A Multichoice Control Strategy for a VSC-HVdc

University dissertation from Stockholm : KTH

Abstract: Utilization of power electronics based controllable systems (or devices) in transmission systems has opened new opportunities for the power industry to optimize utilization of the existing transmission systems, and at the same time to keep high system reliability and security.As a member of these controllable systems, Voltage Source Converters-based High Voltage direct current (VSC-HVdc) systems have the ability to rapidly control the transmitted active power, and also to independently exchange reactive power with transmissions systems. Therefore, VSC-HVdcs with a suitable control scheme can offer an alternative means to enhance transient stability, to improve power oscillations damping, and to provide voltage support. An interesting application of this system is the analysis of a power system when a VSC-HVdc is connected in parallel with ac transmission lines.This thesis presents the derivation of control strategies to damp power oscillations, to enhance the transient stability and to provide voltage support for a VSC-HVdc. The thesis also formulates a multichoice control strategy and its application when the VSC-HVdc is connected in a synchronous system.The control strategy for enhancing transient stability is based on the theory of Control Lyapunov Function. The control strategy for increasing the damping is based on Linear Analysis. A very effective well known way to increase damping in the system is modulating the active power through the HVdc. However, besides the control of active power, the thesis explores an alternative way to mitigate power oscillations by controlling the reactive power. This condition might be very useful when the dc link in the VSC-HVdc system is out of service, but the converter stations are in operating conditions.A simple model of VSC-HVdc is considered in order to test the control strategy. The model represents the VSC-HVdc as an element in the power system that provides adequate interaction with other systems elements. The model is intended for analysis of power flows and electromechanical transients. It is then sufficient to consider the power frequency components of voltages and currents represented by phasors that vary with time during transients. The model is valid for symmetrical conditions, i.e. positive sequence phasors are used for the representation of the electrical state.

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