THE USE OF INSULATED WIRES MILLIKEN CONDUCTORS IN HIGH VOLTAGE POWER TRANSMISSION UNDERGROUND AC LINES. x y s ABSTRACT

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1 THE USE OF INSULATED WIRES MILLIKEN CONDUCTORS IN HIGH VOLTAGE POWER TRANSMISSION UNDERGROUND AC LINES David DUBOIS, NEXANS, (France), Pierre MIREBEAU, NEXANS, (France), ABSTRACT These last years have ointed out a significant increase of the ower that needs to be transorted by underground ower cables. As a consequence, large cross section conductors, u to mm² are now customarily used. The CIGRE grou B1-03 has comleted in 2005 a work that analyses the actual AC resistance of these large cross section conductors as a function of their design. The authors discuss about the imlication of this work on the design of tyical high ower transmission lines, from the cable roduction, accessories, unit length, transortation, civil works, and oeration. They sketch the limits of use of each conductor design as a function of the rojects key arameters. They give the AC resistance measurement results on some conductors of different designs. KEYWORDS HIGH VOLTAGE, CONDUCTOR, POWER LINK INTRODUCTION These last years have ointed out a significant increase of the ower that needs to be transorted by underground ower cables. As a consequence, large cross section conductors, u to mm² are now currently used. These conductors have diameters in the range of 60mm, which is not small as comared to the skin effect deth. CIGRE set u in 2002 a working grou B1-03 to assess the AC resistance of large conductors. Its final reort was released in 2005 in the Technical Brochure 272 [1]. It showed the interest of Milliken conductors with insulated. We will recall the conclusions of this work and give ractical examles of the use of the recommended conductors. AC RESISTANCE OF LARGE CONDUCTORS CIGRE WG B1-03 chose a ragmatic aroach that is based on measurements. It built the following table: Tye of conductor k s Value k Value For coer enamelled and aluminium For coer oxidised (value based on study for unidirectional only) For inter layer insulated coer For uni-directional stranding of coer bare For bi-directional stranding of coer bare The AC to DC resistance of a conductor is given by the formula : R=R'(1+y s +y ) x y s x y with ωμ x ² =. k = with s π. Rdc x dc = x s dc ωμ x ² =. k π. R d c is the conductor diameter and s is the axial distance between conductors. Let's concentrate on the skin effect, as in ractical conditions; the roximity effect is smaller than 1/10 th of the skin effect. The authors have manufactured cables of mm², mm² and mm² cross section, which are in accordance with the last release of IEC The resistance of these cables has been measured using the reviously reorted method [2, 3] The samles length is 12m. All conductor are connected together at each end, the distance between voltage robes is 8m. 2

