Protection strategy in active DC power distribution networks

  • Chunpeng Li

Student thesis: Doctoral Thesis


Environmental incentives to combat climate change are providing the motivation to improvethe energy efficiency of power distribution systems and integrate state-of-the-artrenewable technologies. DC distribution networks are receiving considerable attentionin the literature because they offer a simple and flexible interface between these modernresources and consumers. However, many technical challenges relating to the designand standardisation of DC protection devices still exist that must be overcome prior towidespread adoption. Since DC fault current develops rapidly, many high-speed protectionschemes tailored for DC networks have been proposed. However, few of themhave considered the difficulties in practical implementation. This thesis will presentthe implementation challenges and propose corresponding protection schemes to addressthe issues.In seeking to achieve this aim, the work presented within this thesis makes threemain contributions. This thesis has fi rstly improved the reliability of the high-speedDC differential protection scheme. The main implementation challenge of this schemeis that a short time synchronisation error may cause a signi ficant current differenceerror, resulting in a false-trip problem when a fault occurs outside the protected zone.This thesis has proposed a "multi-sample differential (MSD) protection scheme" to ensurethe protection stability for external zone faults (i.e., the relays must not operate)whilst maintaining sensitivity for internal zone faults (i.e., the relays must operate) byexamining multiples measurement samples. Secondly, the difficulty in realising high-speed DC distance protection is that measurementof rate-of-change of current can be severely affected by even low-level noise,resulting in a failure in fault detection. This thesis has presented the methodologyfor selecting the appropriate sampling time of the numerical derivative as well as thecut-off frequency of low-pass current measurement lfiters.Although high-speed protection schemes can effectively isolate faults quickly, theirimplementation requires many advanced devices, which may not be economical for lowpowerand low-cost DC networks. Finally, this thesis has proposed a "modulated lowfault-energy (MLE) protection scheme" that employs fault current limiters (FCL) atthe grid energy sources and mechanical circuit breakers (MCB) elsewhere throughoutthe distributed network. This deployment can constrain the fault current to a lowenergylevel that enables a longer time window for the downstream MCBs to realiseprotection with a lower total implementation cost.Drawing conclusions from this PhD research, the author advocates that more considerationshould be given to implementation challenges when designing protectionschemes in DC distribution networks. Excessive pursuit of ultrafast fault isolationspeeds can lead to over-cost and protection instability issues in practice. A prospectiveprotection scheme must compromise between the high-speed protection requirementsin theory and the reliable but economical requirements in practice, to accelerate therealisation of large-scale DC grids in future.
Date of Award21 May 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorPatrick Norman (Supervisor) & Pawel Niewczas (Supervisor)

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