IEEE Power & Energy Magazine - March/April 2020 - 91

in my view (continued from p. 96)
wires. The aluminum wires carry 98%
of the electrical current. The steel core
increases the conductor strength and
limits conductor sag under both high ice
and wind loads and at high temperatures
(HTs). ACSRs are limited to continuous operation at about 100 °C or below
to maintain the tensile strength of the
hard-drawn aluminum wires.
In my view, when designing new
overhead transmission lines at 345 kV
and below, HT phase conductors such
as aluminum-conductor steel-supported (ACSS) should be evaluated as an
alternative to conventional ACSRs.
With the same conductor diameter,
ACSSs can yield new lines with 50%
higher thermal ratings and the same
electrical losses at a capital cost typically only 5-10% higher than that for
ACSR. The higher ratings avoid future
power-flow constraints that can otherwise occur due to the system-planning
uncertainties produced by the combination of increased use of renewable
generation, distributed generation, and
open-access transmission.
HT conductors can be operated for
sustained periods of time at temperatures between 150 and 250 °C without changing the conductor's tensile
strength or damaging its connectors,
depending on type. But merely being able
to operate an overhead line at a higher
TCMAX does not imply that a same-size
HT conductor operates hotter than ACSRs under normal system conditions or,
for extended periods of time, under emergency conditions. The temperature of
the phase conductors in most existing
transmission lines is normally only
5-15 °C above air temperature since
system-normal power flows are typically much lower than the line's thermal
rating and normal weather conditions
(i.e., higher cooling wind speeds and
lower air temperatures) produce lower
conductor temperatures than the worstcase weather conditions assumed for
thermal-rating calculations.
An overhead line's thermal limit is
equal to the phase conductor line current
that produces a conductor temperature
equal to TCMAX for an assumed set of
march/april 2020

weather conditions (high air temperature,
low wind speed, full sun). As shown by the
solid curve in Figure 2, an ACSR or HT
conductor with the same outer diameter
(OD) produces a range of thermal ratings
depending on the value of TCMAX. For
Cardinal ACSRs (i.e., 54/7 stranding,
483-mm 2 aluminum area) at 100 °C,
the thermal rating is approximately
1,250 A. For Cardinal ACSSs (also
54/7 and 483-mm 2 aluminum area) at
200 °C, the thermal rating is 1,900 A,
approximately 50% higher. Both conductors yield the same structural transverse wind loads and offer essentially the
same resistance per unit length.

To satisfy the reader's curiosity about
alternatives, the dashed curve shows the
rating of a single larger Falcon ACSR
[806 mm2 (1,590 kcmil), 54/19 stranding, 39.2 mm (1.545-in OD)] with a
thermal rating of 1,760 A at 100 °C
due to its lower resistance (40% lower)
and larger OD. The dotted curve shows
the thermal rating of a two-Cardinal
ACSR bundle with twice the rating of
a single Cardinal conductor. The figure
makes it clear that higher line ratings
can be accomplished by using either a
large ACSR or a two-Cardinal bundle.
In both cases, however, the line project
costs would be significantly higher due

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IEEE Power & Energy Magazine - March/April 2020

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