When Powerformer runs in the condition of open phase， the neutral point is exposed to high over-voltage. The weakest part in the insulation system of Powerformer is the neutral side and the high over-voltage is a serious threat to the neutral point. Hence it is important to study the open phase protection for Powerformer. Based on the analysis of the characteristics of open phase operational state， a novel open phase protection scheme utilizing the zero sequence voltage of the terminal and the negative sequence current of the stator windings is proposed. A simulation model has been constructed using EMTDC， and intensive simulations have been carried out， which demonstrate that the protection scheme is correct and practical， and using the combination of zero sequence voltage and negative sequence current can improve the reliability of the protection.