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THERMAL RATINGS OF HV CABLE ACCESSORIES

J. Becker (Belgium)J-L. Parpal (Canada)M-H. Luton (France)R. Schroth (Germany)B. Parmigiani (Italy)S. Katakai (Japan)H.Geene (Netherlands)J. Svahn (Sweden)J. Head (United Kingdom)

CONTENTS

1 SUMMARY.....................................................................................................................................................................2

2 INTRODUCTION........................................................................................................................................................2

3 THERMAL RATINGS OF ACCESSORIES ........................................................................................................3

3.1 BASIC CONSIDERATIONS............................................................................................................................................ 33.2 CONCLUSIONS............................................................................................................................................................. 3

4 THERMO-MECHANICAL RATINGS OF ACCESSORIES..........................................................................4

4.1 BASIC CONSIDERATIONS............................................................................................................................................ 44.2 CONCLUSIONS............................................................................................................................................................. 4

5 SYSTEMS DESIGN ASPECTS................................................................................................................................5

5.1 THERMAL RATINGS OF ACCESSORIES....................................................................................................................... 55.2 THERMO-MECHANICAL RATINGS OF ACCESSORIES............................................................................................... 5

6 CONCLUSIONS ...........................................................................................................................................................5

ANNEX 1. THERMAL CALCULATIONS IN HV AND EHV CABLES AND JOINTS ...................................7

ANNEX 2. OVERVIEW OF INTERNATIONAL STANDARDS ON THERMAL ASPECTS OFACCESSORIES (AS A RESULT OF A QUESTIONNAIRE UNDER THE MEMBERS OF THE TASKFORCE)....................................................................................................................................................................................11

ANNEX 3. GUIDE TO AID DEVELOPMENT ENGINEERS FOR TESTING THE THERMALPROPERTIES OF JOINTS................................................................................................................................................15

ANNEX 4. GUIDE TO AID DESIGN ENGINEERS IN THE CORRECT DESIGN OFSYSTEMS: THERMAL AND THERMO-MECHANICAL ASPECTS OF ACCESSORYPERFORMANCE..................................................................................................................................................................16

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1 SummaryOn request of IEC TC 20 a Task Force TF 21(B1)-10 was launched by SC 21 in 2001 with thescope to review whether or not existing HV cable test specifications would appropriatelyspecify and verify the crucial thermal and thermo-mechanical characteristics of accessories.TF B1-10 finished its work on schedule in 2003 with the following conclusions:

• Thermal ratings of accessories need not be specified separately from cables, as theyare considered identical due to the presence of cable inside the accessory.

• The successful completion of IEC thermal tests at a complete cable system can beconsidered as simultaneous verification of the adequate thermal design of both, cablesand accessories, provided that comparable or higher conductor temperatures as ratedfor the cable are achieved inside joints. These test conditions shall be realized byapplying only cable conductor current heating.

• The thermal performance of terminations in normal operation is not consideredcritical; therefore they do not have to reach the rated temperature for the cable duringtest.

• External thermo-mechanical forces can be reproduced in the IEC prequalification testonly for the specific installation conditions applied.

• The thermal limits of accessories and external thermo-mechanical forces in serviceoperation cannot be reproduced comprehensively by standardized tests, but have to betaken into account for each individual case by the systems design engineering.

The abstract of the work is published in Electra No 212 February 2004.

2 IntroductionIt is common practice to base the calculation of the current carrying capacity of HV/EHVunderground transmission lines on the thermal ratings of the cable, taking into account thelosses in the cable and the heat transfer to its environment.International standards as IEC specifications 60840 and 62067 have defined the thermalratings of extruded HV/EHV cables by their maximum cable conductor temperatures fordifferent insulations. The thermal ratings of accessories are not explicitly mentioned as theywere generally considered to have equal or better levels than those of the cable.Questions have been raised, whether and how the thermal and thermo-mechanical ratings ofaccessories, i.e. terminations and joints, should be defined and how these should be taken intoaccount in the design of cable systems.

In 2001 a Task Force TF 21-10 was established with the following scope:• to specify the terms “thermal and thermo-mechanical ratings” of accessories• to review existing test specifications with regard to appropriate verification of thermal and

thermo-mechanical performance of accessories• to consider, if applicable, improved or new test procedures for thermal and thermo-

mechanical verification of accessories• to prepare recommendations to IEC, whether and how specific thermal and thermo-

mechanical tests should be combined with and/or integrated into existing testspecifications for extruded HV and EHV cables (e.g. IEC 60840 and IEC 62067)

• to prepare guidelines how to apply the findings of basic laboratory tests to the multitude ofpractical configurations.

