Voltage stability control for long lines black start-up

The paper deals with the stability problems that can determine failures of long lines voltage launch during black start-up. On this concern a new control solution, able to face the phenomena of voltage instability and self-excitation that frequently occur in the alternator-line system at the beginning of the restoration maneuver, is proposed. The paper preliminary analyses the phenomenon, reconstructing and justifying the related voltage instability risk. It is mainly concerned with the influence of alternator and grid electrical parameters on the transient characteristics of the voltage restoration maneuver. They are also described the most significant results of a dynamic study, based on a simulation model of a generalized plant-grid system, suited to the purpose of the investigation. Specific corrective actions, to be essentially taken on the excitation control system, are furthermore outlined. In this context the most effective solution is a multivariable control scheme, involving use of new under-excitation limits or, better yet, a superimposed centralized reactive power control. The objective is to reduce the line extra-voltages, balancing the control efforts of all power plant units while stabilizing the under-excitation operation of the generators and preventing self-excitation wherever possible. Such a control feature could easily be implemented in the power plant making use of the new voltage/reactive power regulator (named REPORT in Italy), required by the Secondary Voltage Regulation (SVR).