  Long-term voltage stability assessment (VSA) of independent transmission
(T-VSA) and distribution (D-VSA)systems have been studied since long to
estimate load margins. However, their impacts on each other have been neglected
due to simplified assumptions i.e. in transmission systems, loads are assumed
to be aggregated, and in distribution systems, substation bus voltage is
assumed to be constant. This work investigates the VSA of integrated
transmission-distribution (TD-VSA) using PV curve superimposition approach and
reveals the possibility that the overall system loadability may be limited by
the distribution system rather than the transmission system. In this paper, we
analyze why T-VSA and D-VSA are not reliable enough to estimate true load
margin of the overall system. The analysis has been verified on an integrated
test system in different scenarios with and without DER penetration. Overall,
the paper points out the need for integrated analysis and further builds a case
why it is essential to develop a realistic co-simulation framework for a
reliable long-term VSA of large-scale coupled T-D systems.
