A new fast-acting backup protection strategy for embedded MVDC links in future distribution networks

Abstract

This paper presents a new fast-acting backup protection strategy for future hybrid ac-dc distribution networks. By examining the impedance measured by an ac-connected distance protection relay, a unique characteristic is established for faults occurring on the dc-side of an embedded medium-voltage dc (MVDC) link, interconnecting two 33 kV distribution network sections. Based on the identified impedance characteristic, appropriate settings are developed and deployed on a verified software model of a commercially available distance protection relay. To remain stable for ac-side faults, it is found that the tripping logic of the device must be altered to provide correct time grading between standard, ac, protection zones and the fast-acting dc region, which can identify faults on the dc system within 40 ms. An additional confirmatory check is also employed to reduce the likelihood of mal-operation. The proposed solution is trialled on a test system derived from an actual distribution network, which employs distance protection, and is shown to provide stable operation for both standard ac-side faults and dc-side pole-pole-ground and pole-pole events.