ANALYSIS OF THE INFLUENCE OF HYDROLOGY AND THE NUMBER OF UNITS IN THE WEIGHTED AVERAGE YIELD

摘要

MSUI, the official model for calculating firm energy in Brazil, uses as input a constant value for the weighted average yield. The values used for this parameter are often derived from Eletrobras guidelines, regardless of the operating conditions of the hydroelectric plant. The analyzes of this paper are based on the criterion for calculating weighted average yield presented in the inform “Metodologia de Cálculo de Parametros Energéticos Médios: Rendimento e Perda Hidráulica” (Calculation of Average Energy Parameters: Yield and Loss Hydraulics), from EPE (Empresa de Pesquisa Energética – Energy Research Enterprise). The article initially presents a simplification of that methodology, which showed results satisfactory close to the original ones. Using the proposed simplified criterion, sensitivity analysis of the performance of the turbine / generator set depending on the variability of the inflows and the number of generating units are presented. The analysis is performed from the HPP Teles Pires, at Teles Pires river, in Mato Grosso. The probability that the inflow is greater than the maximum turbine discharge had a good relationship with the outage and the average loss, as showed in authors’ other articles. This criterion, however, did not produce good results in this case. The best relationship was found relating the weighted average yield increase with the decrease in the coefficient of variation of the inflows time series. In other words, as the variability of affluences diminish, increase the weighted average yield. It was adjusted a straight line in the graphic of yield versus coefficient of variation and a coefficient of determination r2 equal to 0.5926 was found. In statistical terms, this means that 60 % of the variation in the weighted average yield is explained by the coefficient of variation of affluent flows. On the other hand, the increase of installed machines meant an increase in the weighted average yield. This occurs because the employee criterion makes the plant operating each month with a minimum number of units and with the same unitary flow in all units. Thus, the greater the number of machines, most often turbine discharge unit is near its maximum – and this increases the yield most of the time. The authors suggest new studies based on the analyzes made: criticism of EPE criterion (2012) for calculating the weighted average yield; proposition of new criteria based; study of the variation of the weighted average yield to energy operation (peak and of peak operation, with or without monthly drawdown , etc.); analysis of the symmetric operation of machines.