title: Perturbation Observer based Adaptive Passive Control and Applications for VSC-HVDC Systems and FACTS Devices
Creators: name: Yang, Bo
Publisher: name: University of Liverpool Repository
Year: 2016

description: The technology of voltage source converter based high voltage direct current (VSC-HVDC) system and devices used in flexible AC transmission systems (FACTS) has evolved significantly over the past two decades. It is used to effectively enhance power system stability. One of the important issues is how to design an applicable nonlinear adaptive controller for these devices to effectively handle the system nonlinearities and uncertainties. Passive control (PC) has been proposed for the control of nonlinear systems based on Lyapunov theory, which has the potential to improve the system damping as the beneficial system nonlinearities are remained instead of being fully cancelled. However, PC is not applicable in practice as it requires an accurate system model. Adaptive passive control (APC) and robust passive control (RPC) have been developed to handle some specific type of system uncertainties based on strict assumptions on system structure and uncertainty. However, their applications are limited as various system uncertainties exist. This thesis aims to develop a perturbation observer based adaptive passive control (POAPC) to make PC applicable in practice. The combinatorial effect of system nonlinearities, parameter uncertainties, unmodelled dynamics and time-varying external disturbances is aggregated into a perturbation, which is estimated by a perturbation observer (PO). The proposed approach does not require an accurate system model and can handle various system uncertainties. POAPC is applied to two-terminal VSC-HVDC systems to handle various system uncertainties. The VSC-HVDC system model is firstly developed, the proposed controller can inject an extra system damping and only the measurement of direct current (DC) voltage, active and reactive power is needed. The effectiveness ofPOAPC is verified by simulation in comparison with that of passive control (PC) and proportional-integral (PI) control. Moreover, a hardware experiment is carried out to verify its implementation feasibility and applicability. A passive controller is designed for multi-terminal VSC-HVDC (VSC-MTDC) systems via energy shaping, in which the dynamics related to the active power, reactive power, and DC cable voltage is transformed into an output strictly passive form. Then the remained internal dynamics related to DC cable current and common DC voltage is proved to be asymptotically stable in the context of Lyapunov criterion. PC is applied on a four-terminal VSC-MTDC system under eight cases to evaluate its control performance. POAPC is developed on the VSC-MTDC system to maintain a consistent control performance under different operating points and provide a significant robustness to parameter uncertainties, together with other unmodelled dynamics and time-varying external disturbances. Simulation results are provided to evaluate the control performance of POAPC in comparison to that of PI control and PC. Perturbation observer based coordinated adaptive passive control (POCAPC) is proposed for excitation controller (EC) and FACTS controller on both single machine infinite bus (SMIB) systems and multi-machine power systems. Only the range of control Lyapunov function (CLF) is needed and the dependence of an accurate system model can be partially reduced, thus POCAPC can be easily applied to multi-machine power systems. Its control performance is compared with that of conventional proportional-integral-derivative and lead-lag (PID+LL) control, coordinated passive control (CPC) and coordinated adaptive passive control (CAPC) on both an SMIB system and a three-machine power system by simulation. Then a hardware-in-the-loop (HIL) test is undertaken to verify the implementation feasibility of the proposed controller.