  This paper proposes a network decoupling method based on Holomorphic
Embedding (HE) for voltage stability analysis. Using the proposed HE method
with a physical load scaling factor s, it develops a set of decoupled two-bus
circuit channels between a target bus and the swing bus. Accordingly, a
complex-valued virtual index, named {\sigma}(s), is introduced in each channel
to assess the voltage stability of prospective operating conditions with
ensured accuracy. Then the stability margin at each bus can be analyzed by
visualizing its respective {\sigma}(s) index on the {\sigma} plane with a
unified boundary of voltage collapse. Moreover, benefitted by the embedding
with physical loading, the trajectory of {\sigma}(s) for each bus with the
variation of operating conditions can be analytically given about the scaling
factor s. The effectiveness of proposed network decoupling method is verified
on the IEEE 14-bus power system.
