  This paper extends the idea of the Thevenin equivalent into unbalanced
3{\phi} circuits and proposes a 3{\phi} long-term voltage stability indicator
(VSI), that can identify critical loads in a system. Furthermore, in order to
identify whether the voltage stability limit is due to the transmission network
or a distribution network, a transmission-distribution distinguishing index
(TDDI) is proposed. The novelty in the proposed indices is that they can
account for the unbalance in the lines and loads, enabling them to use
unbalanced phasor measurements naturally. This is supported by mathematical
derivations and numerical results. A convex optimization formulation to
estimate the 3{\phi} Thevenin equivalent using PMU & {\mu}PMU measurements is
proposed, making it possible to calculate VSI and TDDI in an online model-free
manner. Numerical simulations performed using co-simulation between Pypower and
GridlabD are presented for the IEEE 9 bus and the 30 bus transmission networks
combined with several modified IEEE 13 node and 37 node distribution networks.
These case studies validate the proposed 3{\phi}-VSI and TDDI over a wide range
of scenarios and demonstrate the importance of {\mu}PMU measurements in
identifying the regions causing long term voltage instability.
