Flex-LCC: A New Grid-Forming HVDC Rectifier for Collecting Large-Scale Renewable Energy

A new grid-forming (GFM) high-voltage direct current (HVdc) rectifier, named flexible line-commutated converter (Flex-LCC), is proposed in this article to collect the power generated by large-scale islanded or weak-grid-supported renewable energy systems. The Flex-LCC uses a series-connected hybrid valve based on the LCC and full-bridge modular multilevel converter (FBMMC), and adopts a new control. As a result, the Flex-LCC works as a GFM rectifier and retains the same dc voltage and power carrying capacity, dc fault clearing, and power reversal functions as an LCC. Particularly, the Flex-LCC has three salient features: the control of the Flex-LCC is decoupled from the LCC part's firing angle, the internal reactive power allocation is flexible, and the valve-side ac voltage of the FBMMC part has a wide operation range due to the decoupling of dc-port voltage and capacitor voltage. These bring reduced interharmonics, enhanced stability, and a smaller capacity boundary to the FBMMC part for the Flex-LCC. Furthermore, utilizing the reasonable design for the actual capacity and valve-side ac voltage of the FBMMC, the Flex-LCC can effectively balance cost, footprint, and weight, and match the current-carrying capacity between the series-connected LCC and FBMMC. The topology, control, characteristics, and design of the Flex-LCC are analyzed in detail in this article. Simulations and experiments validate the theory.