  This two-part paper details a theory of solvability for the power flow
equations in lossless power networks. In Part I, we derive a new formulation of
the lossless power flow equations, which we term the fixed-point power flow.
The model is stated for both meshed and radial networks, and is parameterized
by several graph-theoretic matrices -- the power network stiffness matrices --
which quantify the internal coupling strength of the network. The model leads
immediately to an explicit approximation of the high-voltage power flow
solution. For standard test cases, we find that iterates of the fixed-point
power flow converge rapidly to the high-voltage power flow solution, with the
approximate solution yielding accurate predictions near base case loading. In
Part II, we leverage the fixed-point power flow to study power flow
solvability, and for radial networks we derive conditions guaranteeing the
existence and uniqueness of a high-voltage power flow solution. These
conditions (i) imply exponential convergence of the fixed-point power flow
iteration, and (ii) properly generalize the textbook two-bus system results.
