Mitigation of Remanence Flux in Power Transformers using Predetermined Method of De-Energization
Metadata[+] Show full item record
Energization 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.
Table of Contents
Introduction -- PCAD modelling of remanence in transformers -- Equipment -- Experiment and results -- Implementation of the conclusions