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21, rue d’Artois, F-75008 PARIS B1-103 CIGRE 2014 http: //www.cigre.org

Advanced type testing program in order to assure the reliability of new cable systems

Véronique BEGHIN Christian SZCZEPANSKI Jean BECKER1

Tractebel – GDF SUEZ(B) Elia (B) SART Consult

Belgium Belgium Belgium

SUMMARY

Early 2000 the Belgian network planners asked that the rated power be increased from 250 MVA to 290 MVA in the 150 kV field which led to a new technical stage for the cable system. The potential suppliers offered cables with 2000 mm² Aluminum conductors and reduced XLPE insulation thickness. The dielectric stress at the insulation surface for these new cables was as high as the stresses at the insulation screen of 220 kV cables.

Up to that date the test philosophy was to stick to the international specification, IEC 60840, except for the partial discharge measurements where a sensitivity of 1 pC was prescribed by the Belgian specification instead of 5 pC prescribed by IEC 60840 and except for voltage test where 3.5 U0 is prescribed instead of 2.5 U0. For voltage levels above 150 kV, cables form part of the backbone of the transmission system. Therefore CIGRE WG 21.03 recommended in 1993 a long term ageing test of 1 year on cable systems. This test called “prequalification test” was conducted on a number of 400 kV

cable systems between 1992 and 2000. During these long term tests several failures occurred on accessories which needed to be further improved. These results showed the interest of the long term ageing tests. IEC took over the recommendations of CIGRE and published IEC 62067 in 2001.

As the new generation of 150 kV cables in Belgium had similar dielectric stresses at the interface with joints and terminations as the 220 kV cables, it was considered important to make type tests with 100 cycles instead of 20 requested by IEC 60840, including 20 heating cycles at an even higher temperature to evaluate the behavior of the system in overload conditions. At the end of the test hot impulse tests and partial discharge measurement were considered important to see whether the cable system had not suffered during the ageing test.

The first tests realized on the 2000 mm² cable system gave good results. This was important to give confidence in the future long links installed with this type of cables and accessories.

1 jeanbecker@scarlet.be

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The test philosophy was extrapolated to all future developments of cables and changes in accessories in the range of 70 kV to 150 kV. During the development stage of a number of accessories the 100 heat cycling voltage tests have shown that improvements were necessary.

It was decided to maintain the same philosophy for the 220 kV and 380 kV systems, as for the lower voltage range. There was no reason to doubt that these tests would be useful also in this voltage range, as these tests have given interesting results at lower voltages.

Furthermore the following test sequence has been added on a separate joint for all HV voltage ranges: a cable sample with a joint is submitted to 20 heating cycles followed by a voltage test on metallic screens, 3 short circuit tests, and voltage test on metallic screens again. Finally examination of the cable and accessory with Differential Scanning Calorimetry (DSC) measurement of the insulation near the different connections and tensile strength measurement of the welding of the joint connection

KEYWORDS

High voltage-Cable system-Type testing-Qualification

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1 INTRODUCTION

In 1992 the decision had been taken to install no more overhead lines in the range of 30 kV up to 220 kV in Belgium.[8] Almost all new circuits are underground except in the 400 kV field. Also in the 150 kV field the need for higher transmission capacity came up in the years around 2000, from 225- 250 MVA to 290 MVA and even higher (350 MVA) including also emergency conditions.

For these new transmission needs, the cable suppliers proposed in a first step 150 kV XLPE insulated 2000 mm² aluminum conductor cables. For economic reasons reduced insulation thickness was proposed, based on the good long term test behavior at higher voltages.

Also pre-moulded joints which had been developed mainly for voltages of 220 kV up to 500 kV, were introduced in 150 kV to face the higher dielectric stresses at the interface between joints and cable insulation surface (which, for the 150 kV level, became as high as the stresses encountered on 220 kV and above cable systems). These joints have also been installed to allow the use of Direct Cross-bonding system (no need of Surge Voltage Limiter (SVL)) but higher intrinsic protection of the joints [6]).

Also for the lower voltages, 70 kV and 110 kV, reduced insulation thicknesses were introduced as well as pre-molded joints. Later on, around 2005, the need for 2500 mm² Aluminum conductor cables came up for the 150 kV and 220 kV network and more recently 400 kV cables with 2500 mm² copper conductor (with enameled wires).

