Title
A Reliable Protection Scheme for Fast DC Fault Clearance in a VSC-Based Meshed MTDC Grid
Abstract
A multi-terminal high voltage DC (MTDC) Grid is the optimal solution to minimize the global energy crisis to a great extent. It is a cost-effective transmission system to transmit a bulk amount of sustainable electrical power over long distances with overall lower transmission losses and investments saving land and money. A VSC-based Modular-Multilevel-Converter HVDC/MTDC (VSC-MMCHVDC/MTDC) technology with a key benefit of constant voltage polarity offers considerable benefits and various attractive features to fulfill the basic requirements of the future Super-Grid. However, the protection of a DC system is more challenging and difficult than its counterpart AC system’s protection. In a DC system the fault current reaches to a huge value within a few milliseconds. Major hurdles which prevent the integration and development of the HVDC systems include absence of naturally zero-current points (absence of frequency), minimum impedance, and the lack of high rating DC circuit breakers (DCCBs). Hence, extreme vulnerability of a VSC-HVDC/MTDC system to the DC faults, particularly a solid DC line/cable short circuit fault is a major threat to its operation and development (scalability). Indeed, the core design demand for a feasible MTDC protection scheme capable of clearing the DC fault within a few milliseconds (5 msec. or less) of the critical time limit, has remained a key technical gap in both research and practice so far and needs to be addressed. The proposed scheme utilizes the joint performance of communication-based optical sensing schemes, independent sub-schemes, fast isolation tools, and simple backup into one scheme. In this work, firstly the 3-level, bi-polar half-bridge VSC-MMC-MTDC meshed grids of 3 and 4 terminals are validated in the MATLAB using Sims-Cape Power Systems. Afterwards DC cable P2P and P2G faults are analysed with appropriate simulation results and data. Particularly the DC cable P2P faults are analysed using varied fault distances, fault impedances, and fault locations within the grid. Finally, a comprehensive protection scheme is proposed for a meshed VSC-MTDC system. The scheme utilizes the joint performance of current differential and TW methods based on distributed current measuring units (assumed optical sensor networks), discrete-wavelet-transform (DWT), current derivative data, overcurrent relays, active and passive FCLs, bidirectional Hybrid DCCBs (HDCCBs), half-bridge VSC-MMCs, ACCBs, and other simple backup plans. All the important aspects of the total DC fault clearance time are explored both theoretically and with appropriate simulation results. These important aspects include accurate faulty line/segment determination, quick real-time fault detection, relatively accurate fault location, significant fault current limiting, and fully selective isolation of only the faulty line from the system without shutting down the entire grid. Through this effective fault coordination, the DC fault current is significantly reduced to much below 1.7 kA and the fault clearance time achieved is up to 5.2 ms. The scheme is capable to fulfil all the general requirements of a feasible MTDC protection scheme such as comprehensive, robust, novel, fully selective, cost-effective, reliable, and scalable. Technical feasibility of the proposed concepts can be verified experimentally using the extensive set of simulation results obtained. Further, the scheme is not only applicable to the target meshed VSC-MTDC grid, but its general methodology can be implemented to any meshed MTDC grid with any number of terminals