[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

aluminum shielding, i.e., an oil-immersed air-core reactor with nonmagnetic conductive shielding. In
this case, noise sources are the windings and the shielding.
Mitigation of sound begins at the design stage by ensuring that critical mechanical resonances are
avoided in clamping structure, tank walls, etc., and, where necessary, by utilizing mechanical damping
techniques such as vibration-isolation mounting of core material. Site mitigation measures include the
use of large, tank-supported, external screens with thick acoustic absorbent material, walled compounds,
and barriers made of, for instance, acoustic (resonator) masonry blocks and full acoustic enclosures.
FIGURE 2.9.42 Graph of Q vs. frequency for a natural Q-filter reactor, a filter reactor plus parallel resistor, and a
filter reactor with a de-Q’ing ring system.
2.9.8 Sound Level and Mitigation
2.9.8.1 General
The primary source of sound from dry-type and oil-immersed reactors is related to electromagnetic
forces generated at fundamental power-frequency current and, where applicable, harmonic frequency.
The source of sound in dry-type air-core reactors is the “breathing mode” (expansion/relaxation) vibrations
of the windings resulting from the interaction of the winding currents and the “global” magnetic
field of the reactor. In the case of oil-immersed reactors, the sound sources are more complex and,
depending on design approach, include combinations of and contributions from winding, core (including
air gap), magnetic shields, nonmagnetic shielding, and ancillary equipment such as cooling fans. The
basic mechanism involves the magnetic field at the iron/air interfaces and the resultant “pulling” forces
on the magnetic core material.
2.9.8.2 Oil-Immersed Reactors
Oil-immersed shunt reactors utilize two basic design approaches: air-core magnetically shielded and
distributed air-gap iron core. Unlike power transformers, where magnetostriction in the core material is
the primary source of noise in an unloaded transformer, the major source of noise in a shunt reactor is
vibrational forces resulting from magnetic “pull” effects at iron/air interfaces, primarily at the air gaps.
On a secondary-order, leakage flux penetrates structural components of the reactor, and the resultant
electromagnetic forces generate vibrational movement and audible noise at twice the power frequency.
In the case of oil-immersed magnetically shielded air-core designs, the forces primarily act on the end
shield, producing bending forces in the laminations. The resulting vibrations depend on the geometry
of the laminated iron-core shields and the mechanical clamping structure. Additional forces/vibrations
result from leakage–flux interaction with the tank walls and any ancillary laminated magnetic core
material used to shield the tank walls. Distributed air-gap iron-core reactors produce a major portion of
their noise as a result of the large magnetic attraction forces in the gaps and also, due to similar forces,
at the end-yoke/core-leg interfaces. The avoidance of mechanical resonance is key to minimizing sound
levels. It should be noted that gapped iron-core technology was used in the past for the design of highvoltage
filter reactors for HVDC application, and the issues described above were exacerbated by the
presence of harmonic currents.
Another design approach that can be used for oil-immersed reactors is essentially an air-core reactor
design that is placed in a conducting tank (usually aluminum) or in a steel tank with continuous
2.9.8.3 Air-Core Reactors
For air-core reactors, the primary source of acoustic noise is the radial vibration of the winding due to
the interaction of the current flowing through the winding and the “global” magnetic field. Air-core
reactors carrying only power-frequency current, such as series reactors, shunt reactors, etc., produce noise
at twice the fundamental power-frequency current. In the case of filter reactors and TCRs, the noise
generated by harmonic currents contributes more to the total sound level than the noise resulting from
the fundamental power-frequency current because of the A -frequency weighting of the sound. Because
there are multiple harmonic currents present in reactors employed on HVDC systems, the design for
low operating sound level is a challenge; avoidance of mechanical resonances at the numerous forcing
functions requires excellent design-linked modeling tools.
The winding can be regarded in simplified modeling as a cylinder radiating sound from the surface
due to radial pulsation. A reactor winding has several mechanical self-resonance frequencies. However,
predominately one mode shape — the first tension mode or the so-called breathing mode — will be
excited, since this mode shape coincides with the distribution of the electromagnetic forces. The breathing-mode
mechanical frequency is inversely proportional to the winding diameter. For example, a cylindrical
aluminum winding with a diameter of 1400 mm has a natural breathing-mode mechanical
frequency of approximately 1000 Hz. To avoid dynamic resonance amplification, the model frequency
should be designed so that it is not near the forcing frequency.
The exciting electromagnetic forces are proportional to the square of the current and oscillate with
twice the frequency of the current. If, however, the reactor is simultaneously loaded by several currents
of different frequencies, in addition to vibration modes at double the electrical frequencies, additional
vibration frequencies occur as shown below.
• Loading with two ac currents with frequencies f 1 and f 2 generates acoustic sound with frequencies
of 2f 1 , 2f 2 , f 1 + f 2 , f 1 – f 2 .
• Loading with dc current and one ac current with a frequency f 1 generates acoustic frequencies f 1 ,
2f 1 .
The acoustic-frequency spectrum substantially increases if the reactor’s current spectrum includes
multiple harmonics; “n” harmonic currents can generate at most n 2 forcing frequencies, but the practical
number is usually less because some overlapping occurs.
With the increasing concern for the environment, there are now often stringent sound-level requirements
for many sites. Extensive sound-modeling software and mitigation techniques have been developed
for dry-type air-core reactors. The predictive software allows the design of air-core reactors with mechanical
resonances distant from any major exciting frequency and facilitates the optimum use of component
materials to reduce sound level.
Where extremely low sound levels are required, mitigating methods such as acoustic-foam-lined sound
shields are also available and have been used with great success. Figure 2.9.43 shows an installation of
harmonic filter reactors for an HVDC project with a sound-shield enclosure. Sound-shield enclosures
can typically reduce the sound pressure level by up to 10 to 12 dB. Very low sound levels were required,
as people were living in houses on the hillsides overlooking the HVDC site.
Figure 2.9.44 shows three curves of the sound-pressure level (SPL) plotted as a function of the frequency
of the filter-reactor current for one of the filter reactors shown in Figure 2.9.43. One curve shows the
sound characteristic of the original or natural design. The second curve shows the sound characteristic of
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC