Practical experiences in the analysis of abnormal voltages due to neutral instability

Practical experience has shown that the neutral of an isolated three-phase system may become subject to an instability phenomenon under apparent normal condition. This instability may lead to high abnormal voltages to ground, constituting a possible hazard to system apparatus, besides creating special relaying and synchronizing problems. This paper presents the investigation carried out by CEMIG Companhia Energetica de Minas Gerais and ABB Asea Brown Boveri to analyze the occurrence of this phenomenon in its power system, pointing out the conditions that led to the abnormal behavior of the circuit. A methodology using a simple model of the system to predict a possible occurrence of neutral instability is discussed and proved very effective in the cases analyzed. Also it is shown that overvoltages calculated with alternative transients program (ATP) compared favorably with recorded results. As a conclusion, the paper presents the different successful measures implemented to avoid the disturbance.