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Lookup NU author(s): Professor Vladimir TerzijaORCiD
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Historically, distribution alternating current (AC) networks were designed to operate in a predictable manner, making monitoring, protection, and control relatively straightforward. Radial AC feeders, a dominant source at the substation, and healthy/nominal fault currents gave protection engineers a solid starting point for protection settings and coordination. Time–current curves, fuse-recloser coordination, and a few well-placed directional elements were usually enough to ensure selective, secure, and reliable protection. That world is disappearing. Across the globe, distribution grids are being rewired with power-electronics-based components, including direct current (DC) links, electric vehicle (EV) fast chargers, data centers, and rooftop photovoltaic (PV) and battery systems, all competing for space on networks that were never designed for them. Hybrid AC–DC distribution networks play a key role in this transformation. DC feeders now support EV charging and building microgrids, while low-voltage DC (LVDC) schemes enhance efficiency. Medium-voltage DC (MVDC) branches provide power for large industrial and information and communication technology loads. These DC components are increasingly connected to existing AC networks via a dense layer of converters. These converters change the nature of fault current distribution, waveforms, and magnitudes. Short-circuit currents can be tightly limited; can last only a few milliseconds; and may not appear “large” compared with the normal load, i.e., the nominal current. Power can flow in multiple directions at once. An event on a DC branch can ripple back into the AC system via complex converter controls. Under these conditions, relying on traditional overcurrent-based protection coordination and fuse-recloser philosophies becomes dangerously optimistic. This article examines what protection coordination entails when the distribution network transitions to a hybrid AC–DC system and how this change impacts the principles that have guided protection engineers for generations. We first review the fundamentals of protection coordination in classical AC distribution networks, then explore the impact of hybrid architectures on selectivity requirements. Building on this, we present emerging concepts for zone- and time-based coordination across AC and DC segments, emphasize the crucial role of interface protection at converters, and illustrate these ideas with a stylized urban feeder example. Our goal is to demonstrate how the existing practices must be adapted: from a world where fault current alone determined where to trip, to one where coordination must be deliberately designed across new timescales, understanding how fault currents behave and how new technology can support protection coordination of future distribution networks.
Author(s): Terzija V, Li B, Zhou B, Liu H, Shao M, Dong X
Publication type: Article
Publication status: Published
Journal: IEEE Power and Energy Magazine
Year: 2026
Volume: 24
Issue: 3
Pages: 78-93
Print publication date: 01/05/2026
Online publication date: 20/04/2026
Acceptance date: 02/04/2018
ISSN (print): 1540-7977
ISSN (electronic): 1558-4216
Publisher: IEEE
URL: https://doi.org/10.1109/MPE.2026.3665849
DOI: 10.1109/MPE.2026.3665849
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