B.10.4
8th International Conference on Insulated Power Cables
B.10.4
EVOLUTION IN METHOD AND PERFORMANCE FOR BONDING THE
METAL SCREEN OF UG HV POWER CABLE
David DUBOIS (1), Pierre MIREBEAU (1), Pascal STREIT (1), Didier LIEMANS (1), Mohamed MAMMERI (2),
Franck MICHON(3), Minh NGUYEN TUAN (4), Aude BARRALON(5)
1 – Nexans, david.dubois@nexans.com, pierre.mirebeau@nexans.com, pascal.streit@nexans.com
didier.liemans@nexans.com
2 – Sileccable mohamed.mammeri@sileccable.com
3 – Prysmian franck.michon@prysmian.com
4 – EDF minh-2.nguyen-tuan@edf.fr
5 – RTE aude.barralon@rte-france.com
ABSTRACT
The paper presents the economical reasons for
expanding the cable shipping length, the different
technical obstacles that have to be overcome and the
resulting evolution of the performances required by the
increase in potential stresses applied to the secondary
insulation.
Different novelties in bonding diagram such as “direct
cross bonding” are explained, highlighting their benefit as
well as their prerequisites and drawbacks.
The use of longer shipping lengths as well as of “direct
cross-bonding” method supposes that an accurate and
thorough study of the electrical stress applied to joint
shield break is carried out as part of the basic design of
the link.
In fact the limitation on maximum sheath standing
voltages put a brake to an increase of the unitary shipping
length. Some countries were limiting the maximum
standing to values as low as 65V. Some are still. The
graph in Fig.1 shows that keeping such limit prevents any
kind of change in the maximum allowed distance between
joints. However the demand for UG transmission line
tends toward larger transmission capacity then the
problem of standing voltages has become more and more
penalising.
In France the maximum allowed standing voltage for line
where public access is prohibited is 400V. That particular
feature of the French regulation makes possible the
extension of shipping lengths up to 2000m and more.
New concept of the electrical components around the
cable joint is presented and its benefits on classic design
are explained.
PRESENT AND FUTURE
Economy is leading the world. For many years the
standard shipping length for UG HV and EHV cable was
in the range of 500m, sometimes less.
This value was due to different limitations such as:
•
Production equipment like impregnating tanks, takeup units, etc.
•
Testing equipment like maximum current of test
transformers or maximal capacitance of the test
object
•
Transport in term of drum size and weight
•
Last but not least regulation restriction on the
maximum standing voltages along the metal
sheath/screen.
By having longer lengths per shipping drum the hardware
cost, the labour cost and the civil work cost are reduced.
Costs of maintenance are also linked to the number of
accessories along a UG power line.
• Civil work cost
It is including the cost of joint bays and associated pits
that house the cross-bonding boxes.
• Labour cost
Mounting joint can represent 25 percent of the total
installation cost.
• Hardware cost
Hardware encompasses the joints with their possible
monitoring system as well as the bonding leads and
cabinets with or without SVL’s.
Fig.1 induced voltage versus the ratio of cable spacing
over metal screen diameter
When the obstacle of maximum standing voltage is
removed there is still to address the problem of induced
voltage under short-circuit condition.
Combining longer lengths and short-circuit current that
can reach 63kA and even more makes necessary to take
care of the rating of the SVL that are protecting the screen
interruption of joints.
Another direction to reduce the cost of UG cable power
line is to decrease the amount of protections put on the
different joints along the route, irrespective of the section
length between them. This second track has been called
“direct cross-bonding”.
.
Jicable’11 – 19 – 23 June 2011, Versailles - France
B.10.4
8th International Conference on Insulated Power Cables
Both directions towards more economical UG power lines
have the following prerequisites:
•
A very good and reliable knowledge of the network
(overhead lines and their design, resistance of earth
grids, short-circuit current and duration, etc…),
•
Data on impact of thunder lightnings and their effects
(magnitude, frequency, seasonal and local variation) .
Evolution of the Cross-bonding design in
France
Sectionalised cross-bonding is used [3]. Cables are
transposed to limit induced voltages in neighbouring
networks. The screen interruptions are protected by surge
arresters connected to the shield break joints through
coaxial bonding leads, maximum 10 m in length. These
arresters are star connected, and the neutral point is
grounded. They are located in a manhole, designed to
contain the effect of an internal arcing.
Up to now, the nominal voltage was limited to 6 kV.
In order to increase shipping lengths, a study has been
lead.
The results confirm:
- The possibility to use a 12 kV arrester instead of the
6 kV ;
- The need to test equipments according to the withstand
levels specified in the French standard C-33-254 [4]
given in Table 1.
For impulse levels, international recommendations from
CIGRE and IEC are given in brackets, if they are different.
