When large disturbances occur in interconnected power systems, there exists the danger that the power system states may leave an associated region of stability, if timely corrective action is not taken. Open-loop remedial control actions such as field-forcing, line-tripping, switching of series-capacitors, energizing braking resistors, etc., are helpful in reducing the effects of the disturbance, but do not guarantee that the post-fault power system will be stabilized. Linear controllers are widely used in the power industry, and provide excellent damping when the power system state is close to the equilibrium. In general, they provide conservative regions of stability. This study focuses on the development of nonlinear controllers to enhance the stability of interconnected power systems following large disturbances, and allow stable operation at high power levels. There is currently interest in the power industry in using thyristor-controlled series-capacitors for the dual purpose of exercising tighter control on steady-state power flows and enhancing system stability. This device is used to implement the nonlinear controller in this dissertation. A mathematical model of the power system controlled by the thyristor-controlled series-capacitor is developed for the purpose of controller design. Discrete-time, nonlinear predictive controllers are derived by minimizing criterion functions that are quadratic in the output variables over a finite-horizon of interest, with respect to the control variables. The control policies developed in this manner are centralized in nature. The stabilizing effect of such controllers is discussed. A potential drawback is the need to have large prediction horizons to assure stability. In this context, a coordinated-control policy is proposed, in which the nonlinear predictive controller is designed with a small prediction horizon. For a class of disturbances, such nonlinear predictive controllers return the power system state to a small neighborhood of the post-fault equilibrium, where linear controllers provide asymptotic stabilization and rapid damping. Methods of coordinating the controllers are discussed. Simulation results are provided on a sample four-machine power system model. There exists considerable uncertainty in power system models due to constantly shifting loads and generations, line-switching following disturbances, etc. The performance of fixed-parameter controllers may not be good when the plant description changes considerably from the reference. In this context, a bilinear model-based self-tuning controller is proposed for the stabilization of power systems for a class of faults. A class of generic predictive controllers are presented for use with the self-tuning controller. Simulation results on single-machine and multimachine power systems are provided. / Graduation date: 1995

M. Tech. Electrical Engineering. / Discusses that most real-time applications of synchronised phasor measurement units in power system stability studies are done using RTDS simulator, hardware PMUs and PTP hardware modules with GPS providing the time reference. The major drawbacks of this configuration are both technical and economical. The technical limitations are related to the possible number of outputs that can be used to interface PMU devices, which can be very limited; and also the possible limits on small-time step computation due to the big number of output signals. The economic constraints are mainly due to cost: for a research laboratory, only a limited number of units can be acquired due to budgeting issues. To overcome these difficulties, the realisation of an entirely software-based synchrophasor measurement unit, presents an attractive approach. Such a unit will be capable of deliveringreal-time data by acquiring the voltage and current signals from the Matlab/Simulink simulator, low cost software environment PMUs computing real-time phasors of voltage and current and software only PTP synchronisation protocol. The unit delivers synchrophasors for the application of power system SE and small signal stability analysis, but also taking into consideration the accuracy of the synchronisation protocol on the application of power system SE and small signal stability analysis.

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

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

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.

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Electric power systems are characterized by their immense complexity. The assessment of their security on-line has always been a challenging task. Many possibilities were investigated in the past in an attempt to characterize the secure operating region of a power system. Pattern recognition is thus far the only tool that can take various degrees of network complexity into consideration. / In the present study, an efficient algorithm which learns adaptively the secure operating region is proposed. At each iteration, training operating points are generated sequentially on a piecewise linearly approximated separation surface computed by the one-nearest-neighbor (1-NN) rule. The separation surface so estimated approaches the true one as the number of training points increases. The algorithm not only provides a consistent technique in learning an unknown region, it generates a highly efficient training set. It is found to be effective in reducing the size of the training set without adverse effect to the classifier. / Once the secure region of a power system is available, the task of on-line security monitoring reduces to one of determining whether the current operating point resides in the secure region. As demonstrated in the thesis, both the security status and the security margin of the operating point can be assessed very efficiently. By using the piecewise linearly approximated secure region, the thesis proceeds to give efficient ways of moving an insecure operating point into the secure region. This comprises the problem of security enhancement. / The regionwise methodology via the Voronoi diagram developed in the thesis is also applied to a wide range of problems, such as network planning, coordinating tuning of machine parameters and automatic contingency selection. The major merit is that the dynamics and the nonlinearity of the system no longer present a limitation to solving these problems.

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