2008 International Conference on High Voltage Engineering and Application, Chongqing, China, November 9-13, 2008

Aspects of High Voltage Cable Sections in Modern

Overhead Line Transmission Systems
T. Judendorfer, S. Pack, M. Muhr
Graz University of Technology Institute of High Voltage Engineering and System Management
Inffeldgasse 18 8010 Graz, Austria

Abstract- Electrical power transmission is dominated by have been developed – On the one hand, these are cables
overhead line systems at present. This is mainly based on more which are suitable for an application at high voltages levels
than hundred years of experience of utilities in running overhead
lines. Furthermore, overhead lines have proven their operational and on the other hand there are gas-insulated lines (GIL),
reliability and functional assurance. In the past, cables were used where currently SF6 is used as insulation gas. GILs has been
in distribution networks in urban areas for the most part with some sort of niche product for several reasons. Although this
the exception of direct current submarine cables. New technology has surely a justification, currently only a few
developments of high voltage XLPE cables make it possible to projects have been realized.
use this technology for EHV-level applications in transmission
networks. Within this paper, mixed network configurations, In contrast, the application of cables in the transmission
consisting of overhead lines and high voltage cables, are level is getting more popular these days. According to a study
investigated. An exemplary EHV transmission line with a total [2], in which 28 countries have been researched, the total
length of about 100 km, which is quite typical for Central Europe, length of installed cable lines in the voltage levels between
has been studied. Several different line combinations are 220 kV and 500 kV is 7164 km. In contrast, the total installed
discussed with varied rates between overhead line sections and
cable sections length in practice. The length of the cable sections length of overhead lines in the same voltage level is
are ranging from several kilometers up to lengths of 100 km. 631118 km. Thus, cable lines are currently used in roughly 1
percent of the transmission network in these countries only.
In this paper the work focuses on the transient behavior of
combined 400 kV overhead and cable lines during switching Nevertheless, the application of high voltage cables to the
processes and lightning impacts. A number of calculations were transmission sector can be considered as feasible under certain
carried out to get an overview of the transient stress in numerous conditions nowadays. Although the longest cable line that is
network nodes along the transmission system. Numerical in service at present has a total length of about 40 km (per
programs like ATP/EMTP have been used for these simulations. system), it is supposable that longer lines might appear in
Peak values and wave shapes of the transient voltage stress have
been evaluated, based on different systems and within possible transmission networks in the nearer future. For certain
combinations. In respect of the insulation coordination and projects, like energy transmission through large (railway)
transient stress at network nodes, the voltage time trends are tunnels across the alps for example, long high voltage cable
also analyzed. lines or GILs are the only two technologies that are actually
The combination of high voltage overhead and cable considered as possible technical solution [3, 4]. But besides
transmission systems, especially such with lengths of more than such extraordinary projects, an application of high voltage
about 50 km, are making tightened and extended demands to the cables in combination with overhead lines in transmission
network design, to the operational management and of course to networks might be more feasible in the nearer future.
the network protection also. As an output of this investigations,
the results might influence the strategy in running this new type This paper investigates the behavior of such cable lines in
of combined transmission systems. the transmission network. For this purpose, an exemplary
100 km long, 400 kV overhead line is used as a basis. Parts of
Keywords: overhead line, cable, mixed systems, high voltage,
switching, lightning, insulation coordination the overhead line are then replaced by according high voltage
cables. The length of these cable sections has been varied in
three different application cases, from some kilometers up to
100 km, which means the whole line was simulated as cable
I. INTRODUCTION
line only.
The electrical transmission system is dominated by The behavior of such mixed systems during switching
overhead lines since the very beginning. In the early stages, operation and at lightning strokes is of special interest. A
no adequate insulation technology for voltages above 100 kV multiplicity of conceivable operation states could be
was available. At the other hand, overhead lines have researched within this scope but only some of the important
favorable operational characteristics. Furthermore there are ones could be covered here.
additional facts that make overhead lines really essential for Reactive power compensation and network failure handling
the transport of electrical energy [1]. strongly differ from the operation management that is applied
Huge progress has been made during the last decades in the to overhead lines. But these aspects can be touched only
field of insulation materials and insulation systems research. briefly as this would then go beyond the scope of this paper.
As an outcome of this, two additional transmission systems

