AC-DC optimal power flow implementation: Modeling and application to an HVDC overlay grid

Planning and installation of large onshore and offshore wind farms call for an increase in transmission capacity to enable the transport of the generated renewable power toward the load centers. As power is transported over long distances, HVDC overlay grids become of growing interest. Integrating HVDC overlay grids into AC transmission systems requires a profound understanding of the influence of HVDC converter operating set-points on the AC power flow. The HVDC converter set-points are determined by an integrated AC-DC optimal power flow calculation. In this paper, the implementation steps for the integrated AC-DC optimal power flow calculation are identified and discussed. Special focus is laid on the effect of the converter loss model on the Jacobian matrix as part of the Newton-Raphson method. The optimal power flow calculation is applied to a case study characterized by a high share of wind power and a significant need for power transmission. Results show the integrated operation of the AC-DC grid with optimal HVDC converter set-points.