Small-signal Dynamic Stability Enhancement Of A DC-segmented AC Power System

This thesis proposes a control strategy for small-signal dynamic stability enhancement of a DC-segmented AC power system. This control strategy provides four control schemes based on HVDC supplementary control or modification of the operational condition of the HVDC control system to improve the system stability by (i) damping the oscillations within a segment using supplementary current control of a line-commutated HVDC link, based on the model predictive control (MPC) method (control scheme 1), (ii) minimizing the propagation of dynamics among the segments based on a coordinated linear quadratic Gaussian (LQG)-based supplementary control (control scheme 2), (iii) selectively distributing the oscillations among the segments based on a coordinated LQG-based supplementary control (control scheme 3) and (iv) changing the set-points of the HVDC control system in the direction determined based on the sensitivities of the Hopf stability margin to the HVDC links set-points (control scheme 4). Depending on the system characteristics, one or more of the proposed control schemes may be effective for mitigating the system oscillations. Study results show that (i) control scheme 1 leads to damped low-frequency oscillations and provides fast recovery times after faults, (ii) under control scheme 2, each segment in a DC-segmented system can experience major disturbances without causing adjacent segments to experience the disturbances with the same degree of severity, (iii) control scheme 3 enables the controlled propagation of the oscillations among segments and damps out the oscillatory dynamics in the faulted segment, and (iv) control scheme 4 improves the stability margin for Hopf bifurcations caused by various events. Since power system software tools exhibit limitations for advanced control design, this thesis also presents a methodology based on MATLAB/Simulink software to (i) systematically construct the nonlinear differential-algebraic model of an AC-DC system, and (ii) automatically extract a linearized state space model of the system for the design of the proposed control schemes. The nonlinear model also serves as a platform for the time-domain simulation of power system dynamics. The accuracy of the MATLAB/Simulink-based AC-DC power system model and time-domain simulation platform is validated by comparison against PSS/E.Ph