978-1-4244-2810-6/08/$25.00 ©2008 IEEE 71

 II. SIMULATION SET-UP along the line route, especially not from the viewpoint of
An exemplary but typical overhead transmission line is operational handling and failure treatment.
used for the simulations. As stated before, the line originally Numerical simulation tools (based on EMTP/ATP) have
consists of a 400 kV overhead line in double system been used for these investigations. Several different typical
configuration. It is assumed that the line connects two main application cases have been studied: namely these are
substations with appreciable short-circuit power for that switching processes (switching-on and –off of the whole line
voltage level. The lines were always fed from one side only so and also re-connections) and lightning incidents.
that the far end was in no-load condition to get more A typical lightning discharge with an amplitude of 15 kA
significant transient overvoltages. A straight-forward circuit and a 1,2/50 µs wave shape was used for these studies.
breaker model was used and inner arcing back as well as pole Several different points of impact (near cable bushings) have
scattering has been neglected. been studied.
Furthermore, only one system (3 phases) of the double Earthing impedances (tower footing resistance and cable
system is researched here, as this simplifies the whole process sheath earthing resistance at the cross-bonding connections)
a lot because certain issues with transient voltage phenomena have been assumed with 10 Ω uniformly. The soil resistivity
effecting both systems and load sharing between two cable was set to 100 Ωm along the line route. An increase of both
systems need not to be investigated. values will most likely lead to a worsening of the transient
It is assumed that one system of the overhead line (3 phases behavior of the whole line configuration.
each) is substituted by a suitable cable system (with 3 single Exemplary surge arresters have been applied to the
cables). In reality, this might not fulfill several design substations and at all cable bushings at all phases. The cable
requirements (thermal issues, electromagnetic-fields, etc.) and lines are operated with shunt reactors to keep inrush currents
therefore it is possible that more cable systems are necessary and voltage transients within acceptable limits. Basic shunt
to replace a single overhead line system. Nevertheless, such reactors have been simulated for that reason and each mixed
studies need more data of the actual application case so the configuration setup (Case II, III and IV) has been evaluated
selected substitution seems to be a good general separately.
approximation. So it can not be expected that the results can
be applied to a real use case but the simulations are a suitable III. COMPARISON AND EXPECTED ISSUES
indication for the transient processes that are taking place.
Overhead lines and cables have a very different operational
Based on such a configuration, four system arrangements
characteristic. The main variations are in the current rating
(cases) have been studied. Table I shows these configurations,
and the related thermal limits, reactive power and charging
where OHL is used as an abbreviation for overhead line and
currents, maximum line length and characteristic impedance
CL is used for cable line.
[4].
Table I Therefore, the following points should be considered and
INVESTIGATED SYSTEM CONFIGURATIONS
evaluated very carefully. Firstly, switching transients need to
Case Configuration Description
be examined in detail, especially in view of auto-reclosing.
I 100 km Overhead line (OHL) This brings out an important issue: When a system is operated
II 15 km CL – 70 km OHL – 15 km CL in a mixed configuration, the operational management is
III 20 km OHL – 30 km CL – 50 km OHL likely to be changed completely. Questions like the
permissibility of an auto-reclosure, the hereby connected
IV 100 km Cable line (CL)
waiting period, etc. need to be asked. This is connected with
Case I serves as a reference case with a pure overhead line, issues of de-energization and discharge and also with the
whereas Case IV is exemplary for a line configuration that is repeated switch-on. Basically, auto-reclosing in mixed system
completely accomplished with cables. Besides that, two cases configurations might be possible but this has to be evaluated
with a mixed configuration (Case II and III) have been studied, for each application case separately. This is also a question for
where OHLs and CLs are combined within one transmission the operational management as faults on overhead lines are in
line route. most cases temporarily (so a fast reclosure is permissible and
Case II might be typical when the two substations at the recommended) whereas faults on cables are normally of a
corresponding line ends are in areas which are difficult to permanent nature (reclosure could lead to additional damages
access, like this is the case in urban areas, airports and so on. and faults).
Case III covers the instance of a section along the line route Secondly, resonance conditions need to be studied. As the
where it is not possible to install an overhead line for any characteristics of overhead lines and cables differ strongly, it
reason. This solution is often referenced as “siphon might be possible that a condition of resonance is met, when
configuration”. Of course there is still a multiplicity of for example parts of an overhead line are substituted with
configurations which are not covered here. However, it is not cable lines.
really practical to change from OHL to CL several times

