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Cigré SCB1

Convener: Frank de Wild Secretary: Jos van Rossum

A GUIDE FOR RATING CALCULATIONS OF INSULATED CABLES

TUTORIAL WG B1.35

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Copyright

DISCLAIMER Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of the publication for use other than personal and transfer to a third party; hence circulation on any intranet or other company network is forbidden.

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Contents

1. Introduction

2. Overall issues with cable ratings

3. Starting points for rating calculations

4. Calculation methods and procedures

5. Using calculation tools and techniques

6. Conclusions and recommendations

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Introduction

(1/6)

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Introduction

• Introduction to the workgroup

• Objectives

• Terms of reference / Scope of work

• Introduction to the questionnaire

• Introduction to the technical brochure

• Introduction to cable rating calculations

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Introduction to the workgroup – Workgroup membership

• Frank de Wild (convenor) • Jos van Rossum (secretary) • George Anders (CA) • Rusty Bascom (US) • Bruno Brijs (BE) • Marcio Coelho (BR) • Pietro Corsaro (SU) • Antony Falconer (SA) • Alberto Gonzalez (SP)

• Georg Hülsken (GE) • Nikola Kuljaca (IT) • Bo Martinsson (SE) • Seok-Hyun Nam (KO) • Aleksandra Rakowska (PL) • Christian Rémy (FR) • Tsuguhiro Takahashi (JP) • Francis Waite (UK) • James Pilgrim (UK)

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Introduction to the workgroup – Objectives

The objective of the workgroup:

Reasons: 1. Questions were asked:

o How to calculate if standards do not provide the answer? o How to establish starting points? o How do we actually work when calculating the current rating? o How should we work?

2. Current rating is an important topic to consider 3. Current rating is becoming more important

To be a guide for the user trying to calculate the cable rating of a new or existing power cable

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Introduction to the workgroup – Importance of current ratings

• Cables are installed to transmit power (voltage and current) • Safeguarding the voltage withstand capabilities is extensively

tested in high voltage laboratories • Safeguarding the current rating is typically not done by testing,

only by verification of calculations

Testing the current rating is rather difficult as: • Rating depends on the cable installation • Installation situation immediately after commissioning not

equal to worst case installation situation • Testing would require large current for a relative long period of

time

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Calculations are used in many phases of a cable’s life: • Contractual phase

(current rating evaluated in a contracting phase, before actual design and engineering)

• Design and engineering phase (current rating typically calculated by the supplier in correspondence with requirements set by the user, calculations reviewed by consultants or engineers, typically being the backgrond of the cable’s current rating used in the control room of users)

• Operational phase of power cables (cables often become increasingly loaded, and when the loading comes close to the rating, how sure is it that the cable rating still holds true, sometimes decades after the actual engineering of the circuit?)

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This leads to a need for reliable and representative current rating calculations for each power cable system, regardless of its situation or age Yes, there are standards IEC 60287 and IEC 60853, but these are not fully complete or correct: • New cable and trench designs • Multitudes of installation types • Difficult calculations asking for long development times to

come to verified new calculation methods

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Introduction to the workgroup – Objectives

The objective of the workgroup:

Goals: 1. To discuss starting points for rating calculations 2. To give guidance when calculating the rating, and standards

do not give answers 3. To discuss tools and techniques for rating calculations

To be a guide for the user trying to calculate the cable rating of a new or existing power cable

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Introduction to the workgroup – Terms of reference / Scope of work

1. To collect experiences and information from different countries regarding:

a: aquiring the starting points for rating calc’s b: calculation methods used and experiences

2. To assess and interpret the result of 1a and to make conclusions and recommendations regarding aquiring the starting points to use in a cable rating study

3. To set up a general framework to guide the user in the calculation of the current rating of a cable circuit in any situation, to list special situations in cable circuits and either to recommend an existing calculation method, to highlight possible calculation methods or to give indications for the calculation of the cable rating in every situation

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Introduction to the workgroup – Terms of reference / Scope of work

4. To assess and interpret the result of 1b and to report potential difficulties and problems with the methods, as well as to report recent developments in the methods

5. To prepare the deliverables

Scope of work All AC and DC cables with emphasis on HV and EHV cables, but where possible extended to MV as well

Deliverables Standard (TB, electra paper, tutorial)

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Introduction to the workgroup – Introduction to the questionnaire

Questionnaire was issued to learn how calculations are performed 101 companies answered Note: Answers have not been verified

Company type Result North

America South

America Europe Africa Asia oceani

c number Number number number Numbe

r number

Cable manufacturer

1 1 13 0 7 0

Consultancy company

2 1 8 0 1 0

Utility 11 2 24 2 12 8 Other (specify) 0 0 4 0 4 0 Grand total 14 4 49 2 24 8

101 replies in total

Questionnaire

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Introduction to the workgroup – Introduction to the Technical Brochure

