Harmonic filter reactors for low-voltage
applications are typically dry-type iron-core. No existing standard addresses
harmonic filter reactors, but most manufacturers use IEEE Std C57.12.01-1998 as
a guideline.
Cores are
constructed from silicon sheet steel (such as M-6). The number 6 corresponds to
the approximate power loss per pound of steel at a magnetic flux density of 1.5
T, i.e., M-6 has a loss of “0.6 W/lb” or 1.5 W/kg. M-6 is the typical grade of
silicon steel used, but both lower and higher grades of steel are available.
A manufacturer may choose to use a lower
grade steel and either let the harmonic filter reactor operate with a higher
temperature rise or use more steel. Conversely, a higher grade of steel can be
used and either the harmonic filter reactor may operate with a lower temperature
rise or the harmonic filter reactor could be made smaller.
The construction may be from individual
pieces of cut strip stock or E-I laminations. To create a harmonic filter
reactor, it is necessary to have gaps in the core. These gaps are known as air
gaps, but for physical integrity and rigidity the gaps are filled with hard
insulation.
This insulation will have permeability
similar to air. These gaps can be distributed (many small gaps) or a single
larger gap. A single gap will use E-I laminations whereas a distributed gap
will be made up of individual cut strips. The E-I construction requires less
labor and can be clamped and wedged better than a distributed gap core.
However, the distributed gap core will
significantly reduce fringing. This reduction in fringing helps control the
effective cross-sectional area as well as stray fields that may result in
localized heating of the coil. A “C” core may be used, but it will not offer an
advantage for harmonic filtering.
Reactor coils may be constructed from sheet conductor or magnet wire. Sheet
conductor may be more economical and easier for construction, but reactors with
significant harmonic currents can incur heating problems.
If not properly designed, sheet copper
windings can become annealed due to large localized current densities. Sheet
conductor windings should be used only on harmonic filter reactors that have a
distributed gap well within the boundaries of the coil. Magnet wire, which is
less susceptible to localized heating, is commonly used for harmonic filter
reactors.
In some cases, it may be desirable to have
parallel strands of smaller gauge magnet wire to reduce heating. Although using
parallel strands of such wire significantly complicates winding construction,
the increase in winding complexity can be justified because coil losses may be
significantly reduced.
Clamping is
also very important. If a reactor is not properly clamped, the harmonic current
can cause laminations to vibrate. Lack of proper clamping could result in a
loud audible noise and the breakdown of the insulation coating on the
laminations.
Laminations are clamped with insulated
through-bolts, or bolts that go through clamps, and are external to the
laminations. Clamps that bridge air gaps must be of nonferrous construction.
Such nonmagnetic clamps are used to avoid shunting the air gap with a magnetic
path.
Furthermore, ferrous components need to be
as far from air gaps as possible to prevent inductive heating of the ferrous
material. Coils are held in place with spacers and wedges. The harmonic filter
reactor should be vacuum impregnated, preferably vacuum-pressure impregnated
with a suitable varnish. Impregnation, however, should not be relied upon as
the only mechanical means of support.
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