Autonomous local control and stability of multi-terminal HVDC systems

A new framework is presented for local control design and system stability analysis of high-voltage dc (HVDC) power systems having multiple sending and/or receiving terminals. The terminals are classified as voltage-source (VS) or current-source (CS) terminals. A VS terminal has a large capacitor in parallel with its dc interconnect ports and appears as a voltage source to the dc network, while a CS terminal has a large inductor in series with its dc ports and appears as a current source to the dc network. A local controller is used to regulate 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 using impedance models are also discussed. An offshore wind farm with series-dc collection and HVDC transmission is used as example to demonstrate the proposed methods.