• TB not yet published • TB fully reviewed by SC

B1 and external committee

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• To be used as reference book, next to standards • Single chapters on

o General issues with cable ratings (to be understood by all) o Starting points o Calculation tools and techniques o Conclusions and recommendations

• Multiple chapters on calculation guidance • Quick access to calculation guidance by:

o Lookup table in Appendix C o Mind Maps in Appendix D

• Full answers to questionnaire • 150+ References

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Mind map example (7 exist):

Indication of chapter / section where guidance can be found

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Introduction to the workgroup – Introduction to cable rating calculations

Within the TB, • an understanding on cable ratings is expected / required • the standards IEC 60287 and IEC 60853 are assumed to be

known

Within this tutorial, • a basic understanding on cable ratings and their calculations is

expected • but full details on calculation guidance as in the TB are not

given

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• Heat sources generate heat

o in conductor o in insulation o in metal sheath o in metal armour

• Heat is transported to the far cable surroundings by heat transfer o via “thermal resistances”

• Temperature limits exist: o on conductor o on outer jacket o elsewhere

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43221

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ac

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Central formula in IEC 60287 giving the current rating of a power cable:

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Overall issues with cable rating calculations

(2/6)

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Overall issues with cable rating calculations – Contents

• Circuit rating

• Temperature limitations

• Local differences along the cable route

• Basis for the calculation

• Margin in rating calculations

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Overall issues with cable rating calculations – Circuit rating

• Current rating from busbar to busbar is needed for operation (may contain cable system, OHL, switchgear, CT, transformer)

• Cable may / may not be limiting • Maximum rating of other components governed by other

aspects • Limitation may thus change in time

(cable in winter, OHL in summer)

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Overall issues with cable rating calculations – Temperature limitations

Typical temperature limitations – normal operation: • Conductor temperature limitation to avoid overheating of

insulation material Increasing beyond the maximum operating temperatures will impact degradation & remaining lifetime which typically is irreversible

• Jacket temperature limitation to avoid soil dry-out Soil dry out leads to deterioration of the soil thermal properties, leading to increased cable heating. Runaway can occur. Soil dry out can be irreversible and reversible

• Jacket temperature limitation to avoid safety problems In air-installations where cables could be touched, hot surfaces are avoided (e.g. o avoid burning a hand on a hot cable jacket)

• Air temperature limitation to avoid safety problems In tunnel / cellar installations, the air temperature could be limited to ensure a safe working environment to people

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Overall issues with cable rating calculations – Temperature limitations

Typical temperature limitations – emergency operation: • Conductor temperature allowed to rise higher than normal

operating temperature • Questionnaire outcome:

o Emergency durations are between 10 minutes and 15 days o Emergency temperatures used are between 90 and 130 °C for

XLPE, most popular is 105 °C WG note: Take care not to increase emergency temperature or duration too much. Degradation, or immediate failure may happen

Figure: Overheated cable

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Overall issues with cable rating calculations – Local differences along the cable route

• Chain is as strong as the weakest link • Rating limited by worst case situation along the route • Thus: cable route investigation is required • The thermal bottleneck of the cable circuit may be only small

in length • For reasons of practicality, consider everything >5 m in length

and consider smaller situations in case of extremely worsening environmental changes

Figure: Hotspot vss thermal bottleneck

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Overall issues with cable rating calculations – Basis for the calculations

IEC 60287 always / often used as a basis for calculations by 79% of all users Other basis for calculations are: • IEEE 835 & 848 (North America) • JCS 0501 & 168 (Japan)

Company type Results Always Often Occasional Rare Never Other

A cable manufacturer 13 6 1 1 A consultancy company 7 6 An utility 14 27 9 2 5 2 Other (specify) 5 1 1 1 Grand total 39 40 11 2 6 3 Note: References to commercial cable rating software are given as non-standard IEC method by some respondents.

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Overall issues with cable rating calculations – Basis for the calculations

IEC 60287 is often used, but many problems were indicated. In such a situation, the following approach is followed: • FEM analysis (17%) • Approximation (12%) • Hire consultant (9%) • Ask manufacturer (7%) • Use measurements (3%)

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Overall issues with cable rating calculations – Margin in rating calculations

Almost no cable fails due to overheating. Is there a margin to increase the loading? 1. Up to now, cables are not often loaded close to their rating, but this

may change 2. Margin in current rating is margin in calculation, PLUS effect of

inaccuracies in starting points 3. The latter is 10s of percents or worse, and much larger than the

inaccuracy in calculations 4. When engineering, margin is needed to include worst case and to

cover for inaccuracies in starting points The cable rating should be a reliable, well-considered value, in order to allow network planning and operations. Do note that: • When operating, margin can be reduced by using dynamic rating • The questionnaire learns that safety margins are not commonly

applied

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Overall issues with cable rating calculations – Margin in rating calculations

WG guidance: 1. Balance the accuracy of starting points, calculation method

and calculation tool evenly 2. Discuss the margin in the rating calculation in the bidding

phase, design phase and operation phase

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Starting points for rating calculations