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3 Thermal ratings of accessoriesThe thermal rating of an accessory is defined as “the maximum temperature of the conductoror conductor connector contained within the accessory (whichever is the higher) allowed innormal operation”.

3.1 Basic considerations• Thermal ratings of extruded HV/EHV cables are defined in IEC specifications 60840 and

62067 by maximum cable conductor temperatures for different insulations.• Thermal ratings of accessories are not explicitly mentioned as they were generally

considered to have equal or better thermal ratings than those of the cable.• Thermal aspects are verified in IEC specifications by heating cycle voltage tests and by

prequalification tests on complete EHV cable systems (cable + accessories).• The heating current in the test loop is defined by the maximum temperature in the cable

conductor remote from the accessories.• Thermal designs of accessories are extremely diverse and within the responsibility of

each manufacturer.• Terminations installed in normal conditions are not considered as a hot spot for the cable

due to more effective heat transfer to the environment (e.g. air circulation for outdoorterminations, axial conductor heat transfer in GIS and transformer terminations).

• In steady state test conditions (i.e. when a single cable is installed in air in a test loop)joints will develop higher conductor temperatures than the remote cable, due to theirlarger dimensions (annex 1).

• Longer thermal time constants for joints may result in lower conductor temperature in thefirst part of a heating period than for the cable, but higher temperatures at the end of theheating period, followed by delayed cooling after disconnecting the current (annex 1).

• Depending on the time constants and duration of the cooling period, joints may not cooldown completely to the ambient temperature, resulting in a gradual increase of conductortemperatures in subsequent cycles (annex 1).

• Additional thermal insulation may extend the thermal time constant for the cable.• Most existing standards require current heating for cable and accessories (annex 2).

3.2 Conclusions• Thermal ratings of accessories need not be specified separately from cables, as they are

considered identical due to the presence of cable inside the accessory.• Common test specifications can only assess the basic thermal performance of accessories,

rather than compliance with rated cable conductor temperatures.• Many joints have worse heat dissipation characteristics than cables, thus developing

during thermal IEC tests higher internal conductor temperatures than the remote cables.• Higher than rated conductor (insulation) temperatures are not allowed inside accessories

during service operation.• For type/prequalification tests it is acceptable for maximum temperature in accessories to

be higher than in the cable. The successful completion of IEC thermal tests on a completecable system can be considered as simultaneous verification of the adequate thermaldesign of both cables and accessories, provided that comparable or higher conductortemperatures as rated for the cable are achieved inside joints.

• The thermal performance of terminations in normal operation is not considered critical;therefore they do not have to reach the rated temperature for the cable during test.

• Conductor heating by means of current should be applied during type and prequalificationtests. No external means of heating or sheath current heating should be applied, as this

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will tend to reduce the conductor temperature within a joint in comparison to thetemperature of the remote cable conductor.

• Some cable systems with additional thermal insulation may require extended heating (andcooling) cycles.

4 Thermo-mechanical ratings of accessoriesThe thermo-mechanical rating of an accessory is defined as “the ability of the accessory towithstand mechanical forces, which are developed due to operation of the accessory andassociated cable at the maximum temperature allowed in normal operation. The forces areinternal and external. Only the level of external forces is specified, being symmetrical andasymmetrical conductor forces (arising from the connected cables)” (Fig. 1).

4.1 Basic considerationsThermo-mechanical ratings ofaccessories are neither mentionednor explicitly specified in IECspecifications. Thermo-mechanicalforces in accessories can originatefrom:

• components inside theaccessories: internal forces.

• external sources, mainlycable: external forces.

• Internal thermo-mechanicalforces (expansion, retraction,pressure etc.) are reproduced inactual thermal IEC tests,provided that maximum (rated)conductor temperatures (cycles)are adequately achieved.

• External thermo-mechanicalforces on accessories areinduced by external components and their installation. Depending on the testarrangements used, specific external thermo-mechanical forces can be reproduced by IECprequalification tests. These, however, cannot be representative for the complete varietyof possible service installations.

• The definition of the mechanical withstand strength of each individual accessory againstexternal forces is with the manufacturer. This strength is determined by “mechanical”components such as insulators, anchor joint casings, etc.

