A multivariable new control solution for increased long lines voltage restoration stability during black startup

This paper deals with the stability problems which can determine failures of long lines voltage restoration maneuvers during black startup. Also of concern, a new control solution is shown, capable of facing the phenomena of voltage instability and self-excitation, which frequently occur in the alternator-line system. The paper presents a theoretical analysis, which reconstructs and justifies the described dynamic phenomena. It is mainly concerned with the influence of alternator and grid electrical parameters on the transient characteristics of the voltage restoration maneuver. It also describes 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 underexcitation limits or, better yet, a superimposed centralized reactive power control. The objective is to reduce line energization extra-voltages, balancing the control efforts of all power plant units while stabilizing underexcitation operation of the generators, and preventing self-excitation wherever possible. Such a control feature could easily be implemented, in the Italian network, using the reactive power regulator (REPORT), currently widely employed for secondary voltage regulation (SVR) applications.