Evaluation of High Surge Impedance
Loading (HSIL) solutions for increased
natural capacity of 500 kV Overhead Lines.
Study Committee B2
Working Group WG B2.38
Members
Angel Gallego Del Monte, Danna Liebhaber, Hee-Sung Ahn, Jarlath Doyle,
José Manuel Frenandez Davila, Oswaldo Junior Regis, Paul Penserini,
Tsinghua University, Vivendhra Naidoo.
Abstract
The third generation of 500 kV lines were developed
after several studies of High Surge Impedance Loading
- HSIL, for a new level of 1200MW, applying the concept
of compaction of the distance between phases, or the
concept of expanded bundles, or a mix of both concepts.
This report compares six different 500kV Transmission
Lines Projects, each with four subconductors per phase
and Surge Impedance Loading (natural capacity) of 1200
MW, which are currently operating in more than 10.000
km of extension in Brazil. Transmission Line compaction
and the use of expanded bundle techniques are analyzed
focusing on electrical coupling and its influence on
positive sequence and zero sequence impedance. The
electric field on the conductors’ surface and the electric
and magnetic fields at ground level are evaluated for
each configuration.
2. Theoretical aspects
The basic theory of optimization [1] of HSIL shows
that, for the same voltage level, the equalization and
maximization of the electric field on the sub-conductor’s
surface, and/or an increase of the number of subconductors per phase, increases the Surge Impedance
Loading – SIL of the lines.
1. Introduction
Considering the impedance theory, it is also possible to
show that the Natural Capacity of transmission increases
as the line phases are compacted [2] (i.e. the distance
between the phases is reduced), and as the bundle
dimensions are expanded [3] (i.e. the distance between
sub conductors of a phase is increased).
The Surge Impedance Loading (SIL), also called Natural
Capacity, of an alternating current (AC) transmission
line can be defined as the active power that, being
transported through the line, causes a consumption of
reactive power equal to the value of the reactive power
generated by the capacitance of the same line. Power
flows below the SIL produce minimum voltage drop and
stability problems on the line.
2.1 Diagram of a transmission line
The diagram below represents a transmission line in a
system study, where P is the transmitted power and Vs
and Vr are the sending and receiving end voltages. The
parameters of the line are: C (the shunt capacitance) that
provides reactive power and Z (the series impedance).
The series inductance X consumes reactive power and
is responsible for the drop of the voltage and for stability
problems on the long distance transmission.
The line’s thermal capacity, which concerns premature
aging of conductors and connectors as well as maintaining
minimum electrical safety clearances, is generally higher
than the natural capacity and is not covered here.
The systems analysis view is especially important for
long distance interconnections where increased Surge
Impedance Loading (SIL) of the lines can reduce or even
remove restrictions on power flow.
In Brazil, the first generation of 500kV lines had a
3-conductor bundle per phase and a SIL of 900 MW.
The second generation of 500 kV lines had 4-conductor
bundles per phase and a SIL of 1000 MW. The need for
long lines, mainly interconnections between the north,
northeast and southeast regions, has motivated the
development of lines with higher transmission capacity.
Diagram : Schematic representation of a Transmission
Line in electric systems studies
Cigre Science & Engineering • N°2 June 2015
63
Figure 1 – Racket Tower; the drawings distances and heights are in meters
Figure 2 – Cross-Rope Tower (Chainette)
Figure 3 – VX-Asymmetrical Tower
self impedance and the positive sequence impedance, Zl,
increasing the capacity of the transmission line.
For steady state and transient studies, the TL is
represented by its positive and zero sequence constants.
In the load-flow studies that analyze the voltage control
in the system, only the positive sequence constants are
used. Therefore, a line that has smaller positive sequence
impedance per unit length (Z1) has higher capacity of
transmission for the same length reference.
The mutual impedance (Zm) depends on the distance
between the phases. As the distance between the phases
decreases, the higher will be the line’s mutual impedance.
