The impact of MVDC upon conventional distance protection schemes in hybrid ac-dc distribution networks

Abstract

This paper looks at the protection implications of introducing fully controllable, embedded, medium voltage dc (MVDC) power electronic links into power distribution networks. Studies have indicated, protection notwithstanding, that embedded MVDC can be used to provide economically attractive, enhanced capacity and control of power flows. Through a series of simulation studies, the impact on distance protection schemes (in terms of reach and response time) resulting from the introduction of a controlled MVDC link have been examined for symmetrical faults. The paper also considers under what conditions a dc side fault can be observed from the ac grid.

Hardware in the loop verification of a fast backup protection scheme for embedded MVDC links within distribution networks

“…This could include remote transformer energisation. Converter fault ride through characteristics for ac side faults has been shown to potentially effect clearance times of distance protection devices for both dc links and embedded generation [6].…”

“…The inductive nature of this fault is due to the significant converter filter inductance used to smooth the converter's output. This value is equal to the sum of the impedances through which the current flows to the fault location; including ac network, converter arm, transformer, filter and dc line impedances [6].…”

“…This is true not just for main protection, but also for backup schemes. Previous studies have considered what a dc side fault may look like to conventional ac protection [6]. The role of a modified ac side distance protection scheme to provide sufficiently fast acting backup protection for embedded MVDC links appears promising under a desktop simulation environment; however has been unproven on real hardware.…”

A New Fast-Acting Backup Protection Strategy for Embedded MVDC Links in Future Distribution Networks

“…It is assumed that the MVDC converter does not provide any fault current contribution for ac-side faults (i.e. switching of IGBTs is paused), although it is recognized that significant fault infeed from a converter could influence relays' impedance measurements for ac faults [39].…”