Control-Induced Time-Scale Separation for Multiterminal High-Voltage Direct Current Systems Using Droop Control

This paper introduces a control induced time-scale separation scheme for a multiterminal high-voltage direct current system used for large scale integration of renewable energy sources. The main idea is to provide a detailed theoretical analysis to the long stand practice that consists of the empirical design of two control loops for the terminals. Experience has shown that such loops, i.e., current and voltage control loops, when heuristically tuned, often display very different dynamics. In this paper, singular perturbation theory is applied to give explanation and fundamental analysis on why and how the two control loops work and how to achieve the timescale separation between various state variables. Mathematical analysis is also carried out to illustrate a clear tradeoff between system performance (actuator constraint) and the size of the region of attraction of the controller. Numerical simulations for a system with four terminals are presented to evaluate the system performance and illustrate the theoretical analysis.