• It is considered sufficient to verify the adequate mechanical strength of such componentsby separate certificates rather than testing of complete accessories.

4.2 Conclusions• Internal thermo-mechanical forces can adequately be verified by thermal IEC tests,

provided that maximum conductor temperatures are adequately achieved.• External (thermo-) mechanical forces, in special installation conditions, cannot be

reproduced comprehensively in standardized tests.• Maximum admissible symmetrical and/or asymmetrical forces are indicated by

accessories manufacturers and attested by certificates rather than verified at completeindividual accessories.

Figure 1.Measurement of thermo-mechanical forces

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• Thermo-mechanical ratings of accessories need not be specified explicitly in IECspecifications, but left as a subject of agreement between customer and manufacturer.

5 Systems design aspects

5.1 Thermal ratings of accessoriesThermal limits of accessories in service operation, (i.e. obeying maximum rated cableconductor temperatures in accessories too) have to be secured by systems design engineeringtaking into account:

1. the accessory’s basic thermal characteristics provided by the manufacturer (annex 3).2. appropriate installations such as wider phase spacing (Fig. 2), special backfill,ventilation of manholes, etc. (annex 4).3. severe environmental conditions for terminations (e.g. hot climates, transformerterminations in hot oil).

5.2 Thermo-mechanicalratings of accessories

External thermo-mechanical(and other mechanical) forceson accessories have to beconsidered by systems designengineering taking intoaccount:§ the value of admissible

mechanical forces actingon the accessory, to beprovided by theaccessory’s supplier.

§ the thermo-mechanicalcharacteristics (e.g.bending moments,allowable thrust) of thecables involved, to beprovided by the cable’ssupplier.

§ the actual installation conditions, (annex 4).

6 ConclusionsThermal ratings of accessories need not be specified separately from cables, as they areconsidered identical due to the presence of cable inside the accessory.The thermal performance of terminations in normal operation is not to be considered critical;therefore they do not have to reach the rated temperature for the cable during test.The successful completion of IEC thermal tests at the complete cable system can beconsidered as simultaneous verification of the adequate thermal design of both, cables andaccessories, provided that comparable or higher conductor temperatures as rated for the cableare achieved inside joints. These test conditions will be achieved by applying cable conductorcurrent heating only.

For the heating of cables and accessories during type test, the following clause isrecommended to IEC:

Figure 2. Joints in manholes, installed with wider spacing

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For IEC 62067 clause 12.4.7 becomes:

The cable shall have a U-bend with a diameter as specified in 12.4.4.

The assembly shall be heated by conductor current until the cable conductorreaches a steady temperature 5°C to 10°C above the maximum conductortemperature in normal operation. No additional external heating or heatingby sheath current shall be allowed. If for practical reasons the testtemperature cannot be reached, additional thermal insulation can be applied,which then must be described in the test report.

The heating shall be applied for at least 8h. The conductor temperature shallbe maintained within the stated temperature limits for at least 2h of eachheating period. This shall be followed by at least 16h of natural cooling to aconductor temperature within 15 °C of ambient temperature, with a maximumof 45 °C. The conductor current during the last 2h of each heating periodshall be recorded.

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

The final paragraphs of the section to remain unchanged.

In the case of IEC60840/Ed3/CDV clause 12.3.6: the same wording is recommended as given above except that “12.4.4” ischanged to “12.3.3” and “within 15°C of ambient” is replaced by “within10°C of ambient” in accordance with the existing wording.

For the Impulse voltage test and “hot” partial discharge test, the same method of heating shallbe applied.

External (thermo-) mechanical forces in normal installation condition may be reproduced inthe prequalification test for EHV systems. However, external (thermo-) mechanical (and othermechanical) forces, in special installation conditions, cannot be reproduced comprehensivelyin standardized tests and should therefore be considered by systems design engineering.

Completely new generation of accessories (e.g. dry outdoor terminations) might need furtherconsiderations, regarding thermal and thermo-mechanical aspects.

