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 Shunt Reactors
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 Shunt Reactors
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.
No comments:
Post a Comment