High Frequency Attenuation in Transmission Class Solid Dielectric

High Frequency Attenuation in Transmission Class Solid Dielectric
High Frequency Attenuation in
Transmission Class Solid Dielectric Cable
Jim Jun Guo and Steven Boggs
Institute of Materials Science
University of Connecticut
November 2007
Why Transmission Class Cable?
• Transmission class solid dielectric cable is made
in several structures which differ substantially in
– Materials, i.e., “bedding layers”
g , e.g.,
g , “wire screen” ((neutral wire),
), corrugated
– Design,
sheath, etc.
• PD diagnostics are more important for
transmission class cable than distribution cable,
but the sensitivity to defects is lower as PD
detection sensitivity scales as (size)-1.
Cables Investigated
Bedding Layer & Shields
• The bedding layers of these cables have relatively
low conductivity 0.1 to 10 mS/m or 100 to
10,000 Ω-m.
• The shield layers of these cables have very high
conductivity, ~10 S/m (~0.1 Ω-m) so that the
shield layers can be modeled as purely resistive.
• Thus high frequency loss is likely to be the result
of the bedding layers rather than the shield
Bedding Layer Properties
2 mm bedding layer for concentric neutral cable
0.5 mm bedding layer for corrugated Cu sheath cable
Concentric Neutral Cable
Corrugated Cu Sheath Cable
The “wire screen” (concentric neutral) cable has sufficient wires
that a continuous, coaxial sheath is a good approximation and
predicts well the measured attenuation, which is close to that of a
similar distribution cable.
The Corrugated Cu Sheath Cable is much more complex, as the Cu
sheath contacts and compresses the bedding layers only periodically.
Loss is dominated by high current
density in the bedding layer where the
Cu sheath compresses the bedding tape
Equi-current density plot
Attenuation in Cu Sheath Cable
Comparison with Distribution Cable
The Corrugated Cu Sheath Cable has about twice the attenuation of
concentric neutral transmission class cable or distribution cable
Atten = 20log10 ( e) exp ( −αω L )
Atten = exp ( −αω L )
• The structure and materials of transmission class
solid dielectric cable can have a large impact on
high frequency attenuation and, therefore, PD
detection sensitivity.
• We have provided the basis for computing high
frequency attenuation in corrugated sheath
transmission class solid dielectric cable, including
the effects of the bedding layer.
• Problems of this complexity are great training for
graduate students
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