Annexes

1. Thermal calculations in HV and EHV cables and jointsExample 1: dynamic temperature calculations in a 132kV cable and jointExample 2: thermal behavior of a 400kV joint during IEC loading cycles in air

2. Overview of international standards on thermal aspects of accessories3. Guide to aid development engineers for testing the thermal properties of joints4. Guide to aid design engineers in the correct design of systems: thermal and thermo-

mechanical aspects of accessory performance

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Annex 1. Thermal calculations in HV and EHV cables and joints

Example 1: dynamic temperature calculations in a 132kV cable and joint

Joints and cables have different thermal properties. The basic differences can be expressed in:• thermal resistances• thermal time constants

Tc : conductor temperature in the cable

Tj : connector temperature in the joint

Φr : radial heat flow

Φa : axial heat flow

Tc Tc

Tj

Φa ⇒ ⇐ Φa

Φr

⇓Φr

⇓

2 m

Model for calculations

Tc : conductor temperature in the cable

Tj : connector temperature in the joint

Φr : radial heat flow

Φa : axial heat flow

Tc Tc

Tj

Φa ⇒ ⇐ Φa

Φr

⇓Φr

⇓

2 m

Model for calculations

To verify the differences in the joint and cable temperature during one heating cycle, dynamical calculationswere made for an 800mm2 132kV XLPE cable.First, the heating current is calculated to obtain a stable cable conductor temperature of 95 °C. Based on theheating current, the temperature was calculated inside the joint and cable for a period of 10h after switching onthe current. The results are given in Figure 3 at a conductor heating current of 1800A.

0

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100

110

0 1 2 3 4 5 6 7 8 9 10

Time (hours)

Tem

per

atu

re (°

C)

joint

cable

Figure 3. Typical heating curve of a 132 kV 800 mm² Cu cable and joint

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ConclusionIn stationary conditions, the joint reached a higher temperature than the cable, as a result of higher thermalresistance of the joint. In the first 6 hours of the heating cycle, the temperature in the joint is lower than in thecable due to the longer thermal time constant of the joint.

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Annex 1. (continued)

Example 2: Thermal behavior of a 400 kV joint during IEC loading cycles in air

1 – Introduction

In order to evaluate the temperature difference between a 400 kV cable and the relevant premoulded joint duringthe heating cycle voltage test, specified in IEC 60840 and 62067, thermal calculations have been carried outusing the finite element method.The test loop assumed for the calculation is installed in air and includes 20 m of cable and a 400 kV premouldedjoint complete with its anticorrosion protection. The length of cable is such that the presence of the joint does notaffect the asymptotic temperature of the cable.Due to the use of the FEM method, the joint is subdivided in finite elements where both radial and longitudinalheat transmission is taken into account.During the heating cycle test the cable conductor is heated by conductor current in order to reach in 6 hours aconductor temperature of 95 °C (far from the joint), followed by 2 hours where the current is reduced in order tomaintain the cable conductor temperature between 95 °C and 100 °C, then the current is switched off for 16hours still maintaining the voltage on. This cycle under constant voltage of 2 Uo is repeated for 20 times.The calculation has been made for 400 kV XLPE cables with a 1600 mm2 and a 2500 mm2 copper conductor.Both conductor losses and dielectric losses have been considered in the calculation.

2 – Loading cycle temperature profile calculation

2.1 – 2500 mm2 Cu 400 kV cable and joint

During the loading cycles a constant current of about 3450 A is circulated in the conductor, so that a temperatureof 95 °C is reached in the cable conductor after 6 hours, starting from a uniform ambient temperature of 20 °C.Then the current is reduced in order to maintain the temperature in the cable conductor between 95 °C and 100°C for two hours. Subsequently the conductor current is switched off for 16 hours.The results of the calculations of the cable and joint daily cycles are shown in Figure 4.It can be observed that initially the joint ferrule temperature is slightly lower than 95 °C, then it increases andreaches a stable value of about 101 °C after four cycles.

2.2 – 1600 mm2 Cu 400 kV cable and joint

The initial conductor current in order to bring the cable conductor temperature to 95 °C after 6 hours (includingdielectric losses) is of about 2500 A. The results of the joint and cable load cycle calculation are shown in Figure5.It can be observed that the temperature of the joint has the same behavior as in Figure 4, i.e. the periodicasymptotic temperature in the joint is reached after 4 cycles, but its value is a little smaller (99 °C instead of 101°C).

