This paper presents an average-value model of a line commutated converter-based HVDC system using dynamic phasors. The model represents the low-frequency dynamics of the converter and its ac and dc systems, and has lower computational requirements than a conventional electromagnetic transient (EMT) switching model. The developed dynamic-phasor model is verified against an EMT model of the CIGRE HVDC Benchmark. Simulation results confirm the validity and accuracy of the average-value model in predicting the low-frequency dynamics of both the ac and dc side quantities. Merits and applicability limitations of the average model are highlighted.