Inverter controller for HVDC systems connected to weak AC systems

The paper presents a new control strategy for the inverter side of an HVDC power system. The new control algorithm is intended for stability improvement of an HVDC system connected to a very weak receiving AC system. A root-locus analysis based on a detailed analytical system model determines the eigenvalues which are mostly affected by the reduction in strength of the inverter AC system. The feedback signal for the new controller is selected by considering possible movement of eigenvalues of importance. The controller is designed as a second-order filter placed in the feedback loop. Since the robustness of the system with respect to the AC system SCR changes is the main controller aim, the H_∞ control theory is used for the controller design. Two cases have been studied: the design of a controller to improve the stability of an HVDC system connected to an AC system susceptible to large variations in its strength; and the design of a controller which enables an HVDC system connection to a very weak AC system (SCR=1.0 is considered). The PSCAD/EMTDC simulation results confirm that the controller meets the design objectives