4 – Conclusions

During IEC loading cycles, with only conductor current heating, the joint ferrule temperature of a 400 kV jointfollows the cable conductor temperature with a certain delay due to the higher thermal time constant of the jointcompared to the cable.However, after about 4 cycles also the joint reaches stable periodic conditions with a maximum temperaturegenerally slightly higher than that of the cable conductor.These conclusions depend on the design of the joint. Joints longer than the one considered in these calculationsor with thicker insulation would present a higher time constant, so that stable cycle conditions for the joint wouldbe reached after more than 4 cycles, but certainly within the 20 IEC cycles.The higher periodic temperature in the joint is also due to the fact that, during the type test, the joint is providedwith the thick outer protection foreseen for the underground installation (as in the case of these calculations),while for actual installations in air the casing is normally unprotected.

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0

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20

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80 90 100

Time (hours)

Tem

per

atu

re (°

C)

joint

cable

Figure 4. Loading cycles (8 hours on, 16 hours off) on a 400 kV 2500 mm² Cu cable and joint

0

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20

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80 90 100

Time (hours)

Tem

per

atu

re (°

C)

joint

cable

Figure 5. Loading cycles (8 hours on, 16 hours off) on a 400 kV 1600 mm² Cu cable and joint

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Annex 2. Overview of international standards on thermal aspects of accessories (as a result of a questionnaire under the members of theTask Force)

Standards, relevant statements on thermal ratings and testing of accessories

Standard Contents and comments

BS 7912(UK)

BS 7912 (Implementation of HD 632): the method for heat cycling assemblies containing accessories is“ The assembly shall be heated until the cable conductor reaches a steady temperature 5°C to 10°C above the maximum conductor temperature in normaloperation. The heating arrangements shall be selected so that the cable conductor attains the temperature specified in this sub clause, remote from theaccessories and as far as practicable, also within the accessories.”

EATS 09-16(UK)

EATS 09-16 is the more widely used document which effectively expands on the requirements of the above BS. The latest revision of this, which isstill in draft form states:

“The load cycle test shall be carried out as in clause 16.4 of BS 7912 except that the details of heating given in the last two sentences (i.e. the 2sentences quoted from BS 7912 above) shall be replaced by the following:

The heating arrangements shall be selected so that the cable conductor attains the temperature specified above, remote from the accessories, and as faras practicable, the conductor temperature in accessories shall reach at least the rated temperature of the accessory plus 5oC to 10 oC. The ratedtemperature of the accessory shall have been declared previously by the supplier (the value shall be no lower than the maximum cable conductortemperature in normal operation).

Alternatively, the test installation shall be heated by conductor current alone, until the cable conductor remote from the accessories reaches a steadytemperature of 5 o C to 10 oC above the maximum cable conductor temperature in normal operation. No thermal insulation or means of cooling orfurther forms of heating shall be used.”

NEN 3629(Netherlands)

Thermal propertiesAccording to this standard, in normal operation a conductor temperature of max 90°C and during short-circuit (duration max 5 sec) of 250°C isacceptable.

Heating cycle voltage test with 30 cycles.

IEEE 48-1990(USA)

IEEE standard test procedures and requirements for high-voltage alternating-current cable terminations

4. Rating

Note: regarding the continuous current rating (ampacity). The application of various types of cable terminations requires engineering consideration as tothe ampacity of the completed installation. A cable termination by itself cannot be assigned a design or nominal current or ampacity rating since this

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parameter is completely dependent upon the type of cable insulation, the maximum allowable cable conductor temperature for the type of cableinsulation involved, and the anticipated maximum ambient temperature of the medium surrounding the termination……….The supplier of cable terminating devices or material should be consulted for the ampacity of the design for the intended application with specific typeand size of cable.…….

7.4.2 Cyclic aging test(Conductor heating is required. There shall be no current in the cable metallic shield.)

7.4.2.3During the current-on period, the cable conductor temperature midway between the terminations shall be within 5 °C of the cable’s maximum ratedemergency operating temperature for a period of 6h.

Note: the cyclic aging test is not intended to establish current rating for a termination (see section 4. Rating)IEEE 404-1993(USA)

IEEE standard for cable joints for use with extruded dielectric cable rated 5000-138000V and cable joints for use with laminated dielectriccable rated 2500-50000V

4.2 Unusual service conditions………The manufactures should be consulted for recommendations.

5.2 Current ratings

The current rating of the cable joint shall be equal to or greater than the current rating of the cables for which the cable joint is designed.

5.3 Temperature limitations

The joint shall be designed for operating with the conductor and connector within the joint at the same maximum temperature limitations as those for theconductors of the cables being joined.

