Circulating Current Suppression and Midpoint Potential Balance of Flexible DC T-Type Three-Level Bidirectional Converter for Urban Rail Transit

[Objective] A bidirectional converter integrates traction， feedback， and reactive power compensation and is the primary choice for upgrading the main equipment of an urban rail DC traction power supply system. In the field of low-voltage DC traction power supply， converters have the characteristics of low voltage and high current. The T-type three-level multi-unit parallel connection combined with carrier phase-shifting technology can meet the special requirements of low cost， low harmonic， high power density， high efficiency， and wide DC voltage operation range of urban rail DC traction power supply systems for the main equipment. However， this approach has problems such as excessive zero-sequence circulating currents and unbalanced midpoint potentials. [Methods] First， a zero-sequence circulating current model is established to address the circulating current problem in bidirectional converters. The circulating current is classified and analyzed， and a transformer low-voltage winding splitting， shared neutral line， L-shaped filter， and software control algorithm are proposed to suppress the zero-sequence circulating current. Second， in response to the problem of midpoint potential imbalance and based on the implementation of the equivalent space vector pulse width modulation， the relation of the midpoint potential to the phase voltage， phase current polarity， and space vector type is established by analyzing the T-shaped three-level single-phase topology structure. Accordingly， a feedback-compensation-based midpoint potential balance control method is proposed. [Results] Based on PSCAD/EMTDC simulation testing， it was found that various zero-sequence circulating currents could be effectively suppressed， achieving a midpoint potential balance in three-level converters during four-quadrant operation. [Conclusions] The proposed circulating current suppression method can not only suppress various zero sequence circulating currents， but also help reduce the cost of measuring the midpoint potential of three-level systems. The proposed midpoint potential control method does not require complex space vector modulation processes， has low computational complexity， is easy to implement， and can effectively control the midpoint potential.