IEEE Power & Energy Magazine - March/April 2020 - 96

in my view

Dale A. Douglass

transmission lines
designing for open access transmission

Digital Object Identifier 10.1109/MPE.2019.2954688
Date of current version: 19 February 2020

ieee power & energy magazine

1,000
Line Maximum Power Flow (MVA)

HUNDREDS OF TECHNICAL PApers were written prior to deregulation
in the 1990s regarding the optimization
of new transmission line designs. A
common goal was to minimize electrical losses and capital investment in conductors, structures, and rights of way by
choosing the right conductor size within the boundaries of corona and electromagnetic fields. At the time, it was, to
some extent, possible to predict the line
power flow (load shape and magnitude)
for new lines at least 10-20 years into
the future. Since the advent of open-access transmission, the development of
new power generation by wind and solar
sources, the sometimes rapidly shifting
fuel costs (i.e., gas versus nuclear and
coal), and environmental restrictions on
conventional generation, it is no longer
possible to predict the line-load magnitude and daily load shape for most new
lines over their expected design life of
40 years.
There has been a major shift in transmission expansion planning away from
deterministic methods, as discussed at
length in the July/August 2016 issue of
IEEE Power & Energy Magazine. In
that issue, several articles explained the
difficulties in predicting system power
flows that were increasingly uncertain.
In addition, the daily loading patterns
(capacity factor) that new overhead
lines will experience are more difficult
to predict, especially where wind is the
primary generation source.

Thermal Limit on Power Flow Independent of Length

800

600

400

200

Power Flow Limited by
Electrical Effects for
Long ac Lines

Power-Flow
Limited by
Thermal
Rating for
Shorter Lines

Power Flow Limit 25% of
Thermal Rating for
1,000-km Line
345-kV Transmission Line
Maximum Power-Flow
Dependence on Length

100

200

300

400 500 600
Line Length (km)

700

800

900 1,000

figure 1. Power-flow limits as a function of line length at 345 kV (adapted from
Gutman et al.).

Power flow on overhead lines is limited by electrical effects (e.g., voltage
drop and phase shift) and by the maximum temperature allowed for the phase
conductors (TCMAX). Voltage drop and
phase shift increase with length; thermal
rating does not, as shown in F
- ig --ure 1.
Therefore, short lines are limited thermally, and long lines are limited electrically unless compensated. Power
flow on extrahigh-voltage (EHV) and
ultrahigh-voltage lines may also be
indirectly constrained by the thermal
rating of HV lines due to load reliability (N-1), postcontingency overloading
of the parallel EHV lines, or the use of
lower-voltage lines.
In recent years, many of the solutions to these power-flow limits can be

found in the form of electronic devices
(e.g., flexible ac transmission system
devices) or phase-shifting transformers. In many lines, however, one comes
up against the thermal limit (rating) of
the line at its maximum design temperature. The physical limits due to
such factors as electrical clearance and
conductor system aging are not easily
changed in an existing line.
More than 80% of existing transmission lines around the world use aluminum-conductor steel-reinforced (ACSR)
phase conductors. ACSRs consist of two
or more layers of hard-drawn aluminum
wires, helically stranded around a core
of concentric, helically stranded steel
(continued on p. 91)
march/april 2020

IEEE Power & Energy Magazine - March/April 2020

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