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

The demand for electrical energy in industrialised countries continues to increase steadily. As a result of this growing demand for electrical energy, there is a need for optimisation of the power system in terms of transmission and control. One option could possibly be an increase in transmission facilities to handle the increase in growth; however factors such as environmental issues as well as the possible cost incurred could hamper this particular approach. An alternative resides in loading the existing transmission network beyond its present operating region but below its thermal limit, which would ensure no degradation of the system. For this approach to be realised, improved control of the flow of power in an interconnected network would be advantageous so as to prevent unwanted loop flows and inadvertent overloading of certain lines. This approach can be made possible by the use of Flexible AC Transmission Systems (FACTS) technology. The concept of FACTS incorporates power-electronic compensation devices that can be typically used in an ac power system to enhance the system's power transfer and controllability. There exists a number of FACTS devices, where each device can be utilised differently to achieve the broad objective. One such device is the Thyristor Controlled Series Capacitor (TCSC). The TCSC is a class of FACTS device that makes it possible to alter the net impedance of a particular transmission line in an effort to force the flow of power along a "contract path". This thesis identifies, in the published literature, a set of strategies for the scheduling of power flow by use of variable compensation; such strategies are then considered in more detail in the analysis of the thesis. Firstly, a detailed dynamic model of a TCSC is developed together with its various controls and associated circuitry within the power systems simulation package PSCAD. In addition to this, a power flow controller scheme is then implemented, which exhibits the functionality of the power flow controller strategies reviewed in the literature. In order to test the validity and operation of the TCSC model as well as the analysis of the power flow controller scheme, a single-machine infinite bus (SMIB) study system model is developed and used as part of the investigation. This thesis, firstly, presents a theoretical analysis of two particular modes of power flow control in an interconnected ac transmission system. Secondly it confirms the results of an analytical study in previously published work with the implementation of the two control modes, and further extends the scope of the previous study by examining the impact of the power flow controller's design on the small-signal and transient stability characteristics of the study system. The key findings of this extended investigation are that the power flow controller's mode of operation has an important influence on both small-signal and transient stability characteristics of a power system: in partiCUlar, it is shown that one mode can be detrimental while the other beneficial to both system damping and first swing stability. Finally, the thesis applies the understanding of the power flow controller's operation obtained from the SMIB study system to the problem of inter-area mode oscillations on a well-known, two-area, multi-:generator study system. Real-time simulator results are presented to exhibit the effect of the power flow controller modes and controller design on the oscillatory characteristics of the two-area study system. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005

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This thesis describes a novel harmonic domain approach for assessing the steady state performance of Flexible AC Transmission System (FACTS) devices. Existing harmonic analysis techniques are reviewed and used as the basis for a novel iterative harmonic domain model for PWM FACTS devices. The unified Newton formulation adopted uses a combination of positive frequency real valued harmonic and three-phase fundamental frequency power-flow mismatches to characterise a PWM converter system. A dc side mismatch formulation is employed in order to reduce the solution size, something only possible because of the hard switched nature of PWM converters. This computationally efficient formulation permits the study of generalised systems containing multiple FACTS devices. This modular PWM converter block is applied to series, shunt and multi-converter FACTS topologies, with a variety of basic control schemes. Using a three-phase power-flow initialisation and a fixed harmonic Jacobian provides robust convergence to a solution consistent with time domain simulation. By including the power-flow variables in the full harmonic solution the model avoids unnecessary assumptions regarding a fixed (or linearised) operating point, fully modelling system imbalance and the associated non-characteristic harmonics. The capability of the proposed technique is illustrated by considering a range of harmonic interaction mechanisms, both within and between FACTS devices. In particular, the impact of transmission network modelling and operating point variation is investigated with reference to ac and dc side harmonic interaction. The minor role harmonic distortion and over-modulation play in the PWM switching process is finally considered with reference to the associated reduction in system linearity.

Approaching problems with global warming due to the use of fossil fuels, means that new system solutions have to be investigated. This report investigates the possibility to expand an existing photovoltaic system with a wind turbine generator to simplify the expansion of renewable energy sources. Through an extensive literature study and simulation in SIMULINK the result has been developed and tested. It shows that it is possible to connect a wind turbine generator to the same inverter as the PV-modules if no MPPT algorithm for the PV-modules is integrated. To protect the inverter a dump load has to be connected. Because of the complexity a DC-coupled system brings, AC-coupling is advised when expanding PV-modules with a WTG. The optimal wind turbine is considered to be a permanent magnet synchronous generator connected to the AC-bus through a full-effect inverter. The turbine should be chosen according to the intended location based on wind conditions and desired energy production.

Widespread access to renewable electricity is seen as a viable method to mitigate carbon emissions, although problematic are the issues associated with the integration of the generation systems within current power system configurations. Wind power plants are the primary large-scale renewable generation technology applied globally, but display considerable short-term supply variability that is difficult to predict. Power systems are currently not designed to operate under these conditions, and results in the need to increase operating reserve in order to guarantee stability. Often, operating conventional generation as reserve is both technically and economically inefficient, which can overshadow positive benefits associated with renewable energy exploitation. The purpose of this thesis is to introduce and assess an alternative method of enhancing power system operations through the control of electric loads. In particular, this thesis focuses on managing highly-distributed sustainable demand-side infrastructure, in the form of heat pumps, electric vehicles, and electrolyzers, as dispatchable short-term energy balancing resources. The main contribution of the thesis is an optimal control strategy capable of simultaneously balancing grid- and demand-side objectives. The viability of the load control strategy is assessed through model-based simulations that explicitly track end-use functionality of responsive devices within a power systems analysis typically implemented to observe the effects of integrated wind energy systems. Results indicate that there is great potential for the proposed method to displace the need for increased reserve capacity in systems considering a high penetration of wind energy, thereby allowing conventional generation to operate more efficiently and avoid the need for possible capacity expansions. / Graduate