Reliable protection of modern distribution systems is challenging due to various technical difficulties, mainly related to the need for more efficient use of existing networks (e.g. shorter interruption durations and fewer affected customers, integration of distributed generation, meshed network operation etc.). Such challenges require, among others, the reconsideration of conventional protection techniques. A potential solution under examination concerns the adoption of distance protection for distribution feeders, due to its advantages compared to overcurrent protection. However, as distance relays are mainly designed for transmission networks, there are several issues to deal with in distribution applications, such as the proper setting of the zero-sequence compensation factor (K0). The latter issue is critical in order to guarantee correct relay operation during single-line-ground faults. This paper examines the effect of K0 on the operation accuracy of distance relays protecting inhomogeneous distribution feeders. Theoretical analysis, as well as investigation of various influencing factors, result in the determination of a pattern, which relates relay accuracy to K0. Based on this analysis, a simple methodology for setting K0 properly is proposed, which is implementable with commercially available relays. The methodology is applied on a test distribution feeder and meaningful conclusions are derived.

Future distribution systems are expected to display increased complexity, mainly due to looped/meshed operation, switch between grid-connected and islanded mode and considerable integration of distributed generation. This paper investigates a plug-and-play protection solution for overhead distribution systems with such variable operation conditions, employing existing numerical relay capabilities. This solution is applied by designing plug-and-play, communication-assisted, multifunctional relays with integrated protection element settings, which apply universally to all distribution system conditions, rendering the protection scheme independent of a specific system. Hence, the need for user-defined settings or future revisions due to system changes is eliminated. The scheme ensures coordination between main line relays and backup protection of laterals, without a coordination study. There is no need to replace or modify existing lateral protection means for this purpose; only their known timeovercurrent curves are uploaded to the relays by the user. The scheme is described and evaluated through simulations in two test systems. Meaningful conclusions are finally derived.

This paper introduces a current differential protection scheme, appropriate for application in medium voltage active distribution systems, where it is desired to keep the greatest possible number of loads and DG units energized during a fault. Conventional two-terminal percentage current differential relays are used to form successive, time-current-coordinated, differential protection zones. Multiple time-delayed differential elements in each protection zone guarantee coordination with the zones lateral protection devices, as well as between successive differential protection zones. Sensitive time-delayed differential elements protect against relatively high-resistance faults, while instantaneous differential elements minimize protection speed whenever possible. Additional emergency differential elements deal with post-fault topology changes and breaker failure conditions enhancing the overall schemes performance. The proposed scheme is applied to a model of real medium voltage distribution system with distributed generation, considering a ring topology operation. A detailed simulation-based study proves the applicability and enhanced performance of the proposed scheme.

This study applies distance protection to real mixed overhead‐underground distribution feeders, as an alternative to conventional overcurrent protection schemes. The efficiency of the proposed distance‐based protection scheme is examined both in the radial (normal state) and in the looped operation of the distribution feeders. Moreover, distributed generating units of various types are assumed connected to the feeders to investigate their effect on the efficiency of the protection scheme. The latter proved sufficient to coordinate properly with the other protection means in the system, following a fuse‐blowing philosophy, providing reliable protection to the lines.