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10. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Ljubljana 2011
CIGRE ŠK B1 - 06
SODOBNI TRIŽILNI SN KABLI IN NJIHOV VPLIV NA ZNIŽANJE
SKUPNIH STROŠKOV
Håkan Bringsell
Ericsson AB
Falun, Sweden
hakan.bringsell@ericsson.com
Povzetek – Trižilni kabli vplivajo na znižanje skupnih stroškov s hitrejšo montažo in nižjimi izgubami v
obratovanju. V izogib pojavov vodnega drevesa morajo kabli biti popolnoma vodotesni.
Sodobni trižilni kabli morajo izpolnjevati številne zahteve. Izgube v kablu postajajo z naraščanjem cene električne
energije vse pomembnejši faktor. Pri enožilnih kablih se izgubam v ekranu kabla enostavno ni mogoče izogniti saj je
običajno ozemljevanje ekrana na obeh koncih kabla.
V članku so predstavljene metode kako izkoristiti prednosti trižilnih kablov in nekaj primerov znižanja skupnih
stroškov. Podana je tudi podrobna analiza električnih podatkov in montažnih parametrov.
MODERN 3-CORE MV XLPE CABLES AND THEIR IMPACT ON TCO
Håkan Bringsell
Ericsson AB
Falun, Sweden
hakan.bringsell@ericsson.com
Abstract – 3-core cables lowers the Total Cost of Ownership with shorter installation time and less losses. To avoid
the treeing phenomena the cables has to be totally water tight.
The modern 3-core cables must fulfill many demands of cable laying of today. As power cost increases the losses in
the cables are becoming a more and more important issue. In single-core cables where normally the screen is bonded
to earth in both ends there will always be screen losses.
This paper describes methods on how to achieve the benefits of 3-core cables and some examples of solutions that
have proved successful to lower the total cost of ownership. A more detailed comparison of electrical and installation
data will also be made.
10. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Ljubljana 2011
CIGRE ŠK B1 - 06
I.
INTRODUCTION
The Total Cost of Ownership for a medium voltage
cable in not only the investment cost of the cable it
self. There is also to take in consideration the cost of
during the whole lifetime of the cable. In this paper
the different impacts on Total Cost of Ownership
from a modern 3-core medium voltage cable will be
discussed.
Background
In the time of paper insulated medium voltage cables
a common lead sheath was needed to keep the oil in
place which led to the large use of 3-core medium
voltage cables. With the development of XLPE cables
in the 1960:ies the advantage of a common sheath was
not as obvious and single core cables became more
dominate.
To avoid the treeing phenomena, that were discovered
in the 1970ies, the cables needed to be totally water
tight. The modern 3-core cable should also be flexible
and easy to handle and at the same time tough for the
modern installation techniques. This is solved with an
aluminum-foil around the 3-cores, as seen in figure 1
below, and the outer sheath is reinforced with a hard
layer outside the cable body [1].
Figure 1 Cross section of a modern 3 core cable with
enforces outer sheath
II.
INSTALLATION TECHNIQUES
Ploughing
To make sure that the cable can be safely ploughed
down, without risking damage during the ploughing
time, the installation path should first be preploughed.
There are two ways to plough single core cables, one
way is to have three separate drums for each single
core cable. This has the advantage of being able to use
long length in each drum which might decrease the
number of joints needed. The disadvantages are that a
rather large machine is needed for handling of these
three drums and the preparation time with installation
of the drums on the ploughing machine will take
longer time. The other solution is to bundle the single
core cables into a tripack on one drum for only having
to have one drum on the machine ploughing the cable.
This will on the other hand limit the amount of cable
on each drum leading. For ploughing a 3-core cable
only one drum is needed on the machine and it is
possible to have rather good length of the cable on a
drum. After the cable has been ploughed down in to
the ground the soil only has to be pressed back either
by a tractor bucket or a vibrating foot which results in
a quite low impact on the environment.
Trenching
The other way to lay the cable in the ground is to first
dig a trench then place the cable in the trench. This is
an operation that requires a few more people
compared to ploughing the cable. After the cable is
laid the trench has to be re-filled again which is quite
time and manpower consuming.
III.
LOSSES
As power cost increases the losses in the cables are
becoming a more and more important issue. The main
losses in the cable will always be in the conductor
since all the load is in the conductor. Normally the
screen in medium voltage grids are solidly bonded, a
consequence of this is that a voltage will be induced
in the screen for single core cables with the result of
losses. In a 3-core cable there will be no losses in the
screen since it is enclosing the conductors. To be able
to compare the two different cable solutions, cables
designed according to HD 620 S2:2010 Part 10
Section M will be placed on the same laying depth.
The one core solution is laid in trefoil so the setup
will be rather similar, the cables can also be placed in
flat formation which leads to higher losses in the
screen. The calculations are based on a soil
temperature of 15 ºC, a thermal resistivity of 1,0
ºCm/W and a laying depth of 0,7m.
The current carrying capacity for a single core
solution is slightly higher and therefore the maximum
load for the comparison will be based on the 3-core
solution. A result of this is that the losses in the
conductors in the single core solution will be lower
since the temperature will be lower for the same load.
Even though the losses in the conductor are lower in
the single core solution the losses in the screen will
result in more losses in the single core solution as
seen in figure 3 below.
10. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Ljubljana 2011
CIGRE ŠK B1 - 06
Increased losses in 3 single core cables compared to three
core cables
0,6
0,5
kW/km
0,4
0,3
0,2
0,1
0
0%
20%
40%
60%
80%
100%
Percent of maximum load
Figure 3 Increased losses in a one core cable solution for a conductor area of 240mm2
A typical case
To illustrate the impact of the losses in the screen a
length of 20 km cable is used with the cost of 1 kWh
is 0,1 € and the inflation rate of 1%. The load cycle of
the cable is according to figure 4 and then calculated
with the difference in losses as mentioned before and
seen in figure 3. In a case with 20 years lifecycle the
cost due to higher losses in a single core solution
compared with a three core solution will be
approximately 40 117 €.
As seen in figure 3 the single core will have lower
losses over 90% of the maximum load however a
normal load for a medium voltage cable is quite
modest based in what the cable can handle. In figure 4
on next page is a typical load for a medium voltage
cables taken from Falu Elnät in Sweden during 2005
feeding a area of the city of Falun with approximate
40 000 inhabitants.
Load during 2005
45
40
Percent [%]
35
30
25
20
15
10
5
2005-12-17
2005-12-03
2005-11-19
2005-11-05
2005-10-22
2005-10-08
2005-09-24
2005-09-10
2005-08-27
2005-08-13
2005-07-30
2005-07-16
2005-07-02
2005-06-18
2005-06-04
2005-05-21
2005-05-07
2005-04-23
2005-04-09
2005-03-26
2005-03-12
2005-02-26
2005-02-12
2005-01-29
2005-01-15
2005-01-01
0
Figure 4 A typical load mean load per day for a medium voltage cable feeding the city of Falun in Sweden
IV.
REFERENCES
CONCLUSIONS
The cable will influence the total Cost of Ownership
during it’s whole lifetime not just only the investment
cost.
[1]
[2]
U. Johnsen, L. Efraimsson “Service and
installation experiences from modern 3-core
MV XLPE Cable” Slovenian Cigré Kransja
Gora 2009
U. Johnsen, L. Efraimsson “The contemporary
3-core medium voltage XLPE cables” Croatian
Cigré Cavtat 2007