Mitigation of Remanence Flux in Power Transformers using Predetermined Method of De-Energization
Abstract
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
Degree
M.S.