A Smooth Pre-charging Strategy for an Offshore HVDC System Based on Modular Multilevel Converters

This paper presents a soft pre-charging scheme for a modular multilevel converter-based HVDC transmission system used to interconnect an offshore wind farm to an electrical network. The proposed scheme comprises five stages that are continuously repeated until the dc voltages of the converters reach the peak value of the grid voltage on the side of the HVDC inverter. During the first stage, two thyristors control the current drained from the grid by the HVDC inverter in order to charge the capacitors of two arms of the MMC. Later, during the
$2_{2}^{nd},3^{rd}, 4^{th}$
and
$5^{th}$
stages, the energy stored in the SM capacitors during the first stage is transferred to the others SM dc capacitors of the inverter. In addition, two additional stages, the
$4^{th}$
and
$5^{th}$
, are used to transfer part of the energy stored in the inverter's submodules capacitors to the upper and lower SMs of the rectifier. Results of digital simulations obtained with the PSCAD/EMTDC, as well as the mathematical description of each energizing stage, will be presented to demonstrate the effectiveness of the proposed strategy.