Nonlinear model predictive controller design for rapid earth fault current limiters in compensated distribution networks to mitigate powerline bushfires

Fernando, Warnakulasuriya Sonal Prashenajith (2024) Nonlinear model predictive controller design for rapid earth fault current limiters in compensated distribution networks to mitigate powerline bushfires. Doctoral thesis, Northumbria University.

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Abstract

Powerline bushfires are mainly caused by very high fault currents due to single lineto-ground (SLG) faults and the resonant grounding (RG) technique is utilized to reduce these fault currents to a level that fire risks are mitigated. The fault current in a power distribution network comprises both active and reactive components due to the presence of the leakage resistance and capacitance. Since the RG techniques uses an adjustable inductance, it can compensate only the reactive component of the fault current though it is always difficult to achieve the full compensation. Hence, the uncompensated fault current comprises both reactive and active components (commonly known as the residual current) that can still be significant enough to ignite bushfires. Moreover, the faulty phase voltage. As a result, the residual current compensation (RCC) inverter is required to inject current into the neutral so that the desired fault compensation (i.e., fault current and faulty phase voltage compensation) is achieved. The fault compensation level in compensated distribution networks relies on the control mechanism used for the RCC inverter and this thesis aims to design new nonlinear model predictive controllers to achieve the desired fault compensation.

A nonlinear model predictive controller (NMPC) is designed for the RCC inverter using the first-order model to achieve the desired fault compensation, which is sensitive to system parameter variations and external disturbances. The parameter sensitivity problem of this newly designed NMPC is resolved incorporating a parameter adaptation scheme with the NMPC, i.e., using an adaptive NMPC (ANMPC). A nonlinear extended state observer (NLESO) is proposed to ensure the robustness of the NMPC against external disturbances (i.e., using an NLESO-based
NMPC). Finally, a new robust NMPC is designed by combining the parameter adaptation and disturbance estimation schemes used in the ANMPC and NLESO-based NMPC, respectively to ensure the robustness against both parameter variations and external disturbances.

A new second-order model of the RCC inverter is developed to incorporate distribution network parameters (mainly, the leakage parameters) that directly affect the fault compensation behaviors. For this model, the desired fault compensation is achieved by proposing a new backstepping NMPC (BS-NMPC) offering two-degree freedom, while requiring precise parametric information of the system. The newly proposed BS-NMPC is further enhanced utilizing a parameter adaptation scheme for improving its robustness against variation in system parameters as the proposed adaptation scheme can estimate all parameters (including leakage parameters) appearing in the system model.

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