Therefore, the impedance Z1 will again be smaller cause
Zm has a negative signal in relation (Z1 = Zs - Zm),
increasing the capacity of power transmission.
2.2 Analisys of self and mutual impedance, and zero
and positive sequence
These two techniques, compaction of phase spacing
and expansion of the bundles, were used in different
projects to increase the Natural Power (SIL) of 500 kV
interconnection lines in Brazil, as shown in the next
topic.
The positive sequence impedance Z1 of a line can be
obtained from their self and mutual impedances (Zs and
Zm) by the relation Z1= Zs-Zm.
The self impedance (Zs) depends on the conductor, but
mainly depends on the geometry of the bundle of each
phase. The higher the bundle dimension, the smaller the
While the reduction in phase-phase spacing and the
increase in bundle diameter increases the Natural Power
Cigre Science & Engineering • N°2 June 2015
64
Figure 4 – VX-Symmetrical Tower
Figure 5 – Cat Face Tower
Figure 6 – Monopole tower
Distances and heights are in meters. In all cases, the phase
bundles consist of 4 Rail, 954MCM sub-conductors, but
with different bundle diameters and different distances
between the phases, in order to obtain a Natural Power
of 1200MW.
on the other hand, the influence of those variations of
the self and mutual impedances (Zs and Zm) in the
impedance of zero sequence (Zo) shows a different effect
as the relation is Zo = Zs + 2 x Zm.
Differently from de effect on Z1, the application of
compaction or expansion techniques of the bundles
results in variations of the impedance Zo in both ways,
increasing or decreasing , depending on the case. The
following item (7) shows the Table II with the parameters
of sequence and the analysis of their variations.
3.1 Racket tower
The Racket Tower is aself-supporting structure with
square conventional bundles, 18 inches (0,457m)
on each side. The distance between the phases is very
reduced (bigger Zm) in a triangular arrangement called
Compact Tower. This leads to an increased SIL.
3. Towers and arrangements
analyzed
3.2 Cross-rope tower (chainette)
Pictures of Towers and drawings of phase’s bundles
showing their sub-conductors are presented below.
The Cross-Rope Tower is supported by two guyed poles,
with the electric phases between them. The bundles are
Cigre Science & Engineering • N°2 June 2015
65
Figure 7 – Electric superficial gradients in 500kV
guyed “V” supports, with horizontal phases having
structure masts between them. In order to obtain a
higher SIL, a regular square expanded bundle is used
(lower Zs), 1,20m/side. This allows the use of standard
hardware which the previous design did not. On the
other hand, as it is shown later, the electric field is higher
in some sub-conductors, since it is not the optimized
electric distribution.
conventional (square with 0,457 meters), with a nearly flat
arrangement of phases and with very short distances between
them, which gives a higher SIL due to the compaction.
3.3 VX-ASYMMETRICAL (V guyed tower with
Asymmetrical Expanded Bundle)
The VX-Asymmetrical Tower has a guyed “V” support
for the center phase, with phases on the same level
and structure masts between them, which prevent
compaction. To obtain a higher SIL an asymmetrical
expanded bundle (lower Zs) which optimizes the electric
fields on each sub-conductor’s surface.
3.5 Cat face tower
The Cat Face Tower is self-supported with metal parts
between phases. This limits its compaction. In order
to obtain higher SIL, a regular expanded bundle was
used (lower Zs) with a square form, 1,20m/side. The
phases are arranged in a triangle. This provides a
more equalized distribution of electric fields in the
sub-conductors.
