Development and Supply to Denmark's NKT ofPrefabricated Joints for 420-kV XLPE Cable
by Teruyoshi Tanabe *, Shigeru Suzuki *2, Shin'ichi Gotoh *2,Masahiro Iizumi *2, Takao Nakano *3, Masakazu Ogawa *4,
Shin'ichi Fukano *4, Hiromi Kumagai *5 and Makoto Nakamura *5

In Denmark a massive project is under way to install underground UHV power

ABSTRACT transmission lines to replace aerial lines. In Phase I of this project Furukawa Elec-tric has developed epoxy-unit prefabricated joints (PJs) for 420-kV XLPE cable. Prior to delivery, thePJs successfully passed type approval tests according to IEC standards as well as a 1-year loading
test at CESI (Centro Elettrotecnico Sperimentale Italiano). Cable connection work began in April
1996 involving the installation of 72 phases, and the line went into commercial service in October
1997. These PJs are the first in this voltage class to be used with XLPE cable on long-distancelines.

1.

INTRODUCTIONIn Denmark during the years of 1991 to 1992, there was aheated controversy about the adverse effects of electromagnetic fields on human body, which might be causedby overhead transmission lines. The controversy developed into a social problem that included arguments about
the beauty of supporting structures in addition to depreciation of real estates in the neighborhood of transmission
lines. Against this background, "The Metropolitan PowerProject", a 420-kV power grid construction plan for theeastern part of Denmark, was launched to install underground the transmission line linking the urban area ofCopenhagen with its suburbs.
The construction project started in 1996, in which NKT
Cable Co. and Furukawa Electric were in charge of developing a 420-kV XLPE cable and an epoxy-unit type prefabricated joint (PJ), respectively. The PJ of this type was
the first in the world to be applied for such a long distance
XLPE cable working in the 420-kV class1).

* Power Cable Engineering Dept., Power Cable Div.
* 2 2nd Engineering Sec., Power Cable Engineering Dept.,
Power Cable Div.
* 3 Power Cable Equipment Sec., Chiba Research Lab.,R&D Div.
* 4 1st Quality Assurance Sec., Quality Assurance Dept.,
Power Equipment & Distribution Div.
* 5 Production Engineering Sec., Equipment Production
Dept., Power Equipment & Distribution Div.

2.

OUTLINE OF "THE METROPOLITAN
POWER PROJECT"

Located in the North Sealand of Denmark as shown inFigure 1, the project comprises the southern transmissionline, the northern 420-kV transmission line, and the substations, and it is scheduled to be completed by 2003, byinstalling all the transmission lines underground. "TheMetropolitan Power Project" is planned and promoted
jointly by Copenhagen Lighting Department Co. (CLD), apower utility company in Denmark and North East Electricity Supply Authority (NESA), its counterpart in the northeastern Denmark; and its first phase is the construction ofthe southern transmission line underground that extends
from the city of Copenhagen westward linking to the 420kV overhead transmission line in the suburbs.

Table 1

Outline of the 420 kV XLPE cable line

Voltage

420 kVCable

XLPE 1,600 mm2

Number of joints

72 sets

Number of circuits

1 circuit

Route length

22 kmTransmission capacity

800 MW

Construction period

1996/4~1997/8(1997/10 operation start)

51

The transmission network in Denmark

Lyngby-tårbæk
Stenløse

NESA

VærløseNESATO SWEDENTO NORWAY

Eksisterende 400 kV luftledning
Planlagt 400 kV luftledningPlanlagt 400 kV Kabel

Glentegård
Main transformerstation
Existing line
Planned line

TO SWEDEN

Gentofte

HoveBallerup

Ledøje-Smørum

Herlev Gladsaxe

Gundsø

Københavns
Belsysingsvæsen

København

Høje-Tâstrup

Albertslund

Ishøj

Glostrup

RødovreFrederiksberg

Brønbdy
Vallensbæk

H.C.Ørsted
VærketHvidovre

Ishøj
GREAT
BELTTårnby
Greve

AvedøreDragørThis route

TO GERMANYTO GERMANY

Figure 1

Route diagram of "The Metropolitan Power Project"

4. DEVELOPMENT OF PJ

Figure 2

3.

Cable layout

OUTLINE OF THE CABLE LINE

3.1 Outline
Table 1 gives the outline of the cable line2). The use of oilfilled cable was avoided because of possible environmental contamination in the case of an oil leakage, and XLPE
cable was chosen to be applied for the line. The line isconsiderably large scaled for an XLPE cable of this class,with its 22 km cable length and 72 sets of joints. The cablelaying follows a direct burial method with a layout as shownin Figure 2.
3.2 Structure of Joint
The joint adopted is epoxy-unit type PJ, with its structure
shown in Figure 3 and exterior appearance in Photo 1.
The insulator system of the joint consists of an epoxy-unit
and pre-molded insulators, whereby the latter is compressed against the former by a compression device sothat a specified surface pressure is provided at the interface to ensure insulation. The epoxy-unit, pre-molded insulators, and compression devices are all enclosed in a
metal case, both ends of which are lead soldered onto thelead sheath of the cable.