7.7 Cyclic aging test for extruded dielectric and transition joints

7.7.2 Extruded cable joints rated 46-138kV

(Joints are tested in water and dry. Conductor heating is required.)

There shall be no current in the cable metallic shield.

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The following information shall be recorded in the test report:a) The maximum temperature of the outside of the joint housing in waterb) The maximum temperature of the outside of the joint housing in airc) The temperature of the outside surface of the cable in aird) The temperature at which the joint is qualified

SEN 24 14 34(Sweden)

Joints and terminations, rated voltage 1-420 kV. Testing

Valid since May 1977. Expired 1998. Not replaced by any other Swedish standards.• “...type tests shall be carried out with the largest size of cable conductor...”• “...shall be loaded with the highest permissible current...and sufficiently long time to reach temperature equilibrium.”• “The rise…must not assume values that can damage the materials used, or endanger the environment (fire risk).”• “Cyclic load tests...shall be carried out on terminations...criteria in the cable standard shall then apply.”• “Testing of a complete cable joint...shall be carried out in accordance with the...cable”

SS 424 14 17(Sweden)

Power cables – XLPE-insulated cables with extruded over sheath and rated voltage 12-420 kV – Testing

Load cycling:Conductor temperature: 100-105oC. Achieved by a suitable method (no specifications)Heating during at least 8 h and natural cooling during at least 16 h. The conductor temperature shall be between 100oC and 105oC during thelast 2 h of the heating period.

NF C 33-061September1999(France)

Insulated cables and their accessories for power systems.

Joints for single-core cables with polymeric extruded insulation for rated voltagesabove 30kV (Um = 36kV) up to 500kV (Um = 525 kV)

NF C 33-062September1999(France)

Insulated cables and their accessories for power systems

SF6 insulated metal enclosed terminations for single-core cables with polymericextruded insulation for rated voltages above 30kV (Um = 36kV) up to 500kV (Um =525 kV)

§ 6 Type tests:"…In case of temperature of the conductor is specified, its valueshall be obtained by circulation of adequate current in the cableconductor"" For tests requiring an increase of the cable conductortemperature, this temperature can be obtained, if necessary, bythermal insulation of the cable, in order not to induce a currentampacity either too high or too low, non representative of normaloperation."

Long term test:Thermal cycles:Cycle time: 8h of heating and 16h of cooling.Current applied to raise the conductor to a temperature up to 0°Cto 10°C higher than the maximum normal operatingtemperature, for the first 167 cycles and up to 0°C to 5°C higherthan the maximum emergency overload temperature, duringthe remaining 83 cycles.

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NF C 33-063September1999(France)

Insulated cables and their accessories for power systems

Outdoor terminations, with porcelain insulator, for single-core cables withpolymeric extruded insulation for rated voltages above 30kV (Um = 36kV) up to500kV (Um = 525 kV)

NF C 33-064September1999(France)

Insulated cables and their accessories for power systems

Indoor or outdoor polymeric terminations, without porcelain insulator forsingle-core cables with polymeric extruded insulation for rated voltages above30kV (Um = 36kV) up to 150kV (Um = 170 kV)

C 33-065February2001(France)

Insulated cables and their accessories for power systems

Self supporting outdoor polymeric terminations or terminations with acomposite housing, without porcelain insulator for single-core cables withpolymeric extruded insulation for rated voltages above 150kV (Um = 170kV) up to500kV (Um = 550 kV)

§ 6 Type tests: idem above.Long term testThermal cycles:Cycle time: 8h of heating and 16h of cooling.Current applied to raise the conductor to a temperature up to10°C higher than the maximum normal operating temperaturewith a tolerance of +/-4°C

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Annex 3. Guide to aid development engineers for testing the thermal properties of joints

IntroductionThe aim of the guide is to give the development engineer some recommendations on how to measure the thermalcharacteristics of a prefabricated joint in transient and steady state conditions.

Test installationThe test loop has to contain at least 15 m of cable and one joint. The distance between the joint and cable endshas to be at least 5 m. No direct contact of the cable or joint with the lab floor is allowed. It is important thatcable and joint should be subjected to the same thermal ambient conditions. No additional thermal insulationshould be applied.

