Phase Comparison protection using adaptive carrier modulation

Phase comparison protection has been traditionally applied using a fixed ratio of sequence quantities to modulate the power line carrier at either end of the line. This paper examines a practical method to control this ratio adaptively in order to provide better coverage for high resistance earth faults. The paper also looks at techniques to compensate for phase errors introduced by charging current and signal elongation by the power line carrier communication equipment. Since phase comparison is a normally quiescent protection scheme, fault detection methods based on change in sequence quantities at the instance of a fault are also discussed.