Comparisons of MVAC and MVDC Systems in Dynamic Operation, Fault Protection and Post-Fault Restoration

Abstract

One of the most significant obstacles preventing the large-scale application of direct-current (DC) technology in medium voltage (MV) distribution networks is their fault protection. The existing AC relay protection needs to be changed or redesigned to protect the future overlay MVAC and MVDC distribution networks. Therefore, a comprehensive understanding of the dynamic and fault behavior and post-fault restoration strategies of MVAC and MVDC systems are critically important. Moreover, a comparison of MVAC and MVDC systems during a fault will also contribute to designing the protection systems of hybrid MV AC/DC systems. In this paper, the challenges of protecting DC faults of MVDC systems and possible solutions are first introduced. Then, the fault characteristics and post-fault restoration of MVDC and MVAC distribution systems are compared and investigated through case studies. Time-domain simulations have been conducted in PSCAD/EMTDC. The work in this paper will be valuable for the protection design for future hybrid MV AC/DC systems.

Identifying Opportunities for Medium Voltage DC Systems in Australia

“…In addition to CBs and isolators, surge arrestors and fuses are also important protection components in MVDC systems for providing overvoltage and short-circuit protection [8]. It should be mentioned that MVDC systems have shorter line fault detection, location, isolation and system restoration time compared to MVAC systems, although dc fault currents should be reduced by proper schemes [35].…”

Reliability and Cost-Oriented Analysis, Comparison and Selection of Multi-Level MVdc Converters

“…Therefore, this is a rigid bipolar system without a monopolar operation mode. The required number of components and dclink capacitance of each cell can be calculated using (1)- (3). The hierarchical structure of the RBD of the C3L-NPC is shown in Fig.…”

“…Potential applications of voltage source converter (VSC) based MVdc range from the integration of renewable energy sources [2], [3], traction and shipboard power systems [4], [5], smart distribution systems [6] and future offshore dc collection grids [7], [8]. In particular, the use of VSC technologies at MVdc voltage level is beneficial in terms of their applicability in weak rural and complex urban distribution networks.…”

Offline Reconfigurability Based Substation Converter Sizing for Hybrid AC–DC Distribution Links

“…Since the link conductor reconfigurations are carried out offline using the ac and dc feeder switches once the relevant circuit breakers have isolated the faulty component, the dynamics of the system during the fault is not studied in this paper. Some considerations on dynamic operation during fault protection of components in such systems can be found in [22], [23]. Post-reconfiguration, the healthy MMCs are expected to run in grid connected mode as back-to-back VSC based pointto-point dc link in parallel with the ac link, as shown in [24].…”

“…A possible design is suggested by introducing modularity such that every two dc conductors operate as an independent point-to-point symmetrical monopolar link and the fault is isolated in each with ac-side CB. Emphasizing the importance of proper protection design in future hybrid ac/dc medium voltage links, a recent study [30] compares three possible solutions (i) ac circuit breakers (ii) dc circuit breakers (iii) fault blocking converters with a focus on dynamic operation, fault protection and post-fault restoration. While further research on different protection schemes can potentially influence the magnitude and duration of load shedding during (n-1) contingencies, this aspect is not the focus of the current paper.…”