A New Technology for Ac-Side Cascaded H-Bridge Cells for Multi-Level VSC-HVDC Transmission System

The VSC based HVDC transmission system offers the operational flexibility in terms of active and reactive power control, in addition to improved AC fault ride-through capability. The proposed converter offers a unique feature of DC fault blocking capability (ability to block power exchange between the AC and DC sides during the dc faults, hence no current flows in converter switches). This feature may eliminate the need for a DC-side circuit breaker in applications such as DC power transmission systems. This converter also offers the features available from the modular multilevel converter, such as AC fault ride-through capability, low conversion losses, low harmonic filtering requirement, and extension to a high number of levels without compromising capacitor voltage balancing.

To illustrate the feasibility of the proposed HVDC system, this project assesses its dynamic performance during steady-state and network alterations, including its response to AC and DC side faults. Long extra high voltage (EHV) AC lines cannot be loaded to their thermal limits in order to keep sufficient margin against transient instability. No alterations of conductors, insulator strings, and towers of the original line are needed.

Index Terms—DC fault reverse blocking capability, hybrid multilevel converter with ac side cascaded H-bride cells, modular multilevel converter, voltage-source-converter high-voltage dc (VSCHVDC) transmission system.