Improved dynamics of DC bus control with a zig-zag transformer for bipolar DC distribution systems

The concept of bipolar DC distribution has been shown to possess superior flexibility, efficiency, and quality when compared to the conventional unipolar approach. However, the control dynamics of the power conversion circuit for bipolar DC distribution are typically limited by the presence of DC-side capacitors. To enhance the stability and the reliability of DC power distribution systems, it is essential to ensure stable responses to both load dynamic characteristics and DC faults. This paper proposes a bipolar configuration and control strategy to improve the DC-link response characteristics of DC distribution systems. The proposed circuit utilizes a zig-zag transformer to enhance the DC voltage control dynamics of the converter. The zero-sequence impedance along the common mode path of the converter is effectively reduced by the zig-zag transformer, with an impedance range equivalent to the leakage components of the transformer. The converter of the bipolar structure can independently control the voltages of each DC pole, which enables faster dynamic responses of the voltage controls. Additionally, this structure facilitates the handling of asymmetric loads on each pole. The validity and the operation of the proposed configuration are verified through simulations and experimental results.