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Ferroresonance
(click
here to view the DSTAR
gallery of interesting ferroresonance results, including sound clips)
An inherent consequence of the utility
industry's shift to higher efficiency distribution transformers is an increased tendency
for ferroresonant overvoltages to occur in many everyday situations. DSTAR research has
been focused on both defining the problem as well as exploring solutions. To determine the
system parameters which may result in excessive ferroresonant overvoltages, extensive
full-scale tests were performed on a number of transformers under realistic field
conditions. Literally hundreds of switching events were performed on 12.47, 24.94 and 34.5
kV padmount transformers, including both grounded-wye and delta-primary units. This
project overwhelmingly proved that the efficient transformers now used by the industry are
much more susceptible to ferroresonant overvoltages than those of a decade or more ago.
For example, crest overvoltages 2.35 times normal crest were measured in DSTAR field tests
where a conventional silicon-steel core 225 kVA, 24.94 kV wye-wye padmounted transformer
was switched at the riser pole with only the capacitance provided by 220’ of cable to
support the ferroresonance!
As you can hear in the sound clips
provided in the
DSTAR Ferroresonance Gallery, transformer noise
during ferroresonance is very unusual, and can often lead field crews to believe that the
unit is failing. Actually the noise is due to changes in the core dimensions as the
core goes in and out of saturation.
DSTAR investigations of ferroresonance
disproved several widely-held beliefs regarding this complex overvoltage phenomenon. For
example, a transformer's core loss and not its exciting current is the best indicator of
how much cable can be safely single-phase switched with the transformer. The research
results were the subject of a recent IEEE paper which received considerable attention.
Although this effort yielded a significant
amount of new information regarding ferroresonance susceptibility, mitigation techniques
were also examined. For example, a minimal amount of load is sufficient to eliminate
ferroresonance. Another research project was performed by DSTAR to define the duty placed
on metal-oxide surge arresters by ferroresonant overvoltages. The research has disproved
the widely-held belief that a high ferroresonant overvoltage will immediately fail a
gapless arrester. Because of the very high impedance of the ferroresonant circuit,
arrester heating is slow. The DSTAR investigation has shown that arresters may be
considered as a viable means to control these overvoltages in many circumstances.
Extensive guidelines have been prepared which indicate the limiting system conditions for
which various types of arresters can be expected to survive several minutes of
ferroresonant overvoltages, as during transformer switching, and indefinite exposure, as
might occur from fuse operation which open-phases an unloaded transformer and cable. The
convenient form of these guidelines is depicted in the figure below.
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