3.4 VX-SYMMETRICAL (V guyed tower with
Symmetrical Expanded Bundle)
The VX-Symmetric Tower has also its conception in
Electric field
kV/cm
Cross Rope
Racket
VX Symmetric
Monopole
VX asymmetric
Cat face
Maximum value
18.00
17.76
17.02
16.88
16.05
16.36
Minimum value
15.04
15.00
14.69
14.52
14.67
14.47
Max / Min (%)
19.7
18.4
15.9
16.3
9.4
13.1
Average - phase A
15.7
15.7
15.3
15.1
15.1
15.0
Average - phase B
17.9
17.3
17.0
16.5
16.0
16.0
Average - phase C
15.7
15.7
15.3
15.1
15.1
15.0
Average - General
16.4
16.2
15.9
15.6
15.4
15.3
Table I: Minimum, Maximum and Average values of electric field on conductor’s surface.
Cigre Science & Engineering • N°2 June 2015
66
Figure 8 – Graphic with the audible noise profiles (AN)
Figure 9 – Graphic with radio interference profiles (RI)
3.6 Monopole tower
The Table I below presents, for each configuration,
the values of maximum and minimum gradients, and
the ratio of these values. It also presents the average
of the gradients on sub-conductors of each phase, and
the general average of all sub-conductors of each line
type. Notice that even though the VX asymmetric
tower has the lowest maximum gradient value, the Cat
face tower presents a general average which is slightly
lower.
The Monopole Tower uses guyed supports with a
single lattice pole and structural parts between phases.
Nevertheless, the spacing between phases has been
minimized for the triangular phase arrangement. To
produce a natural capacity of 1200 MW, an expanded
square bundle, 0.90m/side is used. In this case, the SIL
gain was reached as a combination of a light compaction
(higher Zm) and the use of a “semi-expanded” bundle
(lower Zs).
5. Comparative analysis of
audible noise (an) and radio
interference (ri)
4 Comparative analyses of the superficial electric fields
The following graphs, the electric field on each subconductor’s surface, are calculated for a 500 kV line. It is
important to note that when these fields are higher, they
result in more corona activity and, hence, in higher levels
of audible noise (AN) and radio interference (RI) along
the right of way (r.o.w.) and nearby.
The Figure 8 shows the audible noise (AN) profile of all
lines analyzed. It is noticeable that the values of the AN
profile decrease proportionally to the gradients average
of all the sub-conductors, starting with the highest value
of the Cross-Rope, to the smallest value of the Cat Face.
The AN phenomenon generated by the transmission
lines decreases slowly with distance outside of the line
corridor.
Figure 7 is arranged in descending order for the highest
superficial electric field (gradient) calculated with each
design configuration.
Figure 10 – Graphic with the electric field profiles
Figure 11 – Graphic with the magnetic field profiles
Cigre Science & Engineering • N°2 June 2015
67
500 kV TL Parameters
R1 (Ω/km)
Positive Sequence
Zero Sequence
Ratio
VX asymmetric
VX Symmetric
Cat face
Racket
Cross Rope
Monopole
0.0171
0.0174
0.0175
0.0173
0.0173
0.0176
X1 (Ω/km)
0.269
0.261
0.268
0.267
0.268
0.265
C1 (nF/km)
16.57
17.07
16.56
16.29
16.36
16.63
SIL (MW)
1205
1240
1205
1199
1196
1214
R0 (Ω/km)
0.349
0.349
0.351
0.373
0.346
0.369
X0 (Ω/km)
1.336
1.342
1.329
1.478
1.496
1.361
C0 (nF/km)
9.840
9.764
9.708
7.620
7.251
9.347
X0/X1
4.98
5.14
4.96
5.55
5.59
5.13
C1/C0
1.68
1.75
1.71
2.14
2.26
1.78
Table II: Positive and Zero Sequence parameters of 500kV TLs
The Figure 9 shows the Radio Interference profile (RI)
for the lines analyzed. The phenomenon of RI generated
by the transmission lines decreases more rapidly with
distance than Audible Noise.
r.o.w. The other towers have a higher value on the axis,
with variations in the intermediate area, and significant
reduction to the border of r.o.w.
