Cableizer ®

CIGRE TB 880

Introduction

The CIGRE Technical Brochure (TB) 880 is the final report of the CIGRE Working Group (WG) B1.56 and discusses the verification of current rating calculation tools.

The report has been published End of 2022 and provides detailed guidance and case studies with the goal of verifying techniques and tools used for the calculation of the current rating of a power cable. The report is intended to be used by cable specialists who perform calculations themselves or request current rating calculations from others.

The current rating depends not only upon the electrical and thermal parameters of the cable itself but also upon the thermal parameters of the environment in which it is laid. These parameters cannot be known with great detail. It is also worth noting the ambiguity in the forecast of soil temperatures, both for the variations in environmental parameters and for the possible presence of external heat sources nearby to the route. From an engineering standpoint, this justifies the fact that the current rating of a cable is not a unique value but it is more accurately a range. This range must arise from an engineering awareness of the current rating concept and not from arbitrary application of IEC 60287.

With the help of this TB, the user can verify a calculation technique, calculation tool or software tool before using it.

Summary

Cableizer ran all case studies in CIGRE TB 880 and reached almost full agreement with a mean deviation over all 27 tests of < 0.01 %. We are still working on further improvements.

Test date 2025-05-28* 2025-06-16 2025-06-22
Max deviation pos | neg 0.229 % | -0.099 % 0.2289 % | -0.0986 % 0.2321 % | -0.0986 %
Mean | standard deviation 0.009 % ± 0.067 % 0.0092 % ± 0.0667 % 0.0098 % ± 0.0598 %

You can download a complete calculation report for all case studies.


Introductory case studies

The first case study has been prepared by the working group with a configuration as simple as possible, not only in terms of the installation, but also to perform the easiest calculations (small number of formulae, basic equations, parameters independent of the temperature, shared values for all cables):

  • Conductor = 630 mm² in copper, suitable for the installation in a narrow trench, with a stranded design (no specific calculation vs. Milliken conductors, ks = kp).
  • Metal sheath = a single laminated aluminium foil (no copper wires).
  • Voltage level = 132 kV, in a range between high and extra high voltage, which could be operated in almost any transmission utility.
  • Installation = firstly directly buried, to simplify the calculation of the external thermal resistance, especially in the case of touching cables in trefoil formation, for which the T4 value is common to the three cables. No duct, neither backfill nor concrete bank: the soil is considered as homogeneous.
  • Earthing mode = sheaths bonded at both ends of an electric section (with circulating currents losses, with the eddy-current losses ignored).
  • Standard operating conditions, no soil drying. The maximum conductor temperature is given.

The initial configuration is as basic as possible and was then customised in different variants, in order to explore most of the installation and operation modes:

  • Trefoil and flat geometries, with touching cables or not,
  • Cables in ducts, embedded in concrete or laid in a backfill, cables in free air exposed to direct solar radiation, cables in an unfilled trough,
  • Earthing mode with single point bonded cables.
Case study Guidance Point Results Deviations
Nr. Chapter Title Description 6
 8
 20
 31
 47
 CIGRE
[A]		 Cableizer
[A]		 $\Delta$
[A]		 $\epsilon$
[%]
0-1 4 Case 0 directly buried IEC 2015 Test IEC ON 821.78 821.78 0.0000 0
0-1 4.6.1.1 Approximation on depth for T4 IEC ON Test IEC ON 821.81 821.81 0.0000 0
0-1 4.6.3 Variant with single point bonding IEC 2015 Test IEC ON 886.18 886.18 0.0000 0
0-1 4.6.4 Variant with non-neglected eddy-current loss ON 2015 Test ON ON 803.16 803.16 0.0000 0
0-2 4.7 Sub-case study with touching HDPE ducts in ducts IEC 2015 Test IEC Test 682.81 682.81 0.0000 0
0-2 4.7.7 Variant with non-neglected eddy-current losses ON 2015 Test ON Test 679.84 679.84 0.0000 0
0-3 4.8 Sub-case study with PVC ducts in flat formation embedded in concrete ductbank IEC ON Test IEC ON 634.07 633.45 -0.6253 0.1
0-3 4.8.5 Variant with single point bonding IEC ON Test IEC ON 904.55 904.55 0.0009 ~0
0-3 4.8.6 Variant with non-neglected eddy-current losses ON ON Test ON ON 633.04 632.42 -0.6193 0.1
0-4 4.9 Sub-case study with cables laid in free air directly exposed to solar radiation in air IEC ON Test IEC ON 990.94 990.94 -0.0004 ~0
0-4 4.9.5 Variant with cables protected from direct solar radiation IEC ON Test IEC ON 1141.37 1141.37 -0.0007 ~0
0-4 4.9.6 Variant with single point bonding IEC ON Test IEC ON 1046.49 1046.49 -0.0004 ~0
0-4 4.9.7 Variant with non-neglected eddy-current losses ON ON Test ON ON 974.28 974.28 -0.0004 ~0
0-5 4.10 Sub-case study with cables in an unfilled trough in trough IEC ON Test IEC ON 764.56 766.31 1.7498 0.23
0-5 4.10.7 Variant with non-neglected eddy-current losses ON ON Test ON ON 754.86 756.56 1.6967 0.22

Case studies

The following case studies comprise many different cable types and installation conditions. The case studies have been selected on the basis that each follows a different set of calculations and therefore the more case studies that are used, the better the verification will be. For some users, a subset (e.g. power cables for distribution voltages) may be adequate, however we calculated all cases for your reference.