In addition, a.c. withstand levels are also required, and
a.c. tests are carried out as type tests
.
CROSS-BONDING DESIGNS
Generalities
Cross bonding consists essentially in sectionalizing the
cable sheaths along the route into elementary sections
and cross connecting them so as to approximately
neutralize the total induced voltage in three consecutive
sections.
Lightning strokes cause the propagation of surge waves in
underground links they are connected with.
The magnitude of the overvoltages that stress the screen
interruptions at cross-bonding locations decreases with
the distance to the end.
As a consequence, the overvoltages in the middle
sections are likely to be lower than the withstand level of
the screen interruption in shield break joints [1] [2] even if
not protected by SVL.
Nominal voltage of the link (kV)
B.10.4
36/63 (72,5)
52/90 (100)
130/225 (245)
230/400 (420)
325
450
1050
1425
Screen to ground impulse withstand level (kVc)
50 (30)
50 (37,5)
50 (47,5)
62,5
Impulse withstand level for screen interruption (kVc)
80 (60)
80 (75)
100 (95)
125
Screen to ground a.c. withstand level (kV) (15min.)
20
20
20
20
a.c. withstand level for screen interruption (kV 15min.)
25
25
25
35
Lightning impulse voltage for main insulation
(kVc)
Table 1 : Withstand levels specified in French standard C-33-254
Direct cross-bonding
INSULATION COORDINATION STUDIES
In the case of the so called “direct cross bonding”, the
reliability of the system is totally dependent on the
integrity of the insulating shield interruption. The dielectric
strengths, and the ac, impulse, and switching surge
withstand levels of the interruptions have to be
established and coordinated with the calculated
magnitudes of ac and transient voltages for the circuit.
To compare the overvoltages likely to occur in the grid
with the withstand level of shield break joints; studies
were carried out, using EMTP-RV software. Main features
and results are presented below.
Description of the configuration
A single-circuit siphon is considered (see Fig.2).
Span length = 400m
Network
Overhead
Line
Overhead
Line
Fig. 2: studied configuration
Jicable’11 – 19 – 23 June 2011, Versailles - France
B.10.4
8th International Conference on Insulated Power Cables
The overhead line is protected by 2 sky wires with a D.C.
resistance 0.24Ω/km. The towers height is 20m, the
grounding impedance is 10Ω. The withstand voltage of
their insulator strings is 900kV for 225kV grid.
1600mm² Cu XLPE cables are laid in ducts in trefoil
formation (0.2m spacing)
The soil resistivity is 100Ω.m.
The main insulation of the cable is protected by surge
arresters installed at both terminals, connected by a 3m
long lead to the grounding electrode of the first tower.
The screen interruptions are protected by 12kV surge
arresters.
Modelling
Connections between the overhead line and the
underground cable
They were represented as lumped inductances (1 μH/m).
Surge arresters
Surge arresters were modelled as a non-linear element
with U(I) 8/20μs characteristic of the arresters. An
inductance added to the connection account for the
change of characteristic when steeper fronts are
considered.
Surge arrester leads
Lumped inductances were used.
The modelling follows IEC 60071.4 [5] recommendations.
Lightning stroke
The CIGRE Concave [8] model was used.
Representation of the sections of the underground cable
The sections of the underground cable were represented
using the FDQ model [6] [7].
Results
Overhead line
Spans in the vicinity of the siphon were represented using
the FDLINE model. Corona effect has not been taken into
account. Spans far from the siphon have been
represented by single long line avoiding unrealistic
reflections.
Towers were represented as loss-less lines. The
characteristic impedance is 150Ω; the wave velocity has
been taken equal to the velocity of light in vacuum.
The grounding electrodes of the towers were represented
as a constant resistance, except for those just before the
underground cable, which has been represented taking
into account soil ionisation [2].
Air gap
They were represented as an ideal switch closing when
the voltage between its terminals reaches the withstand
voltage of the air gap.
B.10.4
The upper curves show the phase to remote earth voltage
at cross-bonding locations and the lower curves display
the voltage stressing the screen interruptions.
In the first case, SVL are installed only at the 2 ending
major sections; in the second case, SVL are installed
within the 3 sections. The magnitude of the overvoltages
is roughly the same.
The sheath overvoltages within the second major section
exceed 62.5kV only for lightning strokes with high
magnitude. One has to keep in mind that the computed
overvoltages refer to the remote earth, so that
overvoltages applied to the sheath are significantly lower.
The screen interruption overvoltages are lower than
100kV, with or without surge arresters.
Results of same study for 225kV lines were published in
CIGRE Technical Brochure 283 [2]
It is worthy to remind that the median lightning current
magnitude is about 30 kA and the probability of a current
higher than is 150 kA is about 0.05 %.
