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With the penetration of distributed generators (DGs) into active distribution network (ADN), the voltage controlled buses (PV buses) are becoming more common, to maintain a normal voltage level in the distribution network. Moreover, there are many load modes in ADN which need to be taken into account. In this paper, the approximations of three-phase line model have been analyzed and the ZIP loads with star and delta connections were discussed in detail. And then based on the basic three-phase power flow equations, a general three-phase LPF solution under a polar coordinate system has been presented. The proposed method can account for various connections of ZIP loads, multi-phase feeders and single-phase or three-phase DGs. The effectiveness and advantages of the proposed method are validated with the IEEE 13-bus, 37-bus, 123-bus and an improved 615-bus unbalanced networks.