Development of MMC-HVDC grids, as a new generation of VSC-HVDC systems, has been considerable through the past decade. Emerging of multi-terminal MMC-HVDC networks makes integration of multiple large-scale sustainable sources and asynchronous power grids quite feasible. However, protection and control of the multiterminal HVDC grids under fault situations have always been a vital issue. On the other hand, while modern AC grids get benefits of applied Fault-Ride-Through (FRT) operation and capabilities under AC fault conditions, the multi-terminal HVDC grids lack a systematic DC FRT operation. As the multi-terminal HVDC networks are going to become a backbone grid for the future power systems, it is necessary to define grid code requirements and standardizations considering DC FRT regulations. This paper presents potential DC FRT operations and possible profiles from HVDC grid point of view under DC fault conditions. A systematic DC FRT based on voltage against time profile is proposed. Different characteristics of voltage-based DC FRT are investigated in this study and results can be applicable to DC grid code definitions and requirements.
With respect to rapid growth of offshore wind power, multi-terminal HVDC system is going to be an attractive way of the power transmission for remote and large offshore wind farms. Since the HVDC cables are the vital component of the multiterminal HVDC transmission system, an accurate and fast fault detection method is necessary for protection and maintaining uninterrupted power delivery to the main grid. This study presents a novel method to diagnose HVDC cable faults and unbalancing of DC capacitor bank in multi-terminal voltage source converter HVDC systems for offshore wind farms. Transient voltage of cable sheath is investigated as a main parameter for detecting types of cable faults. Extensive simulations are performed on EMTDC-PSCAD platform and the results show the effectiveness of the proposed technique.
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