The efficacy of long-distance and bulk power transmission largely depends on the efficient control and reliable operation of a multi-terminal high voltage direct current (MT-HVDC) grid, more precisely, a meshed HVDC grid. The capability of enduring the DC grid fault eventually enhances the reliability and improves the dynamic performance of the grid. This paper investigates the operation and control of an AC/MTDC system with bipolar topology incorporating the DC grid protection schemes. Based on the scale of a circuit breaker's operating time, the performance of three different protection strategies is compared and analyzed using DIgSILENT PowerFactory. Simulation results explicitly reveal that the dynamic performance of the MTDC grid significantly deteriorates with the slow functioning of the protection schemes, followed by a DC grid fault. Besides, prolonged recovery time causes a substantial loss of power infeed and affects the AC/DC grid's stability. Finally, to assess the frailty of the MTDC grid, a transient energy stability index (TESI) is proposed considering the voltage variation in the pre-state and post-state fault clearing interval. Relevant case studies are performed on the MTDC grid using an analytical approach and non-linear simulation studies to validate the effectiveness of the proposed index.
With the increasing number of renewable generations, the prospects of long-distance bulk power transmission impels the expansion of point-to-point High Voltage Direct Current (HVDC) grid to an emerging Multi-terminal high-voltage Direct Current (MTDC) grid. The DC grid protection with faster selectivity enhances the operational continuity of the MTDC grid. Based on the reactor voltage gradient (RVG), this paper proposes a fast and reliable fault identification technique with precise discrimination of internal and external DC faults. Considering the voltage developed across the modular multilevel converter (MMC) reactor and DC terminal reactor, the RVG is formulated to characterise an internal and external DC fault. With a window of four RVG samples, the fault is detected and discriminated by the proposed main protection scheme amidst a period of five sampling intervals. Depending on the reactor current increment, a backup protection scheme is also proposed to enhance the protection reliability. The performance of the proposed scheme is validated in a four-terminal MTDC grid. The results under meaningful fault events show that the proposed scheme is capable to identify the DC fault within millisecond. Moreover, the evaluation of the protection sensitivity and robustness reveals that the proposed scheme is highly selective for a wide range of fault resistances and locations, higher sampling frequencies, and irrelevant transient events. Furthermore, the comparison results exhibit that the proposed RVG method improves the discrimination performance of the protection scheme and thereby, proves to be a better choice for future DC fault identification.
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