The availability of cheap and powerful microprocessors in recent years has led to their increasing use in all aspects of power engineering. In future years they are expected to play an even greater role in power system protection schemes because of their ability to: (1) compare on-line data and come to a logical or quick decision; (2) store information from past events and make an expert decision; (3) combine various complicated tasks, etc. Thus, the application of microprocessors in power system protection is much more attractive than in many other fields. As an example, this paper develops a high speed digital protection scheme for a power transmission line. A signal processing technique is used to analyze the power spectrum of the transient fault currents, which makes it possible to develop a desired digital filtering algorithm using computer-aided design (CAD) techniques and minimize the potential for false tripping due to the steady-state system harmonics. This design was implemented on a 32-bit MVME133 monoboard microcomputer to demonstrate the protection of a model power transmission line.