published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Generation loss contingencies in electric power systems result in a deviation of system frequency from nominal, a condition which must be corrected promptly in order to prevent further degradation of the power system. Automatic load-shedding using underfrequency relays is one of the techniques used to correct abnormal frequency deviations and prevent the risk of uncontrolled outages. If sufficient load is shed following a contingency to preserve interconnections and keep generators on-line, the system can be restored with relative speed and ease. On the other hand, if a declining frequency condition is not dealt with adequately, a cascading disconnection of generating units may develop, leading to a possible total system blackout. / This thesis develops and tests a new systematic method for setting underfrequency relays offering a number of advantages over conventional methods. A discretized swing equation model is used to evaluate the system frequency following a contingency, and the operational logic of an underfrequency relay is modeled using mixed integer linear programming (MILP) techniques. The proposed approach computes relay settings with respect to a subset of all plausible contingencies for a given system. A method for selecting the subset of contingencies for inclusion in the MILP is presented. The goal of this thesis is to demonstrate that given certain types of degrees of freedom in the relay setting problem, it is possible to obtain a set of relay settings that limits damage or disconnection of generating units for each and every possible generation loss outage in a given system, while attempting to shed the least amount of load for each contingency.

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Flexible AC Transmission Systems (FACTS) involves the incorporation of power-electronic controlled devices into ac power transmission systems in order to extend the power-transfer capability of these systems beyond their traditionally accepted boundaries. One particular category of FACTS devices makes use of high-powered voltage source inverters to insert near-sinusoidal ac compensating voltages into the transmission system. This thesis considers this particular category of inverter-based FACTS devices, namely the static synchronous compensator (STATCOM), static synchronous series compensator (SSSC) and unified power flow controller (UPFC). Although the potential for FACTS devices to enhance the operation of power systems is well known, a device such as a UPFC is itself a complicated subsystem of the overall power system. There is therefore also the possibility that the introduction of such devices could cause adverse interactions with other power system equipment or with existing network resonances. This thesis examines the interactions between inverter-based compensators and a particular form of system resonance, that of subsynchronous resonance between a generator turbine shaft and the electrical transmission network. The thesis presents a review of the theory of operation of high-power, multi-pulse inverter topologies actually used in transmission-level FACTS devices. Detailed simulation models are developed of both two-level and three-level multi-pulse inverters. With appropriate controls, simulation models of both the SSSC and STATCOM, and a full UPFC are then developed using these detailed inverter models and the results from these simulation models compared against other results from the literature. These comparisons show favourable agreement between the detailed FACTS models developed in the thesis and those used by other researchers. However, the models presented in this thesis include a more detailed representation of the actual power-electronic circuitry and firing controls of inverter-based FACTS devices than is the case with other models used in the literature. The thesis then examines the issue of whether the introduction of an SSSC to a transmission system could cause subsynchronous resonance (SSR). SSR is a form of dynamic instability that arises when electrical resonances in a series capacitively compensated transmission line interact with the mechanical resonances of a turbo-generator shaft system. The detailed SSSC simulation model developed in the thesis is used to determine the impedance versus frequency characteristics of a transmission line compensated by an SSSC. The results confirm earlier work by others, this time using more detailed and realistic models, in that the introduction of an SSSC is shown to cause subsynchronous resonance. The thesis then considers the addition of supplementary damping controllers to the SSSC to reduce subsynchronous oscillations caused both by the SSSC itself as well as by a combination of conventional series capacitors and an SSSC in a representative benchmark study system. The results show that subsynchronous oscillations in the transmission system compensated solely by an SSSC can successfully be damped out using a single-mode supplementary damping controller for a range of values of SSSC series compensation. However, in the case of the transmission system compensated by both conventional series capacitors and an SSSC, the nature of the subsynchronous oscillations is shown to be complex and strongly multi-modal in character. The thesis then proposes an extension to the single-mode supplementary damping controller structure that is better suited to damping the multi-modal resonances caused when an SSSC and conventional series capacitors are used together to compensate a transmission line. The results obtained from this multi-modal controller indicate that it is able to stabilise SSR for a range of compensation values, but that the controller design needs to be adjusted to suit different values of compensation. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2006.

Series compensating capacitors were initially introduced in transmission networks mainly to increase the / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009.

Electrical power distribution networks are required to provide power to customers at nearconstant / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2010.