Autonomous Local Control and Stability Analysis of Multiterminal DC Systems

A new approach to local control and system stability analysis of dc power systems having multiple sending and/or receiving terminals is presented. The terminals are classified as voltage-source (VS) and current-source (CS) terminals. A VS terminal has a filter capacitor in parallel with its dc interconnect ports and appears as a VS to the dc network, while a CS terminal has a filter inductor in series with its dc ports and appears as a CS to the dc network. A local controller regulates the current behind the capacitor in a VS terminal and the voltage behind the inductor in a CS terminal. Such local controls are designed based on local operation objectives and stability requirements of individual terminals. The capacitor voltages at VS terminals and inductor currents at CS terminals are not directly controlled, but are determined by the network topology and system dynamics. Supervisory system controls may be incorporated to influence VS terminal voltage and CS terminal current responses by changing local control set points. Different local control methods and their effects on system stability are presented for various network topologies. Small-signal methods to assess system stability are also discussed. An offshore wind farm with series-dc collection and high-voltage dc transmission is used as an example to demonstrate the proposed methods.