A Monopolar Symmetrical Hybrid Cascaded DC/DC Converter for HVDC Interconnections

Abstract

With the rapid development of voltage source converter (VSC) based high voltage direct current (HVDC) transmission, it is an irresistible trend that HVDC grid will come into being. High-voltage and high-power DC/DC converters will serve as DC transformers in HVDC grid to interconnect DC lines with different voltage ratings. This paper proposes a monopolar symmetrical DC/DC converter which is composed of cascaded half-bridge sub-modules (SMs) and series-connected IGBTs. This hybrid topology features low capital costs, high efficiency, small footprint, and bidirectional power transfer capability. Operation principle, parameter design, and the control strategies of this topology are introduced. A 480MW, ±500kV/±160kV monopolar symmetrical DC/DC converter is simulated to verify its performance and evaluate the efficiency. In addition, a downscaled prototype rated at 2.4kW, ±300V/±100V has been built and tested. Experimental results further validate the effectiveness of the proposed DC/DC converter.

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“…DC-DC converters are widely used in DC voltage matching or changing of DC voltage levels, i.e., to perform similar functions as AC transformers. These devices offer several attractive features such as high efficiency and small footprint compared to their predecessor, the linear regulator [1][2][3][4][5]. To maintain a regulated output voltage, it requires a feedback compensation system; which is normally designed employing model-oriented controller techniques.…”

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“…Cascaded structure can cascade two or more converters to make the voltage conversion ratio equal to the product of each stage 12 . Zhang et al 13 proposed a monopolar symmetrical DC–DC converter after introducing cascaded half‐bridge submodules, which featured high step‐up voltage gain and low capital costs. Hu et al 14 put forward a novel hybrid cascaded DC/DC converter based on coupled inductors and voltage multiplier cells, which could achieve a high step‐up voltage ratio and recycle the leakage inductance energy of coupled inductors at the same time.…”