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Starting points for rating calculations – Contents

• Identification of an underground cable system

• Installation of an underground cable system

• Thermal properties of soils and backfills

• Selection of ambient temperature

• Collected practices (questionnaire)

• Discussion and recommendations

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Starting points for rating calculations – Identification of an underground cable system

An understanding about the power cable system is needed before making a rating calculation • Cable type • Cable design • Materials used • Cable system design (changes between cable types, designs,

cross sections etc)

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On cable system design, identify: • Design of joint • Design of termination • Earth continuity conductors carrying current • Other issues influencing cable system current rating

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On material properties, use: • Manufacturer’s data on materials deduced from material

property verification tests • Industry standard properties as e.g. in IEC 60287

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On material dimensions, manufacturer is allowed to make tolerances when making the cable. The data sheet often lists nominal diameters / thicknesses, but these do not match up with the exact cable dimensions

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On material dimensions, use (with decreasing quality): • FAT test results on which measured dimensions are noted

(note that these may change within a production batch) • Factory data sheets • Specifications or industry standards (if other information is

unavailable)

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Starting points for rating calculations – Installation of an underground cable system

Installation conditions have very strong influence (up to ~75%) on the cable rating, therefore cable route must be studied in detail. Use (with decreasing quality): • On-site investigations • As-built information (drawings, studies) • Plan and profile drawings from engineering and design studies

If records are lost, inaccurate or incomplete, either estimations must be used, or investigations must be performed. Note the implication of accuracy differences!

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With this information: Catalogue the installation conditions along the route and list parameters, to find worst case situation at certain moments WG notes: • This situation may change position in time • This situaton may change behaviour over time • This situation may actually not be in the cable system • The situation may be 3 dimensional rather than 2 dimensional • There may be a difference between the worst case situation in

dynamic and stationary calculations Therefore, multiple calculations may be required to find the most limiting situation(s)

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With this information: Catalogue the installation conditions along the route and list parameters, to find worst case situation at certain moments Parameters of importance for direct buried situations (incl pipes/ducts): • Burial depth • Phase and circuit separation • Adjacent circuits • Thermal resistivity of native soil • Extent and characteristics of (special) backfill • Local ambient soil temperature • Dry out possibilities

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Pay attention to: • Soil properties including dry out potential in direct buried

situations • Type of conduits and type of grout / weak mix / concrete used

in conduit installation • Direct buried segments close to risers in conduit installations

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Pay attention to: • Soil properties in submarine cable routes (accuracy?) • The extent of free air ventilation in air-installations • Type and filling of casing pipes in trenchless installations

Possibilities for air pockets / steel casings?

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Pay attention to: • Trough type (open, closed, filled, unfilled, on surface or

buried), completely dry? • Fire countermeasures, solar radiation, cooling in tunnels, shaft

situations in tunnel installations

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Starting points for rating calculations – Thermal properties of soils and backfills

…a most important parameter to consider! Properties required include: • Soil composition (type of material, organic content, bonding) • Layered soils • Texture (fine/coarse, graded/uniform, natural/crushed) • Water content • Dry density • Other factors (dissolved salts, minerals, ageing) Once installed, soil properties except for water content typically do not change, however, note re-excavation, soil settling and backfill ageing

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Effect of water content on thermal resistance of a soil type

Design / engineering performed on worst case water content over lifetime Thus: • Different from actual

situation (influenced by season and weather)

• Different per soil type Establising thermal properties requires measurements and soil testing

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Soil dry-out Combination of heat transfer and moisture migration from the cable environment to elsewhere, transporting water away from the soil surrounding the cable system • Many discussions, conferences and papers exist • Philip – de Vries (PdV) model most generally accepted

Transport of water, water vapour and heat as functions of the gradients of temperature, moisture content and of gravity, depending on some external parameters as e.g. the distance to the groundwater level.

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Soil dry-out Combination of heat transfer and moisture migration from the cable environment to elsewhere, transporting water away from the soil surrounding the cable system • Many discussions, conferences and papers exist • Use the two-layer-model (in IEC) with critical isotherm and

dried out soil inside Model is widely adopted because of its simplicity. It could be shown (Brakelmann - 1984) by means of the Philip/de Vries-model, that the situation of the two-zone-model with a critical isotherm may actually happen in nature

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Soil dry-out Combination of heat transfer and moisture migration from the cable environment to elsewhere, transporting water away from the soil surrounding the cable system • Many discussions, conferences and papers exist • Note: There are groups not accepting the two-layer-model for

partially out-drying soils Their argument is, that the time-dependent transport mechanisms in soil are not governed by temperatures, but by the heat flow density

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How to establish the soil thermal resistivity: 1. Deduce soil type(s) in detail 2. Measure lowest groundwater table 3. Deduce lowest moisture content given 1 and 2 4. Either:

Calculate worst case thermal resistivity based on 1 and 3 Measure worst case thermal resistivity based on 1 and 3

5. And also: establish soil dry-out temperature & effect Note: • Direct TR field measurements may suffer from the fact that

these are not measuring the worst case situation • Soil dry out may lead to thermal runaways

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In case of selected backfills / special backfills • Perform the same as above for the material • Ensure the correct material is delivered on site (do you have

the right soil?) • Consider backfill dimensions and the native soil as well!

