Power Flow of HVAC/FFTS System for Transmission Line Upgrade Based on Modular Multilevel Matrix Converter

Fractional frequency transmission system (FFTS), which also referred as low-frequency ac system (LFAC), is a competitive scheme for transmission line upgrade (TLU). Modular multilevel matrix converter (M³C) is the state-of-the-art frequency converter for FFTS, but the power flow algorithm of M³C-based HVAC/ FFTS is rarely studied. In this paper, the power flow model of M³C station is firstly proposed. The M³C is represented as back-to-back VSC with no physical DC bus, and its control model, loss model and operation limits are deduced by considering the bilateral frequency-component coupling effect. Then a sequential power flow algorithm for HVAC/FFTS system is derived. The FFTS grids and HVAC grids are solved separately, and the convergence is achieved by checking active power balancing at all M³C stations. Therefore, the proposed algorithm can be easily adopted with existing power flow programs. Finally, a case study shows that TLU via FFTS is able to increase transmission capacity, mitigate voltage fluctuations and reduce transmission loss. improve power quality and reduce the power grid loss.