Analysis of distance protection in low frequency AC transmission systems

摘要

This paper extends the analysis of low frequency AC (LFAC) transmission systems and investigates short-circuit fault characteristics and distance protection considerations. Due to operation at a low frequency, transmission line reactance is reduced and thus the power system transfer capability is increased significantly. The transmission line impedance reduction, however, creates a drawback in LFAC systems when faults occur. The analysis shows that fault currents are expected to be much higher than in a 60 Hz system. In addition, LFAC has a longer current wavelength and requires the protection system perform quickly to clear faults. This paper first examines distance protection in LFAC systems and determines that LFAC systems have a larger separation between distance protection zone characteristic impedance and load impedance in comparison to 60 Hz systems. Next, several typical fault types in a power systems are analyzed at different frequencies. A fault clearing time case is also calculated and compared between different systems. The analysis shows that the critical clearing time (in seconds) is less for LFAC systems in comparison to conventional 60 Hz systems.