The selection of the installed capacity of a hydroelectric plant is usually made on the basis of the design lowwater year, which is determined from the design frequency. The assumption, in the project, of an excessively high value of the design frequency results in a reduction of the installed capacity of the plant and a decrease of its economic indices.For the technical-economic justification of the design frequency, a very important quantity is the initial frequency Pi of the guaranteed capacity of the plant. The variation of the frequency in the range from p = 1 to Pi may be justified by the indices of the power system for the given reliability of power supply, and for p < Pi it is necessary to take into consideration the variation of the losses suffered by energy consumers when the reliability of the power supply varies. As a result of the relative cheapness of the turbines and generators, it is convenient to increase the emergency reserve of the power system and at the same time to increase the reliability of power supply.At the present time, in the reliability analyses of power supply, consideration is usually given to the breakdown frequency of the units, the transmission lines, etc. but not to the operating flexibility of the electric plants. It is known that as a result of their greater flexibility, hydroelectric plants substantially increase the reliability of power supply as compared to thermal electric plants. For this reason, in the alternatives considered below for power systems with hydroelectric plants, the reliability of power supply is above the values obtained from analyses in which the operating flexibility of the electric plant is not considered.The initial value of the adequate capacity of a hydroelectric plant may be determined from a comparison of the reliability of power supply obtained for two alternatives for development of the power system, namely: a) on account of the construction of only the replaced thermal electric plant, and b) on account of the construction of the hydroelectric plant and of the replaced thermal electric plant, a part of whose capacity is in this case the replaoed capacity of the hydroelectric plant. The central part of these analyses is the determination of the reliability of power supply in the combined power system. A procedure for carrying out such an analysis as applied to a system with radial links was developed at the Department of Utilization of Water Energy of the M. I. Kalinin Leningrad Polytechnic Institute, and was published in [2].The following initial data are required for the analysis: the capacity of all the generating units in the system, their breakdown rate (probability of the emergency condition with time), the frequency curves of the operating capac- Frequency curve for the emergency conditions of the power system.ities of the hydroelectric plants, the daily load graphs of the power system, the mean standard error in the prediction of the loads, and the breakdown rate of the transmission lines. The results of the analysis are presented in the ...

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