Conditions under which hydromechanical processes proceeded immediately prior to the start of and during the failure of the second generating set at the Sayano-Shushenskaya HPP are examined.Since 2002, the Sayano-Shushenskaya HPP has been drawn into secondary frequency regulation within the power system, and capacity transfers. A signal for a change in the output of the HPP is delivered from the Centralized Dispatching Regulator "Sibir'" to the Group Active-Power Regulator (GAPR) of the hydroelectric plant, and from there to individual turbine-speed regulators controlling the variation in active power through variation in flow rate.The algorithm used for operation of the system for active-power regulation was developed in conformity with characteristics of the turbines at the Sayano-Shushenskaya HPP. It considers that within the range of average capacities, any Francis turbine exhibits increased pressure pulsations in the setting, which may be accompanied by vibration of subassemblies in the generating set (Zone II in Fig. 1). This is dictated by increased flow circulation at the discharge from the impeller, the existence of a low-frequency vortex core in the draft tube, and other transient effects owing to the low efficiency of the turbine at rather high flow rates.The region with increased pressure pulsations is bounded on the left by the basic adjusted range of the turbine (Zone III in Fig. 1), which falls within the limits from 490 to 640 MW with the turbine under the design head of 194 m, and decreases with increasing head. (Prior to the failure of 17 August 2009, the range was all of 70 MW under a head of 212 m -from 570 to 640 MW).Zone I with capacities ranging from 0 to 265 MW acted as the operating zone to broaden the plant's maneuvering potential. Transfer of the sets from operating Zone III to Zone I was carried out alternately in conformity with the priority system adopted. A certain priority ranging from 1 to 10 had been assigned to each set. When the regulated range for unloading into Zone I had been exhausted by the plant, the set with the highest priority was transferred from the operating range into Zone III. Accordingly, when the regulated range for loading had been exhausted, the set with the lowest priority was transferred from operating in Zone I to Zone III.Thus, the switching of Zone I into the operating regimes controlled by the GAPR increased the number of transfers of sets through Zone II during load adjustment. Previous transfers had occurred only during startups and shutdowns. The operation of some sets under individual regulation had also contributed to the increase in the number of the transfers through Zone II, reducing the regulated range of the HPP.In the period preceding the failure, nine generating sets (with the exception of GS6, which was being overhauled) were operating at the Sayano-Shushenskaya HPP. The following control scheme was in effect in conformity with the assignment made by the on-duty shift supervisor: sets GS1,

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A mathematical description of a closed control system with allowance for pressure fluctuations in the head system, which makes it possible to analyze the regime stability of orthogonal generating sets at tidal electric power plants when operating in the complete range of heads, outputs, and rotational speeds, and to select parameters of the control system, is obtained for an orthogonal hydroturbine and a generator with a load regulator.For an orthogonal turbine, constant head variation at tidal power plants (TPP), which is defined by the in-out character of the flow, results in deviations from the optimal regime. At the Kislogubskaya TPP, a high-voltage frequency converted (HVFC) is used to maintain the turbine within the region of maximum efficiency; full-scale tests were performed after installation of this converter.The high-voltage converter varies the output of the generator with respect to the static characteristic, ensuring turbine operation at the optimum of the characteristic. A relationship relating static head to e load was derived from results of the full-scale turbine tests.The generating set at the Kislogubskaya TPP with the frequency converter, and an aeration shaft fabricated from a junction box between units, and a water conduit connecting the turbine to the basin, represent a complex dynamic system. The parameters of this system vary within a broad range as a function of the static head, which assumes values of from 0.6 to 3 m and more in the operating regimes. Figure 1 shows the "static-head/frequency" characteristic, which is encoded in the frequency converted, and which corresponds to its relationship between generator output and static head.Analysis of results of the full-scale tests indicated that the orthogonal set with the high-voltage frequency converter satisfactorily reproduces the assigned optimal relationship between rotational speed and active head on the TPP. Essentially constant positioning of the regime point on the universal turbine characteristic with reduced flow rates Q 11 ranging from 1.62 to 1.69 m 3 /sec and reduced rotational speeds n 11 ranging from 122 to 126 rpm, and a constant efficiency level of the turbine are automatically ensured as the static head varies within the range from 1.2 to 2.4 m.The water conduit connecting the turbine to the basin, and the aeration shaft consisting of a junction box between units represent a fluctuating system, which influences the frequency oscillation and output of the generating set. The full-scale tests, which were conducted in April -May 2010 with the set operating in the power system at a constant and variable rotational speed indicated that gravitational oscillations with a period of 4 sec (correspondingly, at a frequency of 0.25 Hz) develop in the water conduit and aeration shaft. The amplitude of the oscillations increases with increasing load on the set. Figure 2 compares the oscillograms of the output and pressure pulsations on the basin side with the set operating in a straight-through circuit, and via the high-voltage freq...