72
 IV. SWITCHING TRANSIENTS application is quite tricky because surge arresters, cable
Especially at longer cable lines, the switch-on process is bushings and potentially shunt reactors have to be installed
closely connected to large switching transients caused by the “in the wild” then.
specific parameters of the cables. This is also the case for D. Cable Line (Case IV)
switch-off processes. Resonant conditions during such a This is the most unfavorable case in terms of switching
transient network state might be problematically but this has transients as the total cable length is a maximum here. To get
not been investigated here. the overvoltages, caused by switching processes under control,
Table II shows the peak values of the transient voltages the line was separated into two parts (one 40 km long and the
during switching process for all simulated cases. All other 60 km). In between these two sections, adequate shunt
simulation cases containing cable lines are studied with and reactors and also surge arresters have been installed. An
without shunt reactor compensation. exemplary shunt reactor with 150 MVAr was used.
Table II Depending on the grade of compensation, several reactors
PEAK VALUES OF TRANSIENT VOLTAGES DURING SWITCHING need to be installed. Six units have been simulated here, but
Case COMPENSATED WITHOUT COMPENSATION for a practical design more reactors could be needed.
I – 1,2 p.u.
II <1,1 p.u. 1,4 p.u.
III <1,1 p.u. 1,4 p.u.
IV <1,1 p.u. 1,9 p.u.
As it was already noted in the previous chapter, re-
connection/re-closure of long lines can be problematical. The
peak values of the transient voltage depend strongly on the
specific line configuration and operational management. As
an approximate value, these voltages can reach levels of about
2,5 p.u. or even more in certain cases.
Figure 1 TRANSIENT VOLTAGES AT THE FAR, OPEN END OF THE CABLE LINE
A. Overhead Line (Case I) DURING SWITCH-ON PROCESS (LINE UNCOMPENSATED)
A pure overhead line was used as a reference case for a Otherwise, the capacitive inrush current would reach
better comparison. Overhead lines in the extreme high voltage significant heights and the steady-state voltage would be
(EHV) level within lengths of some 100 km are state of the art above the tolerable limits, not to mention power quality.
and in general not to be seen as critically in terms of operation Figure 2 shows the transient time trend of the phase voltages
conditions and performance. This is also the case for during switch on of an uncompensated cable line. The peak
switching processes. The peak values for this pure overhead voltages are near 700 kV which is about some 2 p.u.
line are in the range between 1,1 and 1,2 p.u. V. TRANSIENTS CAUSED BY LIGHTNING
B. Two cable sections with a long overhead line in between (Case Lightning incidents can cause severe danger to electrical
II) equipment, exceptionally to cable lines. Although cable lines
This is a typical scenario for the (inter-)connection of are normally protected against direct lightning impacts, due to
substations located in or near urban areas and industrial connected overhead lines or equipment in substations,
centers, although the rates of overhead and cable sections may significant overvoltages could also reach cable bushings and
vary. Depending on the length of the cable sections, a cable lines themselves. Therefore special precautions have to
compensation with shunt reactors might be necessary. This be taken in mixed configurations. The cable and the bushings
was also the case in this scenario as otherwise problems with must be protected in any case. This can be achieved with
network stability, transient overvoltage and reactive power are modern metal-oxide surge arresters and it is also common
occurring. For short(er) cable lengths it is possible to achieve practice.
good operational characteristics with shunt reactors installed
Table III
in the substations only. This is a huge advantage, as the PEAK VALUES OF TRANSIENT VOLTAGES DURING LIGHTING
installation of such equipment at the connection points Case OVERHEAD LINE CABLE LINE
between overhead line and cable is often very difficult or even
I 2,6 p.u. –
impossible. Good coordination of shunt reactor rating, cable
length and degree of compensation needs to be done to get II 2,2 p.u. 2,1 p.u.
sufficient power quality and transient switching behavior. III 2,6 p.u. 2,3 p.u.

C. “Siphon”-Configuration (Case III) IV – 2,4 p.u.

Case III is a typical usage of cables along the line route
where an overhead line is not feasible for any reason. Such an

