This paper presents a unit protection scheme based on the energy conservation law（ECL）. The signals of real power measurement at both ends of a line or all the ends of a busbar are compared and if the result is more than a pre-specified value， an internal or in-zone fault is indicated. The proposed technique is evaluated using a MATLAB simulator configured to model the effect of faults on typical 400 kV busbars and transmission lines. Results indicate that the method is more reliable than a conventional biased differential protection algorithm and needs fewer calculations and hence less computation time. It can also detect all fault types， including those on long and/or highly capacitive lines.