All these changes in the cable systems from 70 kV up to 400 kV needed to be validated by adequate qualification tests to ensure that their reliability could be considered as acceptable.

2 RECOMMENDATIONS FROM CIGRE FOR CABLE SYSTEM TESTING

2.1 WG 21.03 (Electra 151 and Electra 193)

In 1990 CIGRE Study Committee 21 launched Working Group 21.03[1] with the aim to prepare test recommendations for cable systems based on extending IEC 60840 (1988) up to 400 (420) kV. The WG published the results of this study in Electra 151 in 1993.

In 2000 a task force 21.18 of the same WG presented in Electra 193 an extension of the recommendations up to 500 (550) kV [2].

As cables for rated voltages above 150 (170) kV form part of the backbone of the transmission system, reliability considerations are of prime importance. For these reasons CIGRE recommended that it is necessary to carry out a long term accelerated ageing test to evaluate long term reliability of the cable system, cables and accessories. The recommended test was called “prequalification test”.

A 100 m long cable loop including all types of accessories with a test arrangement representative of installation design conditions is submitted to a 1 years test at 1.7 U0 and to repeated heat cycles where the cable conductor is heated to its maximum operating temperature, maintained there for 2 to 4 hours and then is allowed to cool down.

For type tests the same test sequence as for IEC 60840 has been recommended

The “prequalification test” was conducted on a number of 400 kV cable systems between 1992 and

2000. During these long term tests several failures occurred on accessories which needed to be further improved. These results showed the interest of the long term ageing tests. IEC took over the recommendations of CIGRE and published IEC 62067 in 2001.

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2.2 WG B1.06 (Technical Brochure 303)

In 2001 Study Committee B1 decided to revise qualification procedures for HV and EHV AC extruded underground cable systems with the aim to take into account expected innovations in cable technology and the need to reduce the time to market. Also in HV a “slim-design” generation of cables

with similar technology and stresses to those encountered in EHV XLPE cable systems was being developed. Working group B1.06 published the new recommendations in Technical Brochure 303 in August 2006 [3].

The main recommendations are the following:

- On EHV cable system in case of changes of accessories or major changes in the design of accessories: An extension of qualification, consisting mainly of a prolonged type test: 60 heat cycles followed by a type test.

- On HV cable system for cables where the calculated nominal electrical stresses at the conductor screen is higher than 8,0 kV/mm and/or at the insulation screen higher than 4,0 kV/mm: A prequalification test and an extension of prequalification test is recommended The prequalification test consists of a voltage test at 1,7 U0 and 180 heat cycles performed on a 20 m long cable loop including all types of accessories. Joints installation in rigid or (and) flexible conditions have to be considered.

3 TEST SPECIFICATIONS FROM IEC

3.1 IEC 60840

First edition [4.1]: IEC 60840 started in 1988 with test methods and requirements on cables for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV);

Second edition [4.2]: Accessories were added to the second edition in 1999. This edition covered test methods and requirements on cables alone and on cables with accessories (cable system);

Third edition [4.3]: In the next edition in 2004 tests on accessories alone were added. Tests on cables with longitudinal applied foils were taken into consideration in this specification;

Fourth edition [4.4]: The last edition has been published in November 2011. According to CIGRE Technical Brochure 303, a prequalification and an extension of prequalification test for cables have been added.

3.2 IEC 62067

First edition [5.1]: According to CIGRE recommendations published in Electra 151(1993) [1] and in Electra 193 (2000) [2].