Fig 3: Study for a 400kV siphon – 3 major sections.
Jicable’11 – 19 – 23 June 2011, Versailles - France
B.10.4
8th International Conference on Insulated Power Cables
through the earthings do not lead to excessive
overvoltages at the cross-bonding points of crossbonded links.
POWER FREQUENCY ISSUES
Basic features
For trefoil cables, the voltage of each screen to earth
during a 3-phase symmetrical fault (external to the cables)
is given by the well-known formula Electra 28 [9] or 128
[10]:
E=
ω.μ  2.S 
.Ln
.L.I
2.π
 d 
where ω is the circular frequency, μ is the magnetic
permeability of vacuum, d is the screen mean diameter, S
is the spacing between cables, L is the length of minor
sections and I is the short-circuit current.
Voltages between screens which stress joints screen
interruptions are equal and are given by √3 times the
voltage of each screen to earth E.
In case of phase to phase fault, the theoretical maximum
voltage between screens is 2 times E [9], but, in practical
situations screen currents flow in the screens and reduce
the screen voltages below this level [10].
During a single phase earth fault, external to cables,
screen to earth voltages depend strongly on the earthing
resistances at the ends of the circuit which determine the
return current path, but voltages between screens may be
easily derived since the return current through the screens
divides between the 3 screens in parallel. In that case the
maximum voltage is simply E.
Electra 128 states that, in case of single phase to earth
fault, internal to cables, the maximum voltages between
screens do not exceed those due to external fault.
This is more or less in line with the CIGRE Brochure 347
[11] which states that the maximum voltage may be larger
than the voltage occurring in three -phase faults if:
• The screen resistance is “high”.
• The fault is fed mainly by one side.
• The magnitudes of the 3-phase and the phase to earth
short-circuit currents are similar.
B.10.4
•
In 225 and 400kV links between two substations, EPR
are not excessive provided that earth resistances in
the substations are low and similar (typically less than
4Ω).
•
In 225 and 400kV systems involving an underground
link connected to a substation and an overhead line, or
a siphon, excessive overvoltages are likely to occur at
the faulted underground link to overhead line
transition, if the fault current is close to the nominal
value.
In this latter case, the installation of an earth continuity
conductor is an efficient solution to limit overvoltages that
can appear on sheath SVL.
However, in 400kV links, EPR at underground link to
overhead line transitions lead to overvoltages on cable
oversheath and joint covering exceeding their rating, if the
fault current flowing through the overhead line is higher
than about 30kA. It is then possible to limit the metal
sheath potential rises by reducing significantly the length
of minor sections.
The above conclusions do not apply to underground links
connected to an overhead line without skywire. In such a
configuration, excessive overvoltages may occur even if
earth resistances are low.
NOVELTIES IN BONDING DIAGRAM
Cross-bonding design is usually made by concentric
bonding leads with each conductor connected to each
cable screen in screen interruption HV joints and
connected to a cross-bonding link box.
The link boxes can be either installed underground
(directly buried or inside a dedicated manhole) or on a
frame above the ground (which is not recommended).
Each of these link boxes is equipped with surge arresters
which are here to limit over voltage either on outer sheath
or on joint screen interruption.
So, for design purpose, the highest power frequency
voltage between screens can generally be taken as √3.E.
Earth Potential Rises
As already mentioned, the calculation of the screen to
earth voltages which stress joints coverings and SVL is
easy only in case of 3-phase faults.
Electra 128 and CIGRE Brochure 347 point out some
configurations where earth potential rises may lead to
excessive screen to earth voltage with respect to the
withstand level of SVL if they are star connected with the
star point earthed.
Power-frequency over-voltages likely to occur during
single-phase to earth faults in the French HV and EHV
networks were determined with the Complex Impedance
Matrices method, as presented in CIGRE Brochure 347.
The following conclusions were drawn:
•
Fig. 4. Standard cross-bonding connection with concentric
bonding leads
In 63 and 90 kV links, as the single-phase short-circuit
current is smaller than the three-phase short-circuit
current, EPR caused by the fault current flowing
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Jicable’11 – 19 – 23 June 2011, Versailles - France
B.10.4
8th International Conference on Insulated Power Cables
Advantages:
•
•
•
Short lengths of cable outer sheath can be tested
separately (between each shield break joint)
Easy fault localization (by screen disconnection inside
link boxes)
Disadvantages:
•
•
•
Requires SVL maintenance
Above ground link boxes can be easily damaged
Risks in case of link box internal arcing
B.10.4
Fault localization is more difficult as a test made on
outer sheath integrity change from one phase to
another phase at each shield break joint.
