Resonant Overvoltage Phenomena Associated with Transformer Feeders
System-generated overvoltages are increasingly becoming a critical factor in determining the insulation level of the system, particularly at the higher transmission voltages. The ability to predict these overvoltages for given system conditions is therefore of paramount importance during the planning stage of a power network. This thesis is concerned mainly with excessive transient voltages generated in composite transformer feeders, particularly due to resonance. The thesis commences with a comprehensive review of over-voltages produced in power systems (Chapter 1). Power frequency, sustained and transient overvoltages of internal and external origin are discussed, together with measures employed to protect system equipment against their effects. Factors which are critical to system insulation levels at the higher voltages are established. Chapter 2 is concerned more specifically with the modifying influence of a transformer integrally connected to the feeder on switching overvoltages. Particular consideration is given to conditions under which these overvoltages may be accentuated by resonance and ferroresonance effects. The specific aspects of these phenomena which form the basis of the research effort discussed in later Chapters, are identified. The relative merits of available analogue and digital computer methods which can be used in the study of transients are presented in Chapter 3. The lattice diagram technique adapted for digital computation and the transient network analyser, both of which are utilised in some of the investigations, are described in more detail. Chapter 4 presents the rudiments of an efficient digital computer method based on the compensation theorem. This method is capable of accurately simulating travelling-wave phenomena in transmission lines as well as the transient response of complex lumped and non-linear sub-networks to switching stimuli. Single-phase results of investigations utilising the methods described commence in Chapter 5. The effects of system constants and various circuit arrangements on linear resonant energisation overvoltages in transformer feeders are assessed in Chapters 5 and 6 respectively. Conditions potentially onerous in respect of overvoltage magnitudes are identified and simple analytical techniques for their predictions are derived. A comprehensive mathematical model of three-phase transformer feeder circuits is presented in Chapter 7. This model incorporates mutual effects between the phases of the line and both the electric and magnetic interactions within a transformer of a composite core geometry, including the non-linear saturation characteristics. The compensation method described earlier is extended to three phase circuits and applied to this model to study the effects of sequential pole closure, transformer saturation and winding connections in a resonant network (Chapter 8). The phenomena of resonance and ferroresonance in double circuit transformer feeders following isolation of one circuit from the rest of the system are dealt with in Chapters 9 and 10, respectively. Analytical methods for predicting the occurrence of linear resonance overvoltages in a reactively compensated circuit which is being dropped, and preventive measures, are suggested in Chapter 9. The fundamental aspects of ferroresonant oscillations, their effects on system equipment and methods for their (ii) suppression, are set out in Chapter 10. The compensation method is used to assess the effects of certain circuit constants and characteristics.
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