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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.

performance of high voltage outdoor insulators can be greatly affected by weather conditions. The weather conditions under investigation are cold temperatures coupled with high humidity levels. Weather data from a number of coastal stations around South Africa was analyzed to determine whether surface condensation was likely on outdoor insulators under these weather conditions. A heat transfer equation was used to determine the response of the insulator surface temperature to the environmental temperature. It was found that surface condensation would occur on the insulator surfaces which would lead to sudden, heavy wetting of the surface. Outdoor insulators in coastal environments are often heavily polluted, due to salt spray, and when wet, a conductive layer can form on the insulator surface. This conductive layer can result in appreciable leakage currents flowing on the insulator surface, often leading to premature failure. The finite element method program, Maxwcll, was used to simulate the outdoor insulators both under these polluted, wet conditions and under unpolluted conditions. Both cases were simulated for a silicone rubber, glass cap-and-pin and two EPDM outdoor insulators. The polluted insulators were simulated with varying pollution severities. The results of the simulations are analyzed and the surface resistances of the wet polluted insulators were calculated. An experiment was' set up to mask the environmental weather conditions found which would lead to surface condensation. The insulators under test were placed in a chilled weather chamber which introduced a steam fog to simulate the humidity. The leakage current was measured and recorded for comparison with the simulation results. The results of the weather chamber test showed that surface condensation resulted in more severe wetting than manual wetting. The weather chamber surface resistances calculated were much lower than those calculated by the Maxwcll simulations. This was due to the difference it humid particle temperature in the condensation rate equations used for the Maxwell simulations, and the humid particle temperature of the steam fog used in the weather chamber. Polluted coastal outdoor insulators exposed to the above weather conditions will experience larger than normal leakage currents which will lead to premature failure of the units. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2008.

This dissertation describes the impact of changing network configuration on power quality and performance of existing compensation devices in the transmission network. The underlying theory was assessed and thereafter the Everest substation network case scenario was selected to study the above due to; the number of reconfigurations it has experienced in the past, increased capacitor bank failures and also due to the harmonics problems experienced. The study involved the installation of harmonic current measuring instruments at Everest 132kV feeders to identify the potential sources of harmonics and to determine the dominant harmonics. A dig Silent Power Factory model was then constructed to perform various simulations in order to determine the impact of the changes done on the Everest network as well as the impact of capacitor switching on the harmonics amplification at Everest 132kV Bus-bars. The study also focussed on analysing the performance history of the capacitor banks at Everest and to determine if high harmonic amplification had an impact on capacitor bank performance. The simulation results revealed that network reconfigurations have negatively impacted power quality at Everest. The results showed that there is also a correlation between the switching of the two 72MVAR capacitor banks and the amplification of the harmonics at Everest. The highest amplification occurred when both capacitor banks were switched in and the resonance point occurred around the 5th harmonic which coincided with the data from field measurements. There was a 61% increase in 5th harmonic impedance amplitude after the Everest network was reconfigured, for the condition when both capacitors are switched in. The lowest amplification occurred when none of the capacitor banks were switched in. Three options were assessed to eliminate the problem of harmonics at Everest, the first one was to prohibit the switching in of both capacitors at Everest and utilise other available means around the Everest network for voltage support. The second option was to change the capacitor size, thus moving the resonance point away from the 5th harmonic. The last option was to install a harmonic filter at Everest to filter out the problematic harmonics. The first option is recommended as it has been successfully tested, can be readily implemented and is much more cost effective compared to the others. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.

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