Mitigation of Remanence Flux in Power Transformers using Predetermined Method of De-Energization

Title from PDF of title page viewed October 30, 2017Thesis advisor: Preetham GoliVitaIncludes bibliographical references (pages 37-39)Thesis (M.S)--School of Computing and Engineering. University of Missouri--Kansas City, 2017Energization of large power transformers are subject to many transients that may complicate the successful completion of this process and ultimately reduce the expected life of these critical components. The first-time energization (commissioning), subsequent energizations (operational), methods of energization (abrupt or controlled energizations from the high voltage or low voltage winding) and the possibility/improbability of these transformers being preloaded all affect the transformer’s longevity. The consequences of such energizations during the conditions are inrush currents and voltage stresses on the affected components that may not be foremost on the designer’s mind. The designer may be more concerned with proper parameter application and not the effects of commissioning and operation on these massive components. These behemoths are a bit akin to elephants whose longevity is dependent on the sum of their life experiences and the scars they endure during this period. The reliability of electric system is directly affected by these series connected behemoths. The construction of power transformers has been optimized by the advent of computers (especially finite analyses) to the point that stray flux, eddy current, hysteresis loss and harmonic loss (embodied and represented within the non-linear Rₚ element and known as “core Watt losses”) have all attained significant improvements witnessed by their 99.8+ percent efficiency. The difficulties that remain are magnetizing inrush and remanence embodied within Xₚ which occur dependent on three parameters. The parameters are primary resistance Rₛ (dependent on the location of same for the equivalent circuit used), the time dependent voltage at the point on the voltage wave when the transformer is energized (referred to as “Point of Wave”) and the remanent (or residual flux) and its polarity all at the instant of energization. The magnetizing inrush problem has been thoroughly researched and commercial products exist to mitigate such difficulties by control system add-ons. This research recognizes that knowledge of Point on Wave has effectively mitigated the problems with transformer energization at zero voltage. The results obtained after hundreds of runs confirms a direct relationship between the point of the wave where current is extinguished for a fast acting air switch and minimal to zero remanence flux in a single-phase shell form transformer. This minimal to zero residual flux appears at the peak of the equivalent sinusoidal current wave (increasing or decreasing) without the effects of saturation. The conclusion of the experimental runs was that the use of multiple Hall-Effect transducers (multiple installations suggested for manufacturing errors or wiring failures) within the laminations of a transformer which would be used to confirm the near zero remanent flux once the current was extinguished as described above. These findings and recommendations are still subject to testing at nameplate loads of varying power factors upon three phase transformers of shell and core constructions.Introduction -- PCAD modelling of remanence in transformers -- Equipment -- Experiment and results -- Implementation of the conclusion

Design and Development of Autotransformer Motor Starter for Induced Draft Fan (IDF) of Batangas Sugar Central Inc.

Induction motors as a rotating machine draw a higher current during starting operation than under full load running conditions. With the use of reduced voltage, it limits the adverse effect of the starting current and torque that develops during across the line starting. This is in the case of Batangas Sugar Central Inc. (BSCI) where a wye – delta reduced voltage starting method was utilized in the induced draft fan (IDF) motor. Investigation of the present wye – delta starting method was done to identify the behavior of the voltage, current and torque at different stages of the operation to determine the causes of the operational problem. Performance and characteristics of the current in the present method during the switching transition were found to be defective (5 times the rated value) hence, the reason for analysis. In this study, the characteristics of the various starting method for three phases of induction motors were closely observed and comparison of the stated parameters were made in order to develop a proposed autotransformer motor starter prototype to serve as a basis of starting the IDF motor for the BSCI. Different design requirement based on National Electrical Manufacturers Association (NEMA), Philippine Electrical Code (PEC) and National Electrical Code (NEC) were considered. The voltage, current and torque based on the motor used in the study were computed and the values obtained were compared to the measured value of the parameters using the proposed starting method   in order to ensure reliability, accuracy and applicability. The specification of the autotransformer was computed also based on the actual motor of BSCI and the corresponding values for the input parameters such as voltage, current and torque were then computed and analyzed. The results served as the basis for the BSCI in the implementation of the proposed autotransformer starting method. The computed value of the different input parameters was supported with the measured values using the autotransformer motor starter since the values obtained were almost equal. It showed that the computed value for the starting voltage was almost the same as the actual value. The same thing was true with the computed values of current at different stages in the operation of the motor where it was concluded to be almost equivalent to the measured value using the prototype

Three-Phase Transformer Inrush Current Reduction Strategy Based on Prefluxing and Controlled Switching

n/

Analysis of ac/dc system disturbances.

This thesis describes the development of accurate transient equivalents for the efficient modelling of disturbances in interconnect a.c./d.c. power systems. A survey of existing techniques for the dynamic analysis of such transients is made, and their major limitations are outlined. The most recent utilize diakoptical techniques with state space formulation, which allows detailed representation of all system plant, albeit at the expense of high computation costs. Means of reducing such costs to a level which permits digital simulation to complement/replace physical simulators in the analysis of a.c./d.c. network transients are investigated, and an interactive algorithm is proposed which uses a multimachine a.c./d.c. transient stability programme and a small step dynamic analysis to achieve this purpose. Typical disturbances in both a.c. and d.c. networks are investigated to show the potential of the new technique, and its ability to include accurate representation of the full system throughout the study period. The dynamic analysis programme is also used in preliminary investigations of unit type generator-convertor schemes; the developed model provides a rigorous means of testing the practicalities of such schemes

Electrical transmission systems for large offshore wind farms

Simulations of switching transients were carried out in EMTP-RV. Overvoltages in offshore wind farms ranged from temporary over voltages to very fast front transients. Transient Recovery Voltages of the offshore circuit breakers exceeded IEC 62271 requirements in some situations. The disconnection of an array produced the most severe overvoltages, exceeding IEC 60071 requirements.EThOS - Electronic Theses Online ServiceGBUnited Kingdo