These calculations show that the triangular phase
arrangements generate higher RI values under the line
but lower levels at the edge of right-of-way. 35 meters
from the axis, the Racket, Monopole and Cat Face
designs have a RI value smaller than the others. The VX
Asymmetric design has smaller RI values than the Racket
in the middle area but levels that are 2 dB/1microV/m
higher on at the edge of right-of-way.
7. Comparative analysis of the
electric parameters
As noted before, the techniques of TL compaction or
TL bundles expansion, leading to an increase of their
SIL, have an impact on the values of their electrical
parameters of the positive and zero sequences (see
Table II). It is observed that the values of reactance (X1)
and capacitance (C1) of the positive sequence of each
configuration are very close, since all designs have been
adjusted to a same value of SIL (≈1200 MW).
6. Comparative analysis of the
e&m fields near the ground
On the other hand, it is observed that there are major
differences in the zero sequence. Among the reactance
(Xo) there are variations between the minimum (blue)
and maximum (yellow) value on the order of 13%. In
capacitance (Co), the variation reaches 36%. In the
relations Xo/X1 and C1/Co the variations are similar.
This indicates that the performance under unbalanced
conditions and switching transients will have different
responses depending on the maneuver considered and
the adopted tower.
All the field calculations, whose profiles are shown here,
were made at 1.5 meter above ground level. The minimum
phase height was approximately 10 meters to ground, to
allow comparison of line designs. The recommendations
of the ICNIRP limit values or other international bodies
were not considered since these values must be met
at time of executive project and according to the local
regulations.
In the electric field profile, Figure 10, it is verified that
the more compact towers, i.e., the Racket and the CrossRope, present smaller field values in the middle and side
areas of the r.o.w. The Cat Face and the Monopole tower
have the smallest field values in the line axis, getting
higher at 12m from the axis, and having slightly higher
values than the compact towers at 35m from axis. The
towers of plane configuration and with longer distances
between phases have higher value of electric field under
the phases, with reduced slope until the border, similar
to other alternatives.
8. Conclusion
Six TL design concepts of 500 kV TL have been analyzed,
with Natural Power (SIL) of 1200 MW, and similar
positive sequence electrical parameters for system
studies and steady state conditions.
The zero sequence impedance differs by up to
36% depending on the phases and sub-conductors
arrangements.
In the magnetic field profiles (Figure 11) it is verified that
the compact towers have lower values throughout the
TL electrical studies, such as superficial conductor’s
Cigre Science & Engineering • N°2 June 2015
68
9. References
gradient, environmental interference levels of audible
noise and radio interference, and electromagnetic fields,
showed the advantages and disadvantages of each design.
[1]
Alexandrov, Georgij N., et alii - The Increase of Effectiveness of
Transmission Lines and Their Corridor Utilization - Cigre Paper
38-104, Paris 1996.
[2] Fernandes, José H. et alii – Towers for Compact TL of the Second
Circuit of the 500 kV North-Northeast Interconnection at
Eletronorte – Electric Studies - VIII SNPTEE – 1986 (Paper in
Portuguese).
[3] Regis Jr., Oswaldo; Dart, F. C. et alii – Studies and Application of
Expanded Bundle in 500 kV TL - XIV SNPTEE - 1997 (Paper in
Portuguese)
[4] Machado, Vanderlei G. et alii - 500 kV TL of Third North /
South Interconnection – Tower Solution with Monomast Guyed
Support and Expanded Bundle - XIX SNPTEE – 2007 (Paper in
Portuguese)
In specific applications, where terrain allow the use of
guyed towers or requires self-supporting towers; or in
areas sensitive to audible noise or electric fields at ground
level, the best alternative may be practical.
Finally, many references on switching transients studies
shows that there is a strong dependence of surge levels
of switching procedures (TL energizing and reclosing,
circuit breakers opening under faults, load rejection,
transient recovery voltage) with the sequence parameters
of the TL nearby. This fact suggests further investigations
of these phenomena in order to have an additional
comparison of the behavior of the concepts of lines
presented in this article.
Cigre Science & Engineering • N°2 June 2015
69