IEEE Power & Energy Magazine - March/April 2020 - 32

In practice, an even electric field distribution is not feasible
because each subconductor would need to be equidistant
from other phases, ground, and structures.
The High Surge-Impedance
Loading Concept
The factors limiting the power-transfer capability of a transmission line can be broadly classified into three categories:
✔✔ Ampacity limits refers to a maximum current established to prevent the maximum design temperature
from being exceeded, either for conductor damage or
clearance violation; this limit is relevant for lines of
short length (<100 km).
✔✔ Systemic constraints apply mainly to stability limits related to system configuration and operating point.
✔✔ Voltage drop occurs in medium and long transmission
lines due to the reactive power demanded by the line
inductive reactance.
The system constraints and voltage drop are more commonly
binding in systems with large renewable power sources located
far away from load centers. Historically, this is the case for
remotely located hydropower in Brazil; however, technological solutions are available to deal with such limitations. For
instance, voltage drop is usually addressed by means of series
compensation or flexible ac transmission system.
For such long lines, there is a certain power-transfer level
where reactive power generated in the line capacitance
equals reactive power absorbed in line inductance, such that
no reactive compensation is required. This power rating is
related to the line characteristic power, or surge-impedance
loading (SIL). In circuit theory, when the receiving load
matches the SIL value, a maximum power transfer is reached
for this voltage level. In practice, other factors influence the
maximum power transfer. But for long lines, it can be stated
as a proportion of its SIL.
The R&D projects described here allowed for the design
of optimized lines with very high SIL, called HSIL lines.
The additional transmitted power is due to the maximum
utilization of the electric field around the conductors; the
optimum point is close to the corona-onset threshold.
Because the corona effect also depends on weather conditions, proper knowledge of the statistical distribution of temperature, air density, and humidity enables transmission-line
design closer to its limit, with a margin to avoid overloading
during operation under different weather conditions. The
occurrence of corona in a line is tolerated only under veryhigh-humidity conditions or heavy rain, as an all-weather
corona-free line design is economically unfeasible.
The HSIL concept optimizes the electric field on the surface of each conductor to increase the power-transfer capability with lower cost per megawatt delivered as compared with
32	

ieee power & energy magazine	

conventional line designs. HSIL technology globally optimizes all significant electrical and geometric parameters of a
transmission line, resulting in optimal choices for the diameter of the conductors' cross section, phase spacing, height, and
sag. HSIL design became possible with recent advances in
modeling, calculation methods, and computational resources.
Such tools allow engineers to establish transmission-line configurations optimized with respect to electrical and magnetic
field distributions and, consequently, transmission capacity,
electrical parameters, and current distribution.
The HSIL concept represents a considerable change in
transmission-project development and operating practices,
and its adoption requires the integration of planning, design,
maintenance, and operation to obtain the most benefit. The
HSIL design consists of a geometric (defined by the electric
requirements) and mechanical optimization. The geometric
phase is defined mainly by the surface electric field distribution and the insulation coordination (distance between conductors, conductors and grounded structures, and conductors
and ground). After a number of iterations, a final mechanical
design was obtained for test trials, as shown in Figure 1.
Experience shows that two main geometric factors im---
prove the SIL: compacting the phases and expanding the
conductors' bundles. The concept of compact lines applies
mainly to new designs since the phases' positions are in--
fluenced by the structures, whereas the expanded bundle
(EXB) can be utilized in all types of lines, as the phases'
centers of gravity are unaltered. Both approaches alter
the surface electric field, which could be expressed by a
utilization factor k u, which is the relationship between
the -surface electric field with the theoretical maximum
for each subconductor (see Figure 2). An ideal utilization
(k u = 1) implies a linear power increase in relation to the
number of subconductors. In practice, an even electric
field distribution is not feasible because each subconductor
would need to be equidistant from other phases, ground,
and structures.
Given an assumed horizontal phase arrangement, Figure 2 summarizes the theoretical and achievable SIL as a
function of the number of conductors per bundle. The figure,
based in a hypothetical configuration, also shows how different design approaches for the bundled conductors affect
the SIL and provides examples for bundles of four and eight
subconductors per phase.
When the geometric concept is followed, the insulation coordination analysis must verify the required clearance between
phases and grounded structures, along with the length of the
march/april 2020



IEEE Power & Energy Magazine - March/April 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2020

Contents
IEEE Power & Energy Magazine - March/April 2020 - Contents
IEEE Power & Energy Magazine - March/April 2020 - Cover2
IEEE Power & Energy Magazine - March/April 2020 - 1
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IEEE Power & Energy Magazine - March/April 2020 - Cover3
IEEE Power & Energy Magazine - March/April 2020 - Cover4
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