Which of these soils has better thermal properties? Which one to select, or to avoid?

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In case of submarine soils • Consider the sampling accuracy even more than on land

o accuracy in terms of amount of samples o accuracy in terms of disturbance of samples

• Consider the properties of rock berm / concrete mattrasses, sand bags etc. if applied

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Starting points for rating calculations – Selection of ambient temperature

Ambient temperature: The temperature at the cable location, if the cable system would not be there Influenced by: • Large scale regional differences (solar radiation, air

temperature, terrain differences, surface characteristics, thermal properties of soil)

• Meteorological elements (solar radiation, air temperature, wind, rain)

Ambient temperature: • May change with season / irregular • Harder to predict closer to the surface

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Selecting the ambient temperature • Ideally this value should be measured • IEC 60287 lists values, but exclude specifics Alternatives: • Calculate it from the air temperature / seawater temperature

over the years • Apply increases if

cables are under highly absorbing surfaces (e.g. asphalt ~+5K)

Undisturbed soil temperature in The Netherlands

0

2

4

6

8

10

12

14

16

18

20

jan feb mar apr may jun jul aug sep oct nov dec

Time [month]

Tem

pera

ture

[ºC

]

depth = 63 cm

depth = 125 cm

depth = 251 cm

depth = 502 cm

depth = 753 cm

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In cases of known air temperatures, but unknown soil temperatures, the following formula can be used to derive a theoretical ambient temperature at cable depth :

Taverage = Average annual air temperature, °C A = Difference between maximum and minimum air temperature, °C x = Depth below the earths surface, m a = Thermal diffusivity, m2/hr tO = Length of period, hours t = Time since maximum air temperature occurred, hours. (Williams and Gold, 1976)

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Starting points for rating calculations – Collected practices (questionnaire)

Interpreting the questionnaire to learn what is done in reality compared to what is needed

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Soil ambient temperature: • Assumed by 72% • Measured by 21%

If assumed: • Assumptions made by the designer because of a lack of

information If measured: • Thermocouples are used at various installation depths along

the cable route • DTS use increasing, then soil ambient temperature measured

before cable is put in operation

Using seasonal / regional values: Confirmed by 51% of responses

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Typical soil ambient temperature (land)

0 5

10 15 20 25 30 35 40 45

Soil

tem

pera

ture

(oC

)

-50

-40

-30

-20

-10

0

10

20

30

40

50

Soil

tem

pera

ture

(oC

)

Hot conditions

Cold conditions

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Typical soil ambient temperature (sea) Hot conditions

Cold conditions

0 5

10 15 20 25 30 35 40 45

Seab

ed te

mpe

ratu

re (o

C)

0

5

10

15

20

25

30

Seab

ed te

mpe

ratu

re (o

C)

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Typical assumed air temperature in tunnel Hot conditions

Cold conditions

0

10

20

30

40

50

60

Tunn

el /

galle

ry te

mpe

ratu

re (o

C)

0

10

20

30

40

50

60

Tunn

el /

Gal

lery

tem

pera

ture

(oC

)

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Soil thermal resistivity: • Assumed by 70% • But: 25% indicate a growing interest to measuring TR,

especially for new and larger land and submarine projects

If measured: • A soil sample every 50-500 m is taken, or at suspected

locations, or random • On specific depths and layers in the trench and soil type

changes

Using seasonal / regional values: Not applied by 63 / 55 % of responses

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Soil dry out: • Considered by 63% • Not considered by 36% Critical temperature varies between 20 and 70 °C Reasons to consider dry out: • Know soil characteristic (to dry out), water table, customer

specification, cable class

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Backfill characterisation by: • Thermal resistivity: 88% • Grain size distribution: 47% • Compaction: 57% Control on native soil above backfill: • Done by 42% • Not done by 45% In case of control on native soil, then: • Compaction level is checked: 38% • Thermal resistivity is measured: 29% • Thermal resitivity is cross checked with customers spec: 17%

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Influence of other heat sources: • Calculating exact influence: 68% • Adding a margin to the ambient temperature: 30% • Derate the cable: 26% • Not taken into account: 4% Alternative approach followed by some: • Use minimum separation distance.

Distance quoted: 1m when laid parallel, 500mm when crossing

WG Note: be careful with this!

