Damping in transmission systems under transient conditions

The general practice of neglecting the damping introduced by transmission networks during power-system transient-stability investigations is questioned, and a method is presented for calculating swing curves to include the influence of instantaneous frequency variations on the transmission network. In this consideration, although the system loads are treated as fixed impedances to isolate the effects of the transmission network, the approximation involved still enables the nature and significance of the effects to be demonstrated. An example illustrates that negative damping may be exhibited by a system when account is taken of instantaneous frequency variations in the network, and that consequently not only are the voltage and current distribution less favourable than when assessed by the usual calculations which neglect damping, but the system may be unstable, even though it has been calculated to be stable by conventional methods. As certain power systems, particularly those containing long transmission lines with series-capacitor compensation, have elements which tend to promote negative damping, it is suggested that consideration should be given to the introduction of instantaneous-frequency-deviation effects in transmission networks for transient-stability studies.