Analysis of two-phase operation characteristics of a flexible low-frequency transmission system

Abstract

The two-phase operation mode of a flexible low-frequency transmission system is based on the M3C （modular multilevel matrix converter）. This paper establishes a complex sequence network for the low-frequency system to analyze the steady-state operation characteristics of low-frequency transmission lines and M3C under the conventional control strategy. The analysis shows that： 1） There appears zero-sequence current that shares equal amplitude and opposite phase of the positive-sequence current in transmission lines； 2） The zero-sequence voltage and negative-sequence voltage can also appear in the transmission lines； 3） There is an unbalanced capacitor voltage between M3C sub-converters with fixed power control on the low-frequency side and negative sequence current suppression strategy； 4） There is a double frequency oscillation component in the M3C average capacitor voltage. A simulation model of the dual-terminal flexible low-frequency transmission system is built in PSCAD/EMTDC， and the correctness of the obtained conclusions is verified.

Keywords

• flexible low-frequency transmission
• two-phase operation
• complex sequence network