Adding a margin: +5K: 27% +10K: 20% +15K: 1% +20K: 1%

Derate cable: Using standard: 31% Using derating by software: 15%

Calculating exact influence: By IEC: 20% By commercial software: 19% By FEM: 13%

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Temperature limitations: • Conductor temperature: 85%

Other limitations quoted include: • EMF requirements: 21% • Thermal limitations from other heat sources: 11% • Maximum allowed temperature rise near cables: 7% Recommendations from users: • Use a datasheet between engineering company and cable

user, detailing all starting points for performed calculations • Synchronizing, clarification and teaching of calculation

methods

Cable failures due to overheating By overloading: 4 By incorrect special bonding: 2 By high TR: 2 By changes in operation: 2 By field situation different than assumed: 2

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Starting points for rating calculations – Discussion and recommendations

• Ambient temperature is mostly assumed without a clear reference, occasionally measured

• Thermal resistivity is mostly assumed based on historical data, although it is increasingly being measured

• Soil dry out is taken into consideration by most • Backfill usually has a measured thermal resitivity value Important parameters thus are assumed rather than measured. This did only seldom lead to failures Given the uncertainties in assumed starting points, the need for more accurate starting points will be larger than the need for more detailed calculations

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Calculation methods and procedures

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Calculation methods and procedures – Contents

• Introduction

• Duty aspects

• Cable aspects

• \Installation aspects

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Calculation methods and procedures – Introduction

• Many issues considered o Only some issues are part of this tutorial o Issues can be found via lookup tables, mind maps, or reading

through the TB

• A procedure is followed for each issue considered: o Issue is described o Reference is given to a standard or publication o Or: a suggested solution approach is given o With the goal to help the user as much as possible

• Also meaning: not all answers are complete, not all answers

are ’standardizable’

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Some pages from the table of contents:

report

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Some pages from the table of contents:

report

Only various examples are given

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Calculation methods and procedures – Duty aspects

Duty aspects considered:

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Some issues considered: • Load factors for cyclic loading (we give a few) • Emergency and arbitrary loading • Voltage issues • Frequency issues (harmonics) • Single phase, 2 phase,

3 phase systems • DC systems

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Frequency Effect of frequency on cable rating discussed IEC 60287-1-3 remains valid Curve 3: Rdc + skin & proximity Curve 2: Wd added Curve 1: circ. currents added

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Harmonics • Total heat loss given by sum

of losses at each frequency • Simplification provided:

• With:

(THD = total harmonic distortion)

Notes: • Rn must be known. THD

alone is not enough

• Harmonics are typically damped by transmission in the power cable. The amount of harmonics thus is length dependent

• Harmonics also may be present in DC cables depending on the filtering present in converter stations

å¥

=

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2

21

11

nnn RR

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=2

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nnTHD g

( ) ( )( ) ( ) ( )( )nnnpnsn yyRR 2111' ll ++++=

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Calculation methods and procedures – Cable aspects

Cable aspects considered:

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Armour losses • 145 kV single armour (alternating armour-PE wire) cable measured loss factor:

• 132 kV double armour cable measured loss factor:

Results taken from Jicable 2011, Palmgren D. et al (2011).

Ic [A] 326.1 431.9 640.7 846.7

l1 0.1653 0.1680 0.1739 0.1798

l2 0.1132

0.1355

0.1609

0.1870

l2 (corrected to 90°C conductor temp) 0.0872 0.1044 0.1245 0.1430 l2 (according to IEC 60287) 0.2511

Ic2 303.5 402.6 600.1 789.0 l1 (two layers) 0.0992 0.1001 0.0998 0.1076 l2 (two layers) 0.0655 0.0700 0.0779 0.0712 l2 (corrected to 90°C conductor temp) 0.0500 0.0535 0.0603 0.0538 l2 (according to IEC 60287) 0.1541

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Armour losses • 132 kV single armour cable measured loss factor:

• 400 kV 3 core 1200mm2 AL SL type cable with single layer of steel wire armour, measured loss factor:

Results taken from Jicable 2011, Palmgren D. et al (2011)

Ic1 304.9 404.3 600.4 799.0 l1 (one layer) 0.0966 0.0985 0.1007 0.1043

l2 (one layer) 0.0597 0.0675 0.0816 0.0859 l2 (corrected to 90°C conductor temp)

0.0457 0.0515 0.0627 0.0649 l2 (according to IEC 60287) 0.1790

Ic 302.2 403.9 597.0 790.7 l1 0.3337 0.3272 0.3343 0.3366

l2 0.3774 0.4018 0.4169 0.4281 l2 (corrected to 90°C conductor temp)

0.2402 0.2560 0.2658 0.2722 l2 (according to IEC 60287) 0.476

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Armour losses • Single and double wire armours give rise to similar loss factors

measured o Explanation: inner layer shields magnetically shields the outer layer

• All measurements show measured values ~60% of that of IEC calculations

• Other references also give rise to believe that IEC calculation might be overestimating the armour losses

However, all measurements above made by only 1 cable manufacturer

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Armour losses WG recommendation: • To make additional measurements with similar and different

armour / cable designs and similar and different armour materials • Enough reliable measurements must be available to come to an

empirical model, or there must be fundamental work to come to a sound physical model explaining the measurement results. During the time this takes, the WG recommends to use the existing IEC calculations, unless one is absolutely sure about an alternative based on measurement results

