Abstract: There is a high probability to operate successive load rejections when more than two units share one main pipe, which is generally adopted at pumped storage power stations. This condition could be a control condition of the highest surge in upper surge chamber. This paper derives an implicit formula for calculating the highest surge following load rejection at any time using the basic equation of wave motion of surge chamber, and calculates the surge-tunnel discharge relationship for the condition occurring at the worst surge wave superimposition owing to successive load rejections or other unfavorable superimposition conditions. By the results of the implicit formula taking this relationship as initial condition, we come to the conclusion that the control condition is shifting from load-increment-then-rejection condition toward successive load rejections as the hydraulic resistance coefficient of throttled surge chamber increases. Numerical simulations of an example pump-storage project are included for verification of the conclusion.