General Classification and Comprehensive Performance Assessment of Multi-Objective DC Voltage Control in Multi-Terminal HVDC Networks

The recent massive global movement towards green energy in power systems has raised the efforts of integrating large-scale Renewable Energy Sources (RESs) through Multi-Terminal HVDC (MTDC) systems. The configuration of the MTDC system and the possibility of transnational interconnection impose some challenges and raise the potential of single or multi-objective control for the DC voltage control. Additional requirements from the Transmission System Operator (TSO) and/or AC grids may influence the action taken for the DC voltage control. In this paper, a generalized classification for the DC voltage control methods in an MTDC system is delivered. The DC voltage control methods are classified into conventional control (i.e., reference voltage-based control) and non-conventional control (i.e., virtual resistance-based control) methods. The DC voltage control objective may cover a range of the following targets: power-sharing based on converters' rating capacity, ratio priority of the power distribution, available headroom, and/or loading factor. The control objective may include transmission losses minimization of the MTDC system with optimal or sub-optimal power flow. The design approaches of the control methods for post-contingency operation are presented. The control methods are evaluated and simulated with a 4-terminal radial MTDC network during normal and abnormal system operation. A comprehensive performance assessment is also presented considering the control methods from the perspective of the control method and objective, system efficiency, grid-code violation, communication requirement, and design complexity and flexibility.