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This paper develops and presents a unified and generic three-phase, steady-state, fundamental-frequency, sequence-frame-based model of the voltage-sourced converter (VSC) for power-flow analysis of VSC-interfaced Distributed Energy Resource (DER) units. The model is unified since it represents: 1) three-wire and four-wire VSC configurations; 2) balanced and unbalanced power-flow scenarios; 3) various VSC control strategies and options; and 4) operating limits and constraints of the VSC and its host DER unit. Based on the developed model, a new power-flow algorithm in the sequence-component frame is also developed. To achieve numerical and computational efficiency, the interface-VSC operating limits are accommodated in the power-flow algorithm as an interleaved step. The accuracy of the developed model and the computational efficiency of the power-flow algorithm are demonstrated based on several case studies, and where applicable, the results are validated based on comparison with the exact time-domain solution, using the PSCAD/EMTDC software tool.