Hierarchical System Commutation Failure Prevention Method for UHVDC Based on Turn-Off Area and Commutation Current Area Criteria

In hierarchical UHVDC transmission systems, faults occurring in the inverter-side AC system can lead to simultaneous commutation failures in both high-end converters (HECs) and low-end converters (LECs). To address this, this study presents a commutation failure prevention method on the basis of dual criteria, which considers the voltage and current factors that contribute to commutation failures. The proposed strategy dynamically adjusts turn-off angle settings according to variations in converter bus voltage. Simultaneously, it determines the correction for the thyristor trigger angle based on DC current variations. The approach optimizes the trigger angle selection for each converter layer, effectively preventing simultaneous commutation failures in both HEC and LEC. A simulation model developed in PSCAD/EMTDC was used to validate the proposed prevention strategy. The simulation results demonstrate that the strategy responds rapidly to faults in the inverter-side AC system, minimizing the risk of simultaneous commutation failures in both HEC and LEC. The proposed strategy enhances the overall stability of hierarchical access systems involving both HEC and LEC.