Modified Newton-Raphson load flow analysis for integrated AC/DC power systems

The significant increase in planned offshore wind parks and the tendency towards large parks in considerable distances offshore, make the well established HVDC technology a favorable solution for the connection of these large & distant offshore wind parks to the main power grid onshore. It is therefore necessary to adequately model the HVDC transmission links and integrate them in the load flow analysis of the complete AC-DC system. In this paper, the well known Newton-Raphson method for the load flow analysis is modified to achieve compatibility for AC-DC systems with integrated DC links in the AC network. The elements of the residual vector and the Jacobian matrix for the AC network are kept unchanged and are merely complemented by a new vector and a new matrix, which represent the modifications due to the DC link. The modified Jacobian equation includes the DC real and reactive power at the AC-DC buses, and their dependency on the AC system variables. The modified Newton-Raphson method is evaluated on an AC-DC test system with a load flow computation in Matlab and the results are presented.