A Transformerless DC-DC Modular Multilevel Converter for Hybrid Interconnections in HVDC

In this article, a modular multilevel dc-dc converter philosophy is proposed which can form an effective interlink to exchange power between line commutated converter (LCC) based high voltage direct current (HVdc) link and voltage source converter-based HVdc grid of same or different voltage levels. The converter exhibits symmetric four quadrant operation irrespective of the voltage polarity in the LCC-based HVdc link. In the event of reversal of power flow by reversing the voltage polarity in the LCC-based HVdc link, the power flow direction/magnitude through the interlink can be made to remain unchanged without the need of any extra switching arrangement. The working principle is explained analytically and its operation is validated by simulation as well as experimental results.