Second edition [5.2]: According to CIGRE TB 303, Extension of prequalification has been added. The procedure is the same as for the IEC60840 Ed4

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4 TEST SPECIFICATIONS IN BELGIUM

For cables up to 150 (170) kV, the Belgian type tests and routine test on cable systems, cables and accessories until the year 2000 were based mainly on IEC 60840, but following the feedback from the operation over years, several tests were made more stringent:

- partial discharge test sensitivity was reduced to 1 pC (instead of 5 pC in IEC), as well when performing routine testing as when performing type testing

- the voltage during routine testing was raised to 3.5 U0 (instead of 2.5 U0 in IEC)

Early 2000 the Belgian network planners asked that the rated power be increased from 250 MVA to 290 MVA in the 150 kV field which led to a new technical stage for the cable system. The potential suppliers offered cables with 2000 mm² Aluminum conductors and reduced XLPE insulation thickness. The dielectric stress at the insulation surface for these new cables was as high as the stresses at the insulation screen of 220 kV cables. It was well known that long term ageing (one year) tests in accordance with CIGRE WG 21.03 [1] were conducted between 1992 and 2000 on a number of EHV cable systems and that during these long term tests several failures occurred on accessories indicating that they needed to be further improved. After improvement the accessories passed the long term test. These results showed the interest of the long term ageing tests.

The new generation of 150 kV XLPE cables proposed by the suppliers in Belgium were slimmer and the dielectric stresses at the insulation surface were as high as and even somewhat higher than the 220 kV cables present on the market at that time. Also for the first time pre-moulded joints were proposed. That is why it was decided in Belgium to evaluate the long term heating cycle voltage test behaviour of the system.

Also at lower voltages the cables became slimmer. The same test philosophy for type testing was implemented for all voltage ranges

Hereafter a more detailed presentation of this test concept, which in the meantime, has also been included in the CENELEC Specification HD632 – Part 5 Section C [7]

4.1 ELECTRICAL TYPE TEST PHILOSOPHY OF CABLE SYSTEMS IN

BELGIUM

4.1.1 Electrical type test on complete cable systems

For the different tests only the sequence of tests and the test requirements are presented

4.1.1.1 Bending test on the cable

The bending diameter depends on the cable structure. The bending is followed by installation of the accessories

4.1.1.2 Partial discharge test at ambient temperature

Voltage raised to 2.5 U0 held at this voltage for 10 sec and reduced down to 2 U0

Sensitivity shall be 1 pC or better. The magnitude of the discharge at 2 Uo shall not exceed 2 pC. All partial discharges detected above background noise shall be investigated

Note. In IEC the sensitivity shall be 5 pC or better

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4.1.1.3 Tan measurement

4.1.1.4 Heating cycle voltage test

The assembly is heated by conductor current only, until the cable conductor reaches a steady temperature between 95 °C and 100 °C. The heating is applied for at least 8 h. The conductor temperature shall be maintained within the stated temperature limits for at least 2 h of each heating period. This is followed by at least 16 h of natural cooling.

The cycle of heating and cooling shall be carried out 20 times

4.1.1.5 Lightning impulse voltage test followed by a power frequency voltage test

The assembly is heated by conductor current only, until the cable conductor reaches a steady temperature between 95 °C and 100 °C. The conductor temperature shall be maintained within the stated temperature limits for at least 2 h.

The assembly shall withstand, without failure or flashover, 10 positive and 10 negative impulses of the appropriate value.

After the lightning impulse voltage test, the assembly is subjected to a power frequency voltage test at 2,5 U0 for 15 min, and the voltage test shall be raised gradually to 3.5 U0 and then be held for 30 min. No breakdown of the insulation or flashover shall occur.

4.1.1.6 Additional heating cycle voltage test

The assembly is subjected to 80 additional voltage heating cycles according to 4.1.1.4 from which 20 cycles at a conductor temperature between 100 °C and 105 °C

4.1.1.7 Lightning impulse voltage test followed by a power frequency voltage test

The test is the same as in 4.1.1.5

4.1.1.8 Partial discharge tests

The test is the same as in 4.1.1.2

4.1.1.9 Examination

Examination of the cable by dissection of a sample and, whenever possible, of the accessories by dismantling, with normal or corrected vision without magnification, shall reveal no signs of deterioration (e.g. electrical degradation, leakage, corrosion or harmful shrinkage) which could affect the system in service operation.

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4.1.1.10 Tests of outer protection for buried joints

This test is performed on a joint which has passed the previous test sequence. The test is performed in accordance with IEC 60840.

Nevertheless, if the use of the Direct Cross-Bonding Technology is considered, these impulse voltage tests between bonding leads shall be higher than the IEC values. Specific studies have been performed depending on the voltage ranges in the Belgian Network where this technology is used (i.e. 70 kV and 150 kV fields).

