It can not always be expected that ferroresonance happens.The ferroresonance is rarely seen on the nonlinear resonance of the power system. If the resonance is transient or steady it may result in overvoltage and over current and also degrade insulation of the equipment. It¡¦s been proved that the rare nonlinear resonance causes some unexplainable damages. Due to increasing power capacity, expanding transmission line and underground line of ferroresonance is increased. This thesis presents the three phase power system with analysis. The magnetic curve was obtained from field measurements and using many kinds of parameters which are from the magnetization characteristic and lines to analysis. Especially emphasized is the width of the hysteresis loop and the effects of magnetic hysteresis in the change of the operating point of the transformer. In the simulation, the thesis compares the effects of magnetic hysteresis with the non-effects of magnetic hysteresis, and compare the differences of parameter critical point to avoid ferroresonance. This thesis has presented some results in investigating the impact of different sets of initial condition on the models of behavior of a ferroresonance circuit.

Ferroresonance is a nonlinear phenomenon that could cause damage in the power systems equipment due to a high voltage and current during its sustained period. If the system is under sustained ferroresonance for a long time, it can cause thermal damage to the equipment. Therefore, it is important to eliminate or mitigate ferroresonance. It is shown that different mode of ferroresonance gives different impact to power systems. Thus, being able to detect and classify ferroresonance according to its mode during the transient period could avoid the system from going into the sustained period by initiating appropriate mitigation or elimination procedures. The objective of this research is to develop numerical algorithms for ferroresonance monitoring by analyzing its voltage and current signals using Fourier transform and wavelet transform. The aim of this research is to provide features that can be used in the development of a real time monitoring system that may be incorporated in mitigation or elimination procedures in the future. Ferroresonance voltage and current signals are obtained from the modelling of the ferroresonance circuit in the transient program. The sensitivity studies are performed to obtain different modes of ferroresonance and to observe the sensitivity of ferroresonance towards its initial condition and parameter variation. The signals are then being analyzed using Fourier transforms and wavelet transforms to obtain features that can be used in the classification process. Both the sustained and transient periods of the ferroresonance signals are analyzed. The results show that the ferroresonance voltage signals during sustained period are able to be classified according to their modes, however, the ferroresonance signals of the transient period requires further analysis. The algorithms are tested on the real data and produce the similar results that validate the algorithms.

The onset of a ferroresonance phenomenon in power systems is commonly caused by the reconfiguration of a circuit into the one consisting of capacitances in series and interacting with transformers. The reconfiguration can be due to switching operations of de-energisation or the occurrence of a fault. Sustained ferroresonance without immediate mitigation measures can cause the transformers to stay in a state of saturation leading to excessive flux migrating to transformer tanks via internal accessories. The symptom of such an event can be unwanted humming noises being generated but the real threatening implication is the possible overheating which can result in premature ageing and failures.The main objective of this thesis is to determine the accurate models for transformers, transmission lines, circuit breakers and cables under transient studies, particularly for ferroresonance. The modeling accuracy is validated on a particular 400/275 kV transmission system by comparing the field test recorded voltage and current waveforms with the simulation results obtained using the models. In addition, a second case study involving another 400/275 kV transmission system with two transformers is performed to investigate the likelihood of the occurrence of sustained fundamental frequency ferroresonance mode and a possible quenching mechanism using the 13 kV tertiary connected reactor. A sensitivity study on transmission line lengths was also carriedout to determine the probability function of occurrence of various ferroresonance modes. To reproduce the sustained fundamental and the subharmonic ferroresonance modes, the simulation studies revealed that three main power system components which are involved in ferroresonance, i.e. the circuit breaker, the transmission line and the transformer, can be modeled using time-controlled switch, the PI, Bergeron or Marti line model, and the BCTRAN+ or HYBRID transformer model. Any combination of the above component models can be employed to accurately simulate the ferroresonance system circuit. Simulation studies also revealed that the key circuit parameter to initiate transformer ferroresonance in a transmission system is the circuit-to-circuit capacitance of a double-circuit overhead line. The extensive simulation studies also suggested that the ferroresonance phenomena are far more complex and sensitive to the minor changes of system parameters and circuit breaker operations. Adding with the non-linearity of transformer core characteristics, repeatability is not always guaranteed for simulation and experimental studies. All simulation studies are carried out using an electromagnetic transient program, called ATPDraw.

Diploma thesis deals with ferroresonance phenomena which is present in power networks with saturable inductances and capacitors. A distinction drawn between resonance and ferroresonance will highlight the specific. Lack of knowledge means that it is readily considered responsible for a number of unexplained destructions or malfunctionings of equipment. In the work is a description of such behaviours means risk for electrical power systems. Finally, last part presentat produkt by ABB company intend for damping oscillations and summaries measures on it.

This thesis deals with the ferroresonance phenomenon and ferroresonance overvoltage in electric networks. That’s why the introduction part explains the theoretical basis of ferroresonance and its consequences in the power system. The work then focuses on the power system as a non-linear dynamic system and describes various analytical methods for solving these systems. These methods are further applied to diagnose specific systems. There are a number of various power system configurations susceptible to ferroresonance. The most frequent one is theoretically analyzed in this thesis and a number of recommendations are offered for reducing the ferroresonance phenomenon. The thesis further deals with the selection of suitable software for this computationally chal-lenging task. A number of different tools are considered and efficiently reduced to Matlab Sim-ulink and PSCAD software. Matlab Simulink and its SimPowerSystem Toolbox are used to ana-lyze the effect of hysteresis of the magnetic core of the transformer on the emergence and course of the ferroresonance phenomenon. Using PSCAD, various configurations of the power systems are compared in terms of possible emergence and course of ferroresonance and also to find the limiting conditions that increase the probability of this phenomenon. The possibility of using arti-ficial neural networks for ferroresonance diagnosis is also mentioned in this thesis. The final part provides a number of recommendations for design and operation of both new and existing power systems.

The dissertation studies the nonlinear resonance problems of power systems. Generally speaking, ferroresonance has usually occurred in low-voltage distribution system, especially for potential transformers. Due to the considerable increase of power consumption, the power system is more complex than before. Besides, a number of under-ground cables are used, and transformers¡¦ loss reduce due to improvement of core iron material. These factors could probably result in ferroresnance occurring in extra-high-voltage power system. The dissertation proposed three-phase representation method to analyze unbalance and non-linear system. This method employ magnetically coupled electrical circuit techniques and the original voltage equations can be used without the need for any transformations, which improves significantly computation accuracy. Consequently, it is quite suitable for power system design and incident investigation. Since the traditional d-q-0 model is not well suited for the study of unbalanced faults and requires further transformation, the analytical solution becomes rather complicated and the solutions are still inaccurate. At last we simulated the ferroresonant overvoltages occurring at a nuclear power station in Taiwan in order to investigate the causes and afford mitigation. The simulation results were enough to prove accuracy and practicability of this method.