• For tape armour, no new values are reported leading to the recommendation to use IEC calculations

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Calculation methods and procedures – Installation aspects

Common features considered:

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Evaluation of joint thermal rating • Limited information exists • Higher internal thermal resistance, lower external thermal

resistance, and a higher thermal capacitance • Typically joints have a higher conductor temperature when the

joint environment is the same as the cable environment • Air voids in joints may have significant impact (40 Km/W) • 3D problem, with longitudinal heat flow

89

90

91

92

93

94

95

96

0 0.5 1 1.5 2 2.5

Distance from centerline of joint, m

Tem

pera

ture

, °C

Inside joint outside joint

350 kcmil

1000 kcmil

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Buried installations (not tunnels) considered:

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Buried cables • Multiple thermal resistances in a trench:

o Using IEC (range of 0.33 to 3) o Using El-Kady and Horrocks (extended range) o Using Slanika and Morgan o Using transformal mapping

• Water cooled cable systems and inclusion of hot or cold objects in the ground

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Duct installations Important considerations: • Filling materials • Mutual heating • Inclination of ducts

Soil

Air / water

5 m 5 m

15 m

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Duct installations

Horizontal – air – closed 2D problem, governed by convection and radiation

Horizontal – water – closed 2D problem, governed by convection and radiation

Non-horizontal 3D problem, warmest locations expected to be near higher ends of ducts

Air - open Heat exchange to free air, dependend on geometry, Ambient air temperature plays a role

Water – open (underground) Watch out for airgaps and height differences

Temperature in a horizontal drilling

0

1 1

0 0

2 2

0 0

333333

00

22

00

33

00

22

00

11

00

22

00

11

00

33

00

22

00

22

00

22

00

11

00

444

0

5

10

15

20

25

30

35

9200 9300 9400 9500 9600 9700 9800 9900

Glass fibre length [m]

Tem

pera

ture

[C]

0

1

1

2

2

3

3

4

4

5

5

Tem

pera

tuur

[C]

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Air installations considered:

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Tunnel installations considered: Submarine installations considered:

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Air installations • Energy conservation equations given and explained • Convection coefficients given and referred to

Tunnel installations • Tunnel models discussed

o Naturally ventilated / unventilated tunnels o Forced air ventilated tunnels o Water cooled tunnels o Ambient temperature discussion

( )1 2convW h Aq q= -

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Submarine cables • External thermal resistivity discussion (0.2 – 2.5 Km/W) • Ambient temperature discussion • Cable on top of seabed: assumed to get covered

o Conservative estimate: 0.3 m of sediment with bad TR • J-tube calculation methods overview

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Using calculation tools and techniques

(5/6)

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Using calculation methods and tools – Contents

• Calculation techniques

• Types of tools

• Use of calculation methods and tools

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Using calculation methods and tools – Calculation techniques

Analytical • Most common • Based on electrical equivalent schemes leading to ladder

networks • Accuracy can be very high for simple systems • In complex cases, exact analytical solution may not exist. Then:

o Empirical data o Conformal mapping / transforms

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Empirical • Calculations deduced from experiments • Can be used to high accuracy • Often used in heat transfer coefficients for (combinations of

conduction) convection and radiation terms • Note: experimental conditions MUST match service conditions

Example:

1)0179.0(017.000115.035.110-++×-= wdg r

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Numerical • Using computers to solve complex problems by solving sets of

discretized sets of partial diffential equations by multiple iterations. Typically, a mesh is used, defining how calculation nodes relate to each other. The following types exist: o Finite difference – value at a node is a function of the value at other

nodes, using a uniform grid and typically Taylor series. Superseded by: o Finite element – using a mesh adapted to the geometry under study

and expanding the mathematical techniques to relate between nodes o Boundary element – less often used, solving

equations relating to linear, homogeneous materials o Computational fluid dynamics (CFD) – finite volume

methods (kind of 3D form of finite difference), extremely complex especially in turbulent flows

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Using calculation methods and tools – Types of tools

Two general types of tools exist: • Steady state and transient rating tools

Current ratings are calculated using starting points. These starting points are assumed, rather than actual values. Typically used in design and engineering phase On transient ratings: o 80% of utilities report to use emergency rating tools o 40% of utilities report to use cyclic rating tools

• Dynamic (or real time) rating tools

These tools use real time information as (some) starting points in order to remove some of the assumptions, and generally provide a larger current rating. Typically used in operation phase

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Steady state and transient rating tools • Analytical tools - (exact solutions via formulas)

o Tables (IEC 60502, 60055, IEEE 835) easy but limited. Can be used incorrectly

o Hand calculations with calculator or spreadsheet time consuming, but adaptable to many situations. Verification can be difficult

o Bespoke current rating software (written for a user) can be simple or more complex, but the latter will be difficult to verify

o Commercial current rating software (made available to many) more complex, more functionality. Not always documented, such that it is unclear how the calculation is performed, what assumptions are made, or if the calculation is applicable in the client’s case as well. This makes full verification of the software a requirement

