DC power network post-fault recharging with an H-bridge cascaded multilevel converter

This paper proposes an algorithm for controlled recharge (from the ac side) of the dc link capacitors of a H-bridge cascaded multilevel converter, while ensuring that the voltages across H-bridge cell capacitors remain balanced and tightly within their operating limits. The proposed recharging scheme potentially eliminates any external circuitry needed to start and shutdown the dc grid, and restart following a dc fault. Therefore, it may facilitate HVDC network fast recovery from the DC side faults, with minimal impact on the ac side and risk of power converter failure due to current surge or voltage sag. In addition, this paper presents mathematical analysis that can be used in the development and understanding of DC link capacitor charging and its associated effects. Simulations from detailed switch and average models of the H-bridge cascaded converter are used to validate the proposed charging and discharging algorithm.