is the cost of 1 kW · h of power generated at the replaced power plant, N t.ex and P ex are, respectively, the capacity and power generation of the existing turbine, and N ti and P i are the capacity of and the power generated by the new equipment for the "ith" alternate reconstruction scheme. It is obvious that for this approach, selection of the best alternate reconstruction scheme will be determined by the absolute values of s N and s P .Assessment of the replacement of a hydraulic turbine is, however, insufficient from positions of the attainment of power-producing effects alone. The reliability of the equipment assumes major significance. That which ensures a maximum operational-availability factor K a = K a max for the equipment is considered the optimal alternate reconstruction scheme...
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