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Steady state and transient rating tools • Numerical tools

(set of linked equations needing a number of iterations to come to a converging solution) performed by computer programs, requires trained persons to calculate and to define boundary conditions. In case of fluids, calculations will be difficult

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Dynamic (or real time) rating tools • Repeated calculation based on real time input • Should be a busbar to busbar rating system to be useful to the

operator • Are specific for a certain cable circuit

Inputs • Historic & actual loading • Predicted future loading • Historic & actual ambient temp. • Predicted future ambient temperature • Others (e.g. tunnel air velocity) • Historic and actual system

temperature

Note: accuracy of input should be taken into account

Outputs • Temperature of system • Maximum loading • Rating study output given a scenario • Current rating starting points (e.g. air

temperature in a cable tunnel, or calculated ground thermal resistivity)

Note: changes may occur in the latter deduced starting points!

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Use of dynamic (or real time) rating tools • A DRS should be part of a busbar to busbar system • Main two scenarios studied with DRS are:

o Time limited current rating increase (to schedule outages) o Long term current rating increase (to postpone investments / to

optimally utilize assets) – note starting points and ’worst case’ scenarios!

Verification and testing of DRS No standard methods available, but possibilities are: • Verification against measurements in actual situations • Verification against measurements in laboratory situations • Simulations using a thermal model to compare the predictions of a

DRS

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Using calculation methods and tools – Questionnaire outcomes

Steady state rating calculation tools • Majority uses in-house solutions based on IEC / Neher Mc Grath,

hand or spreadsheet tools

Emergency rating calculation tools Emergency rating performed by 76% of all utilities, the majority using IEC 60583

Calculation method Result

Yes No Blank hand or spreadsheet 57 30 14 rating tables 33 44 22 in house (IEC Neher/McGrath) 57 30 14 commercial (not FEM) 33 51 17 FEM 25 56 20 Other 15 51 35

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Transient rating calculation tools • Majority of utilities do not calculate transient rating themselves • Most of the other users do • The method applied is as follows:

Company type Result Calculated

Not

calculated Blank

Total

Cable manufacturer 16 5 1 22 Consultancy 8 3 2 13 Utility 24 30 4 58 Other 6 2 0 8 Total 54 40 7 101

Emergency rating method

Result Yes No Blank

In house, IEC 31 17 7 In house, CIGRE 11 27 13 Commercial program 25 17 10 Other 13 23 15

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Comparison of calculated and measured cable temperatures for continuous ratings

• 40% made a comparison between calculated and measured

cable temperatures • Majority used DTS system • Some noted steady state condition was not achieved, and a DRS

was needed • Only few comments on the outcome were given (often good)

Company type Result Yes No Blank

Cable manufacturer 7 13 2 Consultancy company 5 6 0 Utility 22 37 1 Other 6 2 0 Total 40 58 3

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Temperature monitoring with glass fibre Reported position of the glass fibre in the cable system:


Inside cable, cable screen 41 21 36 Cable surface 38 20 40 External parallel conduit 33 33 42 Other 8 33 57

Figure: taken from De Wild et al, OFS 2000

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DTS system: • 39 companies applied permanently installed DTS, 20 companies

applied temporary installed DTS, 3 companies used both • About half of the companies verified the accuracy of the DTS

system

• Mostly, good accuracy was found, but problems are reported, especially with longer lengths


Cable manufacturer 11 5 6 Consultancy 4 4 5 Utility 15 20 23 Other 3 2 3 Total 33 31 37

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DTS system: Number of companies reporting limitations or difficult experiences with DTS The following themes are dominant: • Unreliable systems • Difficult to use or to interpret • Accuracy / calibration problems



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DRS systems: Number of companies using DRS or RTTR • Most companies do not use DRS or RTTR, but many stated to

consider using them • Typically, utilities do not develop their own systems, but

manufacturers and consultants do • Utilities predominantly apply commercial available DRS or RTTR

systems

Company type Result Yes No

Cable manufacturer 10 12 Consultancy 5 8 Utility 15 36 Other 1 7 Total 31 66

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DRS systems: • The majority of users of a DRS system use temperatures from a

DTS system as input (~70%) • Only 7 respondends reported to use a safety factor as margin • The majority of systems of respondents are standalone systems,

but amongst utilities, 50% is standalone, 50% is integrated

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DRS systems: Number of respondents reporting difficulties: Difficulties relate to: • Reliability of measurements • Accuracy • The use in a control room • Rating estimation at low loadings



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Conclusions and recommendations

(6/6)

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Conclusions & recommendations

• Guide was produced giving insight and guidance in many areas encountered when calculating the current rating (starting points, calculations and tooling)

• Guide is full with references (150+) to relevant literature • Guide is equipped with lookup tables and mind maps for easy

use

With this guide it is hoped that companies worldwide will use similar approaches when dealing with similar problems, so that in the longer run, even solutions to the most difficult problems become available.

