Distribution-level power electronics : soft open-points

Abstract

This thesis considers the use of medium-voltage power electronic compensation at distribution network voltage levels (11kV) for the improvement of power quality, reliability, and to accommodate growth in customer demand or distributed generation capacity. Specifically, power electronic compensators connecting two or more nodes of previously isolated radial feeders are considered. This type of device can be considered as an alternative to normally-open points, which connect two nodes with mechanical switchgear. Rather than connecting these nodes directly, power-electronics are placed between them. This type of device will be deemed a soft-open point (SOP) in this thesis. Several compensator topologies which can achieve the functionality of a SOP are considered. The feature criteria used to choose which compensators are suitable for use as a SOP are: the ability to transfer active power between feeders; the ability to resupply (post-fault) adjacent feeders connected via the compensator; an inherent or controlled disturbance rejection or fault current limiting between adjacent feeders. Modified versions of some existing flexible AC transmission system (FACTS) or custom power devices o er the potential to meet these criteria. The compensator topologies considered include: static synchronous series compensators, unified power flow controllers, back-to-back connected voltage-sourced converters (VSCs) or multi-terminal connected VSCs. In order to quantify and compare the benefits of these compensator topologies, their relative performance on UK distribution networks is assessed based on load flow and optimal power flow case studies performed on datasets representing several hundred UK distribution networks. Benefits quantified include an increase in customer reliability ratings, prevention or deferral of asset replacement, reduction in conductor losses, accommodation of increased distributed generation, and accommodation of increased customer demand. The benefit analyses show that multi-terminal VSC based SOPs provide the greatest flexibility, but one must recognize that associated cost and right-of-way issues associated with distribution networks can be prohibitive. Series and series-shunt compensators are shown to offer an an adequate amount of control, achieving reasonable levels of load and generation growth with lower overall estimates for cost. Several control strategies and converter topologies are considered for use in SOP implementation under a number of scenarios. The use of multi-terminal VSCs is also verified through implementation in a prototype network and through time-domain simulations. These demonstrations serve as a proof of concept for SOP operation in scenarios relevant to their intended use in distribution networks. Also considered is the use of SOPs to directly compensate overload substation transformers, for which it is found that SOPs can very effectively mitigate overload events at the expense of increased cumulative losses. Different high-level control schemes are shown to reduce the impact of the additional converter losses.