Thermocouples (or other temperature sensors) should be installed in the following positions:• at the cable, at least 5 m from the joint and 5 m from the cable end,

o on the conductoro on the metallic sheatho on plastic outer sheath

• at the joint in the middle,o on connector in the jointo on metallic casing or metallic screeno on insulating cover

• near to the test loop to record the ambient temperature

Thermal testThe heating current shall be applied for at least 24h until the cable conductor, remote from the accessories,reaches a steady temperature of at least 5°C above the maximum conductor temperature in normal operation forthe cable. During the entire heating period, the temperatures shall be recorded. No other means than currentheating shall be used.

Test resultsFrom the recorded heating curves the following characteristics can de calculated:

• The thermal resistance between conductor and outer covering of the joint (deduced from the ratiobetween the steady state temperatures)

• The thermal resistance between the conductor and outer covering of the cable (deduced from the ratiobetween the steady state temperatures)

• Thermal time constant of the cable• Thermal time constant of the joint

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Annex 4. Guide to aid design engineers in the correct design of systems: Thermal andthermo-mechanical aspects of accessory performance

IntroductionThe aim of the guide is to give the design engineer some recommendations regarding important thermal andthermo-mechanical characteristics to check on accessories.When possible or appropriate, the guide indicates the way to control or measure the described characteristics.

ReferencesThis guide has been established taking into account the recommendations and guidelines included in thefollowing documents:

Cigré WG 21-09 : Electra No. 140, February 1992Considerations of ageing factors in extruded insulation cables

Cigré WG 21-17 : Technical brochure No. 194, October 2001Construction, laying and installation techniques for extruded and self contained fluid filledcable systems -

Cigré WG 21-06 : Technical brochure No. 177, February 2001Accessories for HV cables with extruded insulation,

Section 1: A guide to the selection of accessories

Cable systems – Way of layingThermal and thermo-mechanical properties of accessories are directly linked to the way of laying of the cablethey equip.

Cables can be laid in the following environments, which are classified as rigid, flexible or semi-flexible systems.Induced thermo-mechanical thrust on accessories depends strongly of this classification as described moreprecisely in the technical brochure published by WG 21-17.

Rigid systems1. Laid direct and buried in ground2. Troughs3. Tower/ shaft (close cleats)4. Jointing chamber (close cleats)

Flexible systems – cable horizontally snaked or vertically waved5. Tunnel6. Above ground7. Tower/ shaft8. Jointing chamber

Semi-flexible systems – cable constrained9. Bridge10. Unfilled ducts

The cable system including the accessories has to sustain any of the thermal or thermo-mechanical constraints,which could happen under:

1. Normal operation2. Transient operation: Short-circuit (phase to phase or phase to earth)

OverloadEarthquake

Table 1 summarises main properties to take into account by the design engineer to avoid any failure linked to abad design regarding thermal or thermo-mechanical properties.

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Annex 4 (continued)Table 1: Characteristics to control or tests to perform for assessing the reliability of accessories on cable systems

Constraints Constraints Classification Test to perform/ characteristics to control IEC testsMechanical Thermal Thermo -

mechanicalSteady-stateoperation

Mechanical forces linked to snakingLongitudinal dilatation of the cable – axial thrust(conductor thermo-mechanical thrust) andretraction

X Calculations of the thrust (see WG 21-17) orspecific tests made by the cablemanufacturer

Cable components retraction (insulation, metallicscreen, outer sheath)

X Short-term and long term test includingaccessories with thermal cycles and arepresentative configuration of laying andclamping.

Test report has to be clearabout the way of laying (inparticular for the pre-qualification test)

Radial thermal expansion of the cable components X Design of the clampMaximum temperature (surroundings of cable oraccessories)

X To be known and taken into account for thedesign

Vibrations X X Support design, flexible link from powerapparatus and terminations

Overload Same constraints as above for steady-stateoperation but higher level of constraints

Same as previously described for steadystate operation. Adjustment of level ofconstraint and test duration

Earthquake Mechanical properties of the support X § Calculations§ Tests§ Cleating design

Specialenvironments Transformers (raised temperature…)

Outdoor terminationSolar radiation

XX

XOthers

Angle of installation of terminations XTransition between ducts and manholes XTransition between flexible and rigid systems(open air)

X A specific care has to be taken for thetransition part between 2 different ways oflaying as described in the technical brochurefrom WG 21-17 – p 86

Transition between flexible and rigid systems(buried)

X

Note: effects due to short-circuits have not been included in the table but it should be noted that when designing a cable link it is necessary to takeinto account the behavior under short circuit conditions.