This paper introduces a control induced timescale separation scheme for a multi-terminal 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 empirical design of two control loops for the terminals. Experience has shown that such loops, i.e. current and voltage control loop, when heuristically tuned, often display very different dynamics. In the present 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 trade-off 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.
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