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B.10.4 8th International Conference on Insulated Power Cables B.10.4

Jicable’11 – 19 – 23 June 2011, Versailles - France

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, [email protected], [email protected], [email protected]

[email protected] 2 – Sileccable [email protected] 3 – Prysmian [email protected] 4 – EDF [email protected] 5 – RTE [email protected]

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.

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, take-

up 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.

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.

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”. .



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) .

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.

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 .

Nominal voltage of the link (kV) 36/63 (72,5) 52/90 (100) 130/225 (245) 230/400 (420)

Lightning impulse voltage for main insulation (kVc) 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

Table 1 : Withstand levels specified in French standard C-33-254

Direct cross-bonding

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.

INSULATION COORDINATION STUDIES

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).

Overhead Line

Overhead Line

Span length = 400m

Network

Fig. 2: studied configuration



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 The modelling follows IEC 60071.4 [5] recommendations. Representation of the sections of the underground cable The sections of the underground cable were represented using the FDQ model [6] [7]. 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.

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. Lightning stroke The CIGRE Concave [8] model was used.

Results 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.


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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]:

ILdSLnE ...2.

.2

.

=

πμω

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.

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:

• 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

through the earthings do not lead to excessive overvoltages at the cross-bonding points of cross-bonded links.

• 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.

Fig. 4. Standard cross-bonding connection with concentric bonding leads


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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

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. 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:

• Outer sheath tests cannot be segmented.

• 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.

Fig. 6 Bonding to earth through earthing box using concentric leads with possible disconnection

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.

Fig 7. Earthing System using Direct Cross Bonding


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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

Shield break joint

Shield break joint

Shield break joint

Shield break joint

Shield break joint

Earthingjoint

Earthingjoint

Cross bonding cabinet

Cros

s bon

ding

ca

bine

t

Cros

s bon

ding

ca

bine

t

Unit section repeated N times

Earthingcabinet

Earthingcabinet

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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