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Starting points • Although cables do rarely fail from overloading, this may be

due to the often moderate loading of cables. This may change in future!

• As rating evaluation studies come at a certain limited accuracy, there is need for margin in cable rating calcuations. This margin is proposed to be discussed and agreed between parties in any cable project

• The most important starting points seem often to be assumed rather than measured or investigated. This leads to large uncertainties. o Pay attention to measuring / investigating starting ponts o Evenly balance the accuracy in starting points, calculations and

tools

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Calculations • Guidance for calculations in many situations has been given. • Sometimes, too little international experience is available to

propose guidance. More abstract guidance has been given there and the WG recommends experts to publish their information on: o Ratings for HVDC cables o Effect of air / oil pockets in cables with e.g. corrugated sheaths o Armour losses o Joint and termination ratings o Ratings of cables in inclined water / air filled ducts

Tools and techniques: • Dynamic rating slowly atracts more attention • The user needs to verify the calculation tool before using it

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Questions?

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End of tutorial on cable ratings

Thank you for your attention!

For more information: [email protected]

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Extra – HVDC cables Electrical stress and temperature

• In an AC cable, electric stress (kV/mm) depends on capacitive effects. Therefore the permittivity of the material determines the electrical stress. This permittivity is rather stable

• In a DC cable, electric stress depends on resistive effects. Therefore the resistivity of the material determines the electrical stress. This resistivity is very temperature dependent

• If the circuit is not in operation: uniform temperature, highest stress near the conductor

• If the circuit is in operation with high current: temperature higher near the conductor, resistivity lower, highest stress near the sheath

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Extra – HVDC cables Electrical stress and temperature

Formula for resistivity: where ρ0 = resistivity at reference temperature (Ω m) θ = difference in temperature between the actual and reference temperatures (K) α = temperature coefficient of electrical resistivity (per K) β = stress coefficient of electrical resistivity (per MV/m) E = electrical stress in the insulation (MV/m)

( ) ( )Ebaqrr --= expexp0

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Extra – HVDC cables Current rating

The current rating of a HVDC cable is limited by: • The limits on electrical stress

(! – also note these may be dynamic) • The maximum operating temperature of the

insulation material reasons for such maximum operating temperature: o slow ageing of materials, e.g.:

• ‘breathing’ and cavities of MIND cables • chemical decomposition of PE

o fast material destruction, e.g.: • Large cavities in MIND cables, lead

sheath problems • Softening of PE and possibility of forces

acting • Melting

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Extra – HVDC cables Current rating

Excluding the electric field dependency (!), IEC 60287 gives formulae for calculation of the current rating:

where: I : continuous permissible current [A] Ɵc: maximum conductor temperature Ɵe : ambient temperature of the environment Rdc : DC resistance at maximum conductor temperature n : number of conductors in the cable T1, T2, T3, T4 : thermal resistances àT4 represents the cable environment up to the earth surface, typically soil

( )( ) ú

û

ùêë

é+++

-=

][ 4321 TTTnTRI

dc

ec qqTth

Tth

Tth

Tth

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Extra – HVDC cables Further rating increases

• Decrease resistance: towards larger conductors 3200, 4000 mm2? o Manufacturing and installation with heavier cables is cumbersome

• Decrease resistivity of the cable environment: use better soils? o For land application, this is a realistic possibility. Soils exist with thermal resistivity

under 0.4 Km/W, which is a significant improvement compared to soils with a more common thermal resistivity of 0.8 – 1.2 Km/W for native soils (which also may dry-out).

o For submarine application, this is not a viable option.

• Decrease amount of thermal bottlenecks o Unfortunately, investigations learn that often the transport capacity is limited by a

short section in the cable connection (thermal bottleneck). Improving the environment in that bottleneck, improves the current rating of the complete system. There were per design bottlenecks in e.g. a shore crossing limits the transport capacity of the complete system, it may be interesting to investigate if this bottleneck indeed is unavoidable.

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Extra – HVDC cables Further rating increases

• Make usage of dynamic rating systems (DRS) to increase current without surpassing temperature limitations o For cable systems experiencing a time varying transport current (e.g.:

wind farm, merchant line, utility asset, but not a base load generator feeder) it is often quickly useful to apply DRS. One can gain tens of percents of loading, given the specific limitations to the cable’s usage.

• Increase operating temperature: extruded already at 90ºC à change materials? o Work is ongoing on in R&D scale

• Influence the coupling of electric stress and thermal gradient by the insulation electric resistivity. Remove thermal gradient limitations? o Especially when the transport current is very dynamic and in cases where

DRS are applied, it may be of importance to ensure that the thermal gradients over the cable do not lead to electric stresses beyond their design values, which may happen e.g. during ramp up or ramp down. If there are thermal gradient limitations, these should either be very well known, or removed from being limiting to the cable usage.

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