In view of the inability of traditional distance protection zone Ⅲ to correctly identify the difference between overload and fault

this paper specifically analyses the differences in electrical characteristics such as overload and fault from current asymmetry degree

Ucos φ and the variation of voltage phase. A new method for identification of overload and fault is proposed

which is based on current asymmetry degree and the difference between fault arc voltage and the minimum system voltage and voltage phase change. This method not only realizes the identification of symmetric overload and fault

but also solves the identification of the asymmetric overload and the high resistance grounding fault by comparison of changes in voltage phase. Through real-time digital system (RTDS) simulation

the results show that the method is capable of preventing maloperation of distance protection zone Ⅲ caused by overload while unblocking distance protection zone Ⅲ when fault does happen. This work is supported by State Grid Corporation of China

Major Projects on Planning and Operation Control of Large Scale Grid (No. SGCC-MPLG003-2012).