To avoid this drawback the following departures are
applied
• For very long links, in order to divide the link in shorter
segments some earthed joints with screen interruption
have to be used. The 6 single-core bonding leads
(concentric cables can be used as well) are connected
to earth via an earthing box.
As described in the previous section this design needs to
remain on first major section on each end of long
underground cable links.
But studies have shown that such cross link boxes can be
avoided between the 2 extreme major sections by doing a
so called "direct cross-bonding".
This design consists to use single core bonding leads to
make joint screen cross-bonding directly from one joint to
the others inside the joint-bay.
Fig. 6 Bonding to earth through earthing box using
concentric leads with possible disconnection
Fig. 5 Direct cross-bonding connections
One positive feature of the standard cross-bonding
system is that owing the different link boxes it is easy to
locate sheath fault. A fully direct cross-bonding system
would not provide easy fault location because:
•
Practical experience with direct cross bonding has been
achieved on 2000mm² Alu XLPE 150kV links as shown in
figure 7. The shield break joint has been successfully
tested according IEC 60 840 tests of outer protection
(Annex H) where the sheath sectionalizing insulation has
been submitted to 150kVp impulses level instead to
75kVp.
Outer sheath tests cannot be segmented.
Fig 7. Earthing System using Direct Cross Bonding
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Jicable’11 – 19 – 23 June 2011, Versailles - France
B.10.4
8th International Conference on Insulated Power Cables
B.10.4
NEW CONCEPT FOR ACCESSORIES
The use of the direct cross-bonding system makes
necessary new types of accessory as shown on the
following schematic diagram
Shield
break
joint
Earthing
joint
Shield
break
joint
Shield
break
joint
Earthing
joint
Earthing
cabinet
Earthing
cabinet
Cross bonding
cabinet
Shield
break
joint
Shield
break
joint
Cross bonding
cabinet
Cross bonding
cabinet
Shield
break
joint
Unit section repeated N times
Fig. .8. Schematic diagram of a UG power cable line featuring some direct cross-bonding sections [12]
Sections between A to D and G to J are with a standard
cross-bonding system. Sections between D and G use the
direct cross-bonding system. Three different types of
joints and their associated hardware are represented:
•
•
Joints with direct connection to earth
Joints with cross-bonding connections : the screen
interruption is protected with SVL’s
•
Joints with direct cross-bonding connections : the
screen interruption is not protected against
overvoltages.
Different technological solutions can be applied to
address the functional requirements listed above. Three
different specifications for bonding leads are identified:
•
Single core bonding lead directly connected to earth
•
Single core bonding lead to connect two joints
•
Coaxial bonding lead for classic cross-bonding
connection between joint and cross-bonding box.
These three types have a different technical specification
based on their functional requirements.
In principle they have to feature the same performances
as the different components which they are bonded to.
CONCLUSION
To achieve more economical long HV UG lines on the
French network the design rules have been studied to
cover longer cable length between accessories and less
protection hardware for the screen interruptions.
Meanwhile the performances of the material has been
improved and so the tests to check them. Namely a.c.
voltage is now part of the test program.
To verify that the incurred increase of potential stresses
on the joints remains acceptable thorough studies of the
induced voltages under surges conditions were
necessary.
RTE is currently studying their optimisation to the French
power transmission system.
GLOSSARY
UG: Underground
HV-EHV: High Voltage, Extra High Voltage
SVL: Surge Voltage Limiter or surge arrester.
EPR: Earth Potential Rise
REFERENCES
[1] A.Gille et al “Double 150 kV link, 32km long, in
Belgium : design and construction” - 2004 – CIGRE
report B1-305
[2] CIGRE Technical Brochure 283 Special Bonding of
High Voltage Cable Oct. 2005
[3] CIGRE 2000 Dorison et al - 400 kV underground links
for bulk power transmission. New developments in
the field of XLPE insulated cables
[4] French standard NFC 33.254 Insulated cables and
their accessories for power systems
[5] IEC 60071.4 – “Computational Guide to Insulation
Co-ordination and Modelling of Electrical Networks”
2004
[6] EMTP-RV online documentation
[7] Hermann Dommel – EMTP Theory Book
[8] CIGRE 63 brochure – Guide to procedures for
estimating the lightning performance of transmission
lines
[9] The design of specially bonded cable systems
against sheath overvoltages – Electra 28 – 1973
[10] Guide to the protection of specially bonded cable
systems against sheath overvoltage – Electra 128–
1990
[11] CIGRE Technical Brochure 347 Earth Potential Rises
in specially bonded systems - June 2008
[12] French dispositions for bonding the metal screen of
UG HV power cable – RST part 7 – October 2010
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Jicable’11 – 19 – 23 June 2011, Versailles - France