We propose two approaches to formulate and convexify optimal power flow (OPF) problems in multiphase power distribution networks with delta connections. The first approach is based on SDP relaxation of both branch flow variables and delta-configured nodal voltages and currents. The second approach is based on the conversion between delta-connected loads/energy resources to equivalent wye-connected power injections. This conversion is linear when three-phase voltages are balanced, which approximately holds in practice such that an OPF with delta connections can be approximately converted to an OPF with wye connections only. We evaluate performance of the two proposed approaches through numerical experiments on IEEE distribution test feeders.