  The branch flow based optimal power flow(OPF) problem in radianlly operated
distribution networks can be exactly relazed to a second order cone programming
(SOCP) model without considering transformers. However, the introdution of
nonlinear transformer models will make the OPF model non-convex. This paper
presents an exact linearized transformer's OLTC model to keep the OPF model
convex via binary expanstion scheme and big-M method. Validity of the proposed
method is verified using IEEE 33-bus test system.
