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.