With the increasing penetration of renewable generation in power systems and the electrification of heat and transport, LV distribution networks are under pressure to host a growing number of low carbon technologies. Alternative Low Voltage Direct Current(LVDC) distribution networks have been considered as an effective approach to release the pressure on existing LV distribution networks. Meanwhile, the fast development of power electronics and increasing LVDC applications facilitate the transition to LVDC distribution networks. However, DC fault protection and location have been identified as key technical challenges by a number of research works and industrial groups. This thesis works toward the development of a fast and selective protection scheme and a reliable and accurate fault location technique that are to ensure secure and reliable LVDC operations, thereby facilitating the transition towards the widespread implementation of LVDC distribution networks. As LVDC protection solutions are influenced by AC/DC interface converters, existing protection solutions are mainly based on conventional two-level voltage source converters. Recently, new converter technologies such as solid-state transformers (SST) have been implemented, their unique fault characteristics introducing new requirements in understanding the impact of new converters. This is presenting the need for new recommendations and guidelines, and for developing new protection solutions. Therefore, this thesis presents an approach for fault characterisation and protection evaluation of future LVDC distribution networks. The SST is selected in this thesis to conduct case studies due to its unique fault characteristics, enhanced fault current control capabilities, great potential to replace existing LV transformers, and additional ancillary services. The derived understanding of the impact of SST on DC fault response and the effectiveness of existing protection solutions gives recommendations for future LVDC protection design. These help the development of a novel voltage-based protection scheme that uses the combination of sign of current derivative, voltage magnitude, and voltage concavity. The proposed protection scheme only relies on local measurements without any communications. The enhanced protection selectivity and the fast protection speed have been proven by simulation studies. The improved performance of the proposed protection scheme compared to existing methods allows LVDC distribution networks to have faster power restoration during fault events thus enabling resilient LVDC operations. In addition, a fault let-through energy (FLTE) based fault location technique is developed that uses FLTE in conjunction with a 'critical point' concept that is based on the capacitor ratio of local and remote converters. This achieves improved accuracy and reliability over the most of the proposed fault distance estimation techniques during the fault transient period. The enhanced performance of the proposed fault location technique has been verified based on simulation studies. This improved accuracy and reliability allows DC faults to be accurately located and facilitates rapid network reconfiguration and post fault cable maintenance.
|Date of Award||4 Dec 2020|
- University Of Strathclyde
|Supervisor||Abdullah Emhemed (Supervisor) & Graeme Burt (Supervisor)|