An adaptive time-graded control method for VSC-MTDC networks

Abstract

Power flow and voltage control, beside other challenges such as DC faults, are the main concerns in the operation of multi-terminal HVDC (MTDC) networks. This paper proposes a novel DC voltage control method, which is inspired from the principles of power system protection, for VSC-based MTDC networks. This time-graded control method periodically ranks all converters; the highest priority one is dedicated to regulate the network voltage while the others operate in the power control mode. In abnormal conditions in which the grid-forming converter cannot solely control the network voltage, a sufficient number of converters are sequentially turned to the voltage control mode according to their priority. Upon recovering from the abnormal condition, the proposed controller with its conditional reclosing ability returns these mode-changed converters to the power control mode. Since the proposed method regulates the voltage of MTDC network by the minimum number of converters, the maximum ones can be employed in power flow control. Proposing an adaptive procedure to calculate appropriate mode change delays is another contribution of this communication-less control method. Simulation of a test network in MATLAB/Simulink under various conditions with both proposed and conventional droop controllers verifies the outstanding abilities of the proposed method.