52

4.1 Selection of joint type
Two types of joints, extrusion-molded and prefabricated,were investigated as candidates for cable joint to be applied in this project. Extrusion-molded joints for 275 kV
rating are already in practical use in actual lines and thedevelopment of those for 500 kV have completed successfully. In contrast, although PJs for 275 kV rating are in service in actual lines, those for 500 kV are now under development. However, we decided to employ PJ for the present
project for the following reasons.
(1) Experienced jointers are not needed.
(2) Facilities for the work are not very expensive.
(3) Working hours can be shortened.
(4) Constituent parts are manufactured under quality control at the factory. Main insulators, the epoxy-unit andpre-molded insulators, are manufactured at the factory
and are totally tested.
(5) PJ is equivalent to extrusion-molded joint in total performance.
4.2 Design of PJ
4.2.1 Insulation Design
At the insulation design of 420-kV PJ, its electric stress
distribution for each region was designed with reference
to the stress obtained with the 275-kV PJs3). In the designof PJ, electric field stresses at epoxy-unit body (τ 1), at theinterface between pre-molded insulator and epoxy-unit (τ2), and at the rise-up region (τ 3) are major design factors.
Table 2 shows the comparison. Insulator geometry wasfinally determined by electric field analysis based on thedata hitherto obtained. Figure 4 shows the results of electric field analysis.
4.2.2 Water Proof Design
Since the line is installed directly underground, the entire
PJ is housed in an FRP coffin box prepared by NKT, whichis then filled with water proof compound.

Furukawa Review, No. 18. 1999

Mark

Parts nameMark

Parts name1

Ferrule6

Insulating flange

2

Epoxy-unit

7

Intermediate flange

3

Fixture for epoxy-unit

8Compression device A4Pre-molded insulator

9Compression device B

5Compression pipe

10Anti-corrosion tapeFigure 3

Structure of 420-kV prefabricated joint

Table 2

Interface field strength of prefabricated joints

PositionStress at working voltage or normal
maximum voltage (kV/mm)
275kV

420kV

1: Epoxy5.8

5.8

2: Interface between
pre-molded insulator
and epoxy-unit

1.4

1.9

3: Rise-up region

5.5

6.0

Epoxy

Electrode
Premolded insulator

CableFigure 4

Photo 1420-kV prefabricated joint

6.
5.

Electric field distribution for 420-kV prefabricatedjoint

TESTS OF PJ

Type tests of PJ were conducted by Furukawa in accordance with IEC (International Electrotechnical Commission) standards. Having succeeded in the test, the PJ wassubjected to long term reliability tests at CESI (CentroElettrotecnico Sperimentale Italiano) for one year according to CIGRE (Conference Internationale Des Grands
Reseauz Electriques) recommendations. Good results
were obtained, as shown in Table 3, validating the performance of the PJ.

WORK AT SITE

The installation of PJs was carried out by NKT Cable Co.
It was required to do the installation work under a safeand sufficient level of quality control, so that the functions
that the PJ was originally provided with were maintained.
Following measures were taken prudentially.
Prior to the start of work, the jointers of NKT Cable Co.
received a course of training, in which lectures and practices were given in terms of crucial checking points of quality control at installation. The skill of every jointer was evaluated to be satisfactory by checking the electrical performance of his achievements. Furthermore, at actual instal-53

Table 3Results of type and pre-qualification tests
Contents of test

Kind of test

Result
Test conditions

Test item
Partial discharge test

. Should be 5 pC or lower at AC 330 kV,
after a loading of AC 385 kV for 1 min

Good(Noise was 4 pC)

tan. Should be 0.1% or lower at a conductor
temperature of 95~100 ˚CGood(Voltage was 220 kV)

Heat cycle test

. Voltage: AC 440 kV. Heat cycle:
8hr on/16hr off x 20 cycles
Conductor temperature: 95~100˚C

GoodPartial discharge test

. Should be 5 pC or lower at AC 330 kV,
after a loading of AC 385 kV for 1 min

Good(Noise was 3 pC)

Impulse withstandvoltage test

. ±1425 kV x 10 repetitions. Conductor temperature: 95~100˚C

GoodAC withstand voltage. AC 550 kV x 15 minGoodGoodLong term reliability tests. AC 375 kV x 1 year
. Heat cycle:
16hr on/32hr off
Conductor temperature: 90±3˚C. ±1425 kV x 10 repetitions. Conductor temperature 95~100˚C

Good

Type test (IEC 840)

Pre-qualification test
(CIGRE recommendations)

The PJ developed this time is the first in the world thathas been applied to a long distance transmission line ofXLPE cable. While the product is expected to be capableof meeting the needs of long transmission lines of thisvoltage class, we are determined to proceed further to thedevelopment of PJs of 500 kV class.
Finally, we would like to express our deep appreciation
to the personnel of NKT, CLD, and NESA for their guidance and cooperation that they offered at the development and supply of the PJ.

REFERENCE
Photo 2Exterior view of joint house with the staff of NKT
Cable

lation, a joint house with a simple clean room was used tocontrol the temperature and humidity of the joint site. Photo2 shows the external view of a joint house at actual site.
In this way, a multitude of 72 sets were successfully assembled and installed by NKT jointers, and the line went
into operation in October, 1997.

(1) Tanabe et al. : Development of Prefabricated Joint for 420-kV
XLPE Cable, 76th Regular General Assembly of Electrical
Society of Japan, Study on Generation, New Energy, Transmission, and Transformation, 7 (1997). (in Japanese)(2) P. Andersen, T. Tanabe, S. Suzuki et al. : Development of a420-kV XLPE Cable System for The Metropolitan Power
Project in Copenhagen, CIGRE 21-201 (1996).
(3) Kobashi et al. : Development of Prefabricated Joint Box for
275-kV CV Cable, Furukawa Electric Review No.93 (1993).
(in Japanese)
Manuscript received on August 3, 1998.

7.

CONCLUSION

The development and supply of prefabricated joint of 420kV XLPE cable has been reported. We were successful inobtaining satisfactory results, which were confirmed byvarious tests including long-term reliability tests lasting for
one year. After the assembly and installation of PJs by
NKT Cable Co., the line passed the commissioning test
carried out based on CIGRE recommendations. The transmission line went into commercial service in October 1997,
and has since been in operation without any abnormalities.

54

Furukawa Review, No. 18. 1999