2 The following table gives our measurements, at 90 C, as comared to IEC and CIGRE formulas. ( AC90resistance) ( DC resistance) 90 CONDUCTOR TYPE Round Standard Milliken Bi-directional stranding milliken IEC CIGRE* Authors IEC CIGRE authors IEC CIGRE Authors** mm² 1.30 na mm² 1.0 na mm² 1.53 na *out of the scoe of WG B1-03 ** authors recision of results is ± 0.05 (the main cause of error is the conductor temerature recision and homogeneity). The table shows the quality of the CIGRE WG work. Calculated skin effect is very close to measurement. The IEC formula when alied to the large cross section gives otimistic results. IEC secifies that the given arameters are valid for cross sections less than mm². COMPARISON OF DIFFERENT SPECIFICATIONS To evaluate the benefit of imroved conductor construction; it is necessary to comare the erformance of standard design (bare coer wire) and insulated wire design (enamelled wire in this study) cables. The cables are 00kV cables, XLPE insulated, butt to butt welded aluminium laminate screen, HDPE oversheath. Such a comarison will serve as a guideline throughout this aer and for each feature the main conclusions shall be drawn. Table for comarison In the following table are given the main characteristics and erformance in the same laying arrangement of different cross section cables. The conductors are suosed to be with a Milliken construction, bi-directional stranding, with 6 segments. The calculation of the a.c. resistance of conductor have been based on the skin effect and roximity effect coefficients k s and k as recommended in the CIGRE Technical Brochure n 272 [1] i.e.: o k s = 0.8 and k = 0.37 for bare coer conductor o k s = 0.25 and k = 0.15 for coer enamelled The laying arrangement is the same for every cable tye: o Two arallel circuits o Flat formation, cable sacing: 350 mm o Burial deth: 1300 mm o Soil thermal resistivity: 1.0 K.m/W o Ground temerature: 20 C Table 1: Raw data In the following table the e suffix stands for enamelled wire i.e. a conductor having in its construction a sufficient number of insulated to revent any electrical contact between adjacent. Losses are given at rated current for one circuit. Conducor area and tye mm² e e e Overall Diameter mm Weight kg/m 20,3 20,3 2,85 2,85 29,27 29,28 35,8 36,38 39,7 39,7 Current carrying caacity A 1208,0 1271,9 1351,0 163,0 160,9 1626,5 1558,5 1780,0 1630,0 1907,5 Transmitted ower MVA 836,9 881,2 935,9 1013,0 1012,1 1126,9 1079,7 1233, 1129,6 1321,5 Losses at maximum current kw/km 106,9 107,0 109,9 110,2 113,3 113,7 116,0 117,1 118,1 119,1 AC resistance at 90 C µω/m 21,73 19,52 17,501 1,69 15,055 11,928 13,116 9,72 11,916 8,315 Rac/Rdc - 1,129 1,01 1,215 1,025 1,312 1,039 1,3 1,061 1,55 1,087 length over one drum m

3 CABLE OVERALL DIAMETER Excet for the mm² the construction of which is different for the insulated wire tye introducing enamelled in the conductor does not change the conductor diameter (small thickness of the enamel layer). This is shown on the following bar grah. e e e CABLE WEIGHT Cable Overall Diameter Diameter (mm) For the same reason there is little change in cable weight between the two designs (small weight of the enamel later). CURRENT CARRYING CAPACITY This is the most interesting feature of cable with insulated wire conductor. Due to the lower skin and roximity effect factors the amacity of the cable with insulated is often higher than the one of the bare coer cable with one crosssection higher. E.g. a mm² coer with insulated cable exhibits a higher current carrying caacity than a mm² standard cable. For cross-section lower than mm² this rule does not aly. But for all section including mm² and above the enamelled construction is always better than the bare coer construction with one cross-section uer. Conductor area (mm²) e e e Current carrying caacity e e e Cable Weight Bare Coer Current (A) LENGTH OVER ONE DRUM In order to evaluate the benefit of having a smaller cable for the same current carrying caacity we have comared the cable length that can be accommodated on a tyical drum whose dimensions are: o Flange diameter:.5 m o Overall width: 2.5 m The barrel diameter DB of that drum has been adated to each cable according to the following rule: o DB 20 x cable OD Weight (kg/m)

4 e e e Length er individual drum Length on one drum (m) Again there the great advantage of insulated is using a smaller cable for the same amacity. It allows lengths between joint to be increased by u to 0%. CABLE LOSSES In table 1 all the losses are calculated for the maximum ermissible current that is to say for the current that lead to a conductor temerature of 90 C. It is remarkable that the conductor losses at 90 C do not vary very much neither with the cable size nor with the current. All cables have very similar thermal internal resistances and the quantity of heat they can dissiate in a given environment is almost constant. In fact it only varies with the cable external diameter. Cable losses vs cable diameter at 90 C conductor temerature Cable losses for 3 hases (W/m) 120,0 118,0 116,0 11,0 112,0 110,0 108,0 106, Cable diameter (mm) almost in the same roortion as the a.c. resistance. e e e A.C. resistance at 90 C OVERLOAD CAPACITY A.C. resistance µohm/m One might wonder whether the overload caacity of a cable with insulated is as high as the one of a standard cable of same current carrying caacity. Indeed cables with insulated being smaller and lighter their thermal caacitance is smaller. However their losses being lower than those of a cable with a standard conductor (with one cross-section bigger) their overload caacity is very close and even better for the very large cross-sections. This is ilustrated in the two following grahs. e e e Overload caacity Overload for one hour (A) Insulated For a given current, the total losses can be reduced by much when using an enamelled wire conductor, in fact

