The two general types of construction used
for shunt reactors are dry-type and oil-immersed. The construction features of
each type, along with variations in design, are discussed under the headings
which follow.
Dry Type
Dry-type shunt reactors generally are
limited to voltages through 34.5 kV and are usually applied on the tertiary of
a transformer which is connected to the transmission line being compensated.
The reactors are of the air-core (coreless) type, open to the atmosphere,
suitable for indoor or outdoor application. Natural convection of ambient air
is generally used for cooling the unit by arranging the windings so as to
permit free circulation of air between layers and turns.
The layers and turns are supported
mechanically by bracing members or supports made from materials such as
ceramics, glass polyester, and concrete. The reactors are constructed as
single-phase units and are mounted on base insulators or insulating pedestals
which provide the insulation to ground and the support for the reactor.
Because the dry-type shunt reactor has no
housing or shielding, a high-intensity external magnetic field is produced when
the reactor is energized. Care is thus required in specifying the clearances
and arrangement of the reactor units, mounting pad, station structure, and any
metal enclosure around the reactor or in the proximity of the reactor.
A closed metallic loop in the vicinity of
the reactor produces losses, heating, and arcing at poor joints; therefore, it
is important to avoid these loops and to maintain sufficient separation
distances. Shielding may be required when it is not possible to arrange
dry-type units in an equilateral-triangle configuration isolated from external
magnetic influences. This shielding is required to limit the impedance
deviation between phases. Deviation from impedance values for reactors will
result in a deviation from the actual MVAR rating.
For the same range of applications, the
primary advantages of dry-type air-core reactors, compared to oil-immersed
types, are lower initial and operating costs, lower weight, lower losses, and
the absence of insulating oil and its maintenance. The main disadvantages of
dry-type reactors are limitations on voltage and kVA ratings and the high
intensity external magnetic field mentioned above. Because these reactors do
not have an iron core, there is no magnetizing inrush current when the reactor
is energized.
Oil-Immersed
The two design configurations of
oil-immersed shunt reactors are coreless type and gapped iron-core type. Both
designs are subject to low-frequency longtime constant currents during
de-energizing, determined by the parallel combination of the reactor's
inductance and line capacitance. However, the gapped iron-core design is
subject to more severe energizing inrush than the coreless type.
Most coreless shunt reactor designs have a
magnetic circuit (magnetic shield) which surrounds the coil to contain the flux
within the reactor tank. The steel core-leg that normally provides a magnetic
flux path through the coil of a power transformer is replaced (when
constructing coreless reactors) by insulating support structures. This type of
construction results in an inductor that is linear with respect to voltage.
The magnetic circuit of a gapped iron-core
reactor is constructed in a manner very similar to that used for power
transformers with the exception that small gaps are introduced in the iron core
to improve the linearity of inductance of the reactor and to reduce residual or
remanent flux when compared to a reactor without a gapped core.
Oil-immersed shunt reactors can be
constructed as single-phase or three-phase units and are very similar in
external appearance to that of conventional power transformers. They are
designed for either self cooling or forced cooling.
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