TRANSIENT VOLTAGE OF NEUTRAL POINT IN THE THREE PHASE TRANSFORMERS

Neutral point displacement of a power transformer is mostly posed in the unbalanced load and line impedances. However, the neutral point may have a non-zero voltage even when the primary and secondary sides of a Y-Y non-grounded transformer are connected to a balanced three phase voltage and load respectively. This can be significant during switching transient period and its value is being effectively influenced by the magnetic characteristics of the transformer core. In this paper the comprehensive model of a transformer is simulated using the Preisach model of hysteresis. In this simulation the hysteresis model of the core is coupled with the electrical equations of the terminal circuits. A transient load switching problem is studied and the neutral point displacement of the secondary side is investigated while the primary of a laboratory built Y-Y transformer is grounded. This study is imperative regarding the protection and safety issues.