73
 Table III shows the peak values in the overhead and cable
sections respectively at the studied lightning cases. It can be
derived that the voltage stress during lightning on overhead
lines is higher than on cable lines. This is mainly founded in
the lower characteristic impedance of the cables.
The highest stress for the cables occurred in all cases at the
transition from the overhead line to the cable line and the
bushing. Along the cable and the far end, the stress was
subsequently lower, more in the region of less than 2 p.u.
A. Overhead Line (Case I)
Overhead lines are protected against direct lightning
impacts by the application of earth wires in general.
Figure 2 Transient voltage time trend at a cable bushing
Nevertheless, in case of comparable low lightning currents for Lighting-Induced overvoltages at the connected overhead line
example, a so-called shielding failure might occur. In such a (Lightning stroke near the far end)
case, the simulated 15 kA lightning current is a reasonable
value. D. Cable Line (Case IV)
The consequences of lightning on high voltage overhead In such a configuration it is very unlikely that lightning is
lines are normally low. This is due to the long distances that an issue, especially when the substations are located indoor.
are needed for fulfilling the requirements of the insulation But even with outdoor-switchgear the risk for transferring
coordination. Furthermore, the insulating air can refresh lightning overvoltages into the cable is generally low, as the
quickly, which means in the case of a flashover the line can substations are directly protected by good lightning protection
go into service again promptly, of course under the systems and furthermore by station-class surge arresters.
precondition that no equipment damage occurred. A possible issue could be the compensation sites in the
B. Two cable sections with a long overhead line in between (Case middle of the line route. Special attention needs to be paid to
II) overvoltage and lightning protection, especially when the
In general, the longer the overhead line is, the higher is the facility is constructed outdoors. It is also important to achieve
risk of lightning incidents on that line. As the cable needs to good earthing conditions within such sites. If this is the case,
be protected by surge arresters at the bushings anyway, the it is not very likely that the overvoltages caused by lightning
risk of failure under normal conditions is low. In terms of reach values of more than 2,5 p.u.
lightning performance this case is a bit similar to Case III
where lightning incidents are discussed a bit more detailed. VI. CONCLUSION
C. “Siphon”-Configuration (Case III) Until now, the application of cable lines in transmission
This configuration needs special care in regard of lightning networks is quite limited. Currently, cables are in most cases
strike consequences. Apparently, transient overvoltages can only used to connect overhead lines to substations in urban
reach the cable and the cable bushings easily via the areas or in regions where overhead lines are unfeasible for
connected overhead line. Therefore the cable entries need to example. This might change in the near future. Two
be protected, which can be difficult as the connection site is exemplary networks with mixed configurations have been
not within substation or a comparable “protected” site. studied. A comparable line consisting only of an overhead line
Even when the change-over from overhead line to cable is and a cable line respectively have been used as reference
protected by surge arresters, significant overvoltages at the cases.
bushing and in the cable line can occur. Figure 2 shows the The simulations have been done on a very fundamental
time trend of the transient voltages at the cable bushing. The level with idealized values and required assumptions. This
source of the overvoltage was a direct lightning hit of one was necessary to reduce the computation effort and to get
phase conductor on the overhead line far away from the site. comparable results. Therefore a direct applicability to an
The low characteristic impedance is beneficial for the actual use case might not be practical. Nevertheless, it is
overvoltage behavior as the peak values are reduced when possible to draw some basic conclusions about mixed network
compared to overhead lines. Nonetheless, steep impulses can configurations.
appear inside the cable line. From the technical point of view, long cable lines might be
feasible. Switching processes and lightning incidents can be
handled also in mixed configurations. But it has to be said that
the effort for cable lines and for mixed systems is higher than
for pure overhead lines. Additional equipment needs to be
installed, mainly surge arresters and shunt reactors to get the
reactive power and transient overvoltages under control.

74
 As this is a basic study, not all aspects of operational characteristic and network stability need to be evaluated
conditions could be covered. In relation with substation carefully.
connections and other network nodes, critical network
REFERENCES
conditions could occur. Fault conditions and short-circuit
currents can be an issue, just to name a few. Furthermore, the [1] CIGRE Joint WG 21/22.01, “Comparison of High Voltage Overhead
Lines and Underground Cables – Report and Guidelines,” Cigre
load flow in a meshed network - as the transmission network Technical Brochure 110, December 1996
normally is - could be changed completely by the insertion of [2] CIGRE WG B1.07, “Statistics of AC Underground Cables in Power
cable lines caused by the different characteristic impedance. Networks,” Cigre Technical Brochure 338, December 2007
[3] M. Laussegger et al., “Italy-Austria GIL in the new planned railway
Finally, decisions about future lines in the transmission galleries Fortezza-Innsbruck under Brenner pass”, Cigre Session 2006,
sector can not be done in regard of one parameter like Paper B1-304
technical feasibility only. Especially ecological and eco- [4] Gatta, F.M.; Lauria, S., "Very long EHV cables and mixed overhead-
cable lines. Steady-state operation," Power Tech, 2005 IEEE Russia,
nomical issues as well as impacts on the operational pp.1-7, 27-30 June 2005

75