5 e e Overload caacity conductor need secial techniques, however this is by far comensated by the lower rice of cable and by the reduced number of accessories with is a direct consequence of longer unitary drum length. The cable relative cost er transmitted MVA show a clear advantage for the insulated wire Milliken conductor design. If the economics of the comlete link had been taken into account, including transortation, civil and installation works, the benefit would have been even higher. Relative cost er transmitted MVA e Overload for five hours (A) Insulated The following hyotheses were taken: Start from maximum load (90 C at conductor). Maximum overload conductor temerature: 100 C. The calculations were carried out according to IEC CABLE COST Cable cost is very sensitive to coer cost and also to the other materials cost. The benefit of the insulated wire technology increases when the raw materials are exensive. This is the case at resent time. relative cost (mm² = 100) 20,0 220,0 200,0 180,0 160,0 10,0 120,0 100,0 bare coer Milliken insulated w ires Milliken 800,0 1000,0,0 100,0 MVA Cable cost e e e % Bare Coer In the above diagram it is shown that conductor from mm² to mm² with enamelled wire are less exensive than one cross-section above cables. Installation cost er accessory are slightly higher for enamelled conductor because the connection of enamelled

6 ENVIRONMENTAL IMPACT The Environmental Imact has been comuted using the EIME software. Its resentation can be found in ref. []. We considered only the making hase. We comared the mm² bare coer cable with the mm² enamelled one. manufacturing environmental imact Hazardous Waste Production Water Eutrohication Water Toxicity Air Acidification Photochemical Ozone Creation Air Toxicity Ozone Deletion Global Warming Water Deletion Energy Deletion Raw Material Deletion 0,8 0,85 0,9 0,95 1 1,05 1,1 relative imact (1=mm² cable) The making of the enamelled cable has globally a lower imact on the environment than the bare coer one. This effect is even more ronounced when taking into account the installation and civil works (smaller cable diameter, longer delivery length). The only drawback at the manufacturing stage regards the hazardous waste roduction. Although this waste roduction is very limited. Studies are in rogress to reduce it even further. REFERENCES [1] CIGRE Working Grou B1.03, TB 272, June 2005, "Large Cross-sections and Comosite Screens Design". [2] 00 kv underground links for bulk ower transmission. New develoments in the field of crosslinked olyethylene insulated cables. E. DORISON - Y. MAUGAIN - P. ARGAUT - P.M. DEJEAN - P. MIREBEAU Reort CIGRE. [3] A new method for the measurement of ower cables AC resistance. BOURGEAT X. - SANTANA J. - FUSTIER A. - DEJEAN P.M. B 8.1. Reort - JICABLE 1999 [] Use of an ecoconcetion software to design a hv cable connection. Pierre MIREBEAU, Martine.DUVIVIER, Laurent BENARD, François LE BOURDON, Francis POILLEAUX, Bernard.POISSON, JICABLE 2007 GLOSSARY Amacity: Neologism for current carrying caacity. UG: Under ground CONCLUSION By using imroved Milliken conductors with insulated gains are achieved in almost all technical and economical asects of ower UG cable transmission: For a given requirement in terms of current carrying caacity (i.e. transmitted ower), we obtain: o Smaller cable o Lighter cable o Higher current carrying caacity o Lower losses o Same or better overload caacity o Lower cost o Reduced Environmental Imact conductor connection need secial techniques but this is widely comensated by the saving in cable rice and number of joints. This new technology has been long term tested, tye tested and imlemented for a mm² XLPE 00 kv cable roject in Abu Dhabi including 12.7 km of circuit. Because of a high ower demand (1000MVA er 3-hase circuit) and a very tough environment (0 C soil temerature) this roject would not have been ossible without this new technology.