The electrical constraints on Belgium’s 150 kV network have been calculated before, and have

resulted in imposing the interruption of the elements in the joint in order to withstand 150 kV surge voltage, and 75 kV surge voltage between each element and the earth.

In 70 kV field, these values are respectively 100 kV and 50 kV.

4.1.2 Short circuit test on joints

When the electrical type test is successfully completed, a short circuit test on a separate joint is performed the following way: A cable sample with a joint is submitted to 20 heating cycles followed by a voltage test on metallic screens and 3 short circuit tests. After that the test of outer protection for buried joints (see 4.1.1.10) is carried out again to evaluate whether the joint connections have suffered or not from the heating or mechanical effect of the short circuit.

Finally an examination of the cable and accessory is performed. A DSC measurement of the insulation near the different connections is performed to get an indication of the temperature rise of the cable insulation near the connections. Tensile strength measurement is performed on the welding of the joint connections.

4.2 Test philosophy for 220 kV cable systems and 380 kV cable systems

It was decided to maintain the same philosophy for the 220 kV and 380 kV systems, as for the lower voltage range. There was no reason to doubt that these tests would be useful also in this voltage range, as these tests have given interesting results at lower voltages.

However, the Direct Cross-bonding has not been considered.

4.3 Test results

In the HV range a number of type tests have been performed. In some cases the 20 higher temperature heat cycles between 100 °C and 105 °C revealed weak points. Corrections could be brought to the joints and subsequent type test were satisfactory.

In the EHV range the test results are still limited. Nonetheless short circuit tests at 50 kA-1sec, (following the highest value in the Belgian Grid Code), have shown in some cases weak points due to very high dynamic mechanical stresses. The corrections were successful

4.4 Conclusion

Advanced type testing program is now well established in Belgium. The 100 cycles type test with partly higher temperatures during testing has revealed the interest of this test philosophy. During early stage of the evaluation of a number of accessories the long term tests have shown that improvements were necessary. Corrections could be brought to those components. The main purpose of this

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extended type test program is that the reliability of the systems is improved and that the long term performance of the complete system is assured

BIBLIOGRAPHY

[1] Electra No. 151, December 1993 Recommendations of the CIGRE WG 21.03 “Recommendations for electrical tests (prequalification and development) on extruded cables and

accessories at voltages 150 (170) kV and ≤ 400 (420 kV)”

and

“Recommendations for electrical tests,( type, sample and routine) on extruded cables and accessories at voltages 150 (170) kV and ≤ 400 (420 kV)”

[2] Recommendations of the CIGRE, task force 21.18 of WG 21.03 published in Electra 193 December 2000: “Recommendations for electrical tests (prequalification and development) on extruded cables and accessories at voltages 150 (170) kV and ≤ 500 (550 kV)”

and

“Recommendations for electrical tests, type, special and routine) on extruded cables and accessories at voltages 150 (170) kV and ≤ 500 (550 kV”

[3] CIGRE WG B1-06 2006: Technical Brochure 303 “Revision of qualification procedures for HV and EVH AC extruded underground cable systems”

[4] IEC 60840 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) –Test methods and requirements [4.1] Ed1 1988 [4.2] Ed2 2/1999 [4.3] Ed 3 4/2004 [4.4] Ed4 11/2011

[5] 62067 First edition 10-2001: power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um=170 kV) up to 500 kV (Um=550 kV) – Test methods and requirements [5.1] First Edition 10/2001 Edition 1.1, 03/2006 Edition 1 completed with amendment1 from 01/2006 [5.2] Ed2 11-2011

[6] CIGRE 2004 Paper B1 – 305 - Double 150 kV Link, 32 km long in Belgium: Design and Construction

[7] CENELEC HD 632 – Part 5 – Section C - Power cables with extruded insulation and their accessories for rated voltages above 36 kV (Um = 42 kV) up to 150 kV (Um = 170 kV) –Test methods and requirements – for lead or lead alloy sheath, or aluminium or copper sheath and their accessories

[8] CIGRE 2000 Paper 21-101 150 kV Underground Links in Belgium. A new Technical Stage