  To solve the AC optimal power flow problem, it is proposed in [1,2] that a
convex conic approximation to branch flow model (BFM) can be obtained if we
first eliminate phase angles of voltages and currents and then relax a set of
equality constraints to second order conic ones. In particular, provided a set
of sufficient conditions are satisfied, the conic relaxation is exact. We note,
however, that those conditions do not always guarantee the exactness. In this
letter, we analyze the argument of exactness and include a new condition that
there is no line with negative reactance to ensure the conic formulation's
exactness.
