The purpose of this study is to propose a novel differential protection scheme for DC-AC converters. The current differential relay is widely used to protect transformers, generators, busbars and transmission lines. However, it cannot be applied to a DC-AC inverter, which is used in an AC-DC transmission line system. This paper presents a novel differential protection scheme for DC-AC converters, using Tellegen's Theorem. Tellegen's Theorem is a simple theorem of electric power proved from Kirchhoff's current law and Kirchhoff's voltage law. Under the theorem, we define three kinds of electric power as power flow. The proposed technique is evaluated using simulation studies based on an ideal inverter circuit which transforms DC to AC. The test results show the validity of the proposed protection.