Case study GP & preferences Results Deviations
Nr. Chapter Title Description 8
 20
 Mutual heating CIGRE
[A]		 Cableizer
[A]		 $\Delta$
[A]		 $\epsilon$
[%]
1-1 5.1 Direct buried 132kV cables trefoil trefoil, touching, solid bonded ON Test 990.54 990.54 -0.0008 ~0
1-2 5.2 Direct buried 132kV cables flat flat, spaced, cross-bonding ON Test apply $\Delta\theta_{p}$
apply $F_{mh}$
apply $\Delta\theta_{p}$ equal losses
apply $F_{mh}$ equal losses		 1460.45 1460.40
1459.67
1457.74
1457.74		 -0.0547
-0.7806
-2.7098
-2.7137		 ~0
0.05
0.19
0.19
2 6 A 30kV submarine array cable three-core steel armour ON Test 838.34 838.34 0.0000 0
3 7 A 230kV HPFF cable in backfill with cyclic rating Test
ON		 Test 1187.36 1187.38
1184.32		 0.0290
-3.0377		 ~0
0.26
4 8 A 33kV land cable trefoil, touching, solid bonded 2015 Test 537.46 537.46 0.0000 0
5-1 9.2 A 400kV LPOF cable trefoil trefoil, touching, solid bonded 2015 Test 903.62 903.63 0.0122 ~0
5-2 9.3 A 400kV LPOF cable flat flat, spaced, cross-bonding ON Test apply $\Delta\theta_{p}$
apply $F_{mh}$
apply $\Delta\theta_{p}$ equal losses
apply $F_{mh}$ equal losses		 1590.23 1590.24
1585.20
1570.64
1570.64		 0.0197
-5.0221
-19.5891
-19.5900		 ~0
0.32
1.23
1.23
6 10 A 400kV single core AC submarine cable circuit three cables flat wide spacing ON Test apply $\Delta\theta_{p}$
apply $F_{mh}$
apply $\Delta\theta_{p}$ equal losses
apply $F_{mh}$ equal losses		 1039.43 1039.23
1039.34
1039.23
1039.23		 -0.2030
-0.0898
-0.2060
-0.2085		 0.02
0.01
0.02
0.02
7 11 A 320kV HVDC submarine bipole two cables flat spaced ON Test apply $\Delta\theta_{p}$
apply $F_{mh}$		 2311.10 2311.10
2311.10		 -0.0001
-0.00001		 0
0
8 12 A 220kV 3-core submarine export cable three-core stainless steel armour ON Test 1134.82 1134.82 0.0012 ~0
9 13 A 110kV retrofitted cable triplex cable in pipe ON Test 572.43 572.43 0.0000 0
10 14 A 10kV three core PILC cable three-core sector-shaped ON Test 165.74 165.74 0.0000 0

Interpretation of differences

The verification of our software against TB 880 found differences the results presented in the brochure and those from the software. Any difference has been investigated with the aim to understanding its origins and possibly correcting the calculation tool. In case of a difference, it was checked if all input parameters and all formulas are exactly correct.

The differences which still exist have to do with methodology that is applied differently:

  • Differences of less than 0.0001% were found in case study 0-4 with cables in air. This is due to a different approach of calculating the increase in surface temperature. The approach according to IEC-Standard, which was used by TB 880 as well, is unreliable for cables in common ducts and for non-cable objects and it does not allow different values of $\Delta\theta_{s}$ for different phases. We therefore consider our approach superior.
  • We calculate backfills with different equations and consider correction factors which allow to have multiple different systems, including of non-cable type, inside the same backfill area. This was explained in a blog post Buried projects with backfills from 2020. This leads to a difference of 0.1% in Case study 0-3 and 0.26% in Case study 3. We introduced a special test setting to use an almost identical approach for the specific case study 3 with cyclic loading which reduced the difference to 0.002%.
  • The difference of 0.22% in case study 0-5 of cables in touching flat arrangement with solid bonding is unclear.
  • For multiple non-touching cables we investigated with different methods, using $\Delta\theta_{p}$ or $F_{mh}$. In the preferences the user can select which method to apply.
    • Applying $\Delta\theta_{p}$ gives very good agreement for all cases. We recommend always using this method.
    • In the case study 1-2, direct buried 132kV cables, considering $\Delta\theta_{p}$ shows very good agreement with an error of 0.0037%. By applying $F_{mh}$ this increases to 0.0535%.
    • In the case study 5-2, 400kV LPOF cable flat, considering $\Delta\theta_{p}$ shows very good agreement with an error of 0.0012%. By applying $F_{mh}$ this increases significantly to 0.3158%.
    • In the case study 6, 400kV single core AC submarine cable circuit with very large spacing, considering $\Delta\theta_{p}$ gives good agreement with an error of 0.0195%. By applying $F_{mh}$ this reduces to 0.0086%.
    • In the case study 7, HVDC bipol in flat arrangement, considering $\Delta\theta_{p}$ gives very good agreement with an error of 0.0001%. By applying $F_{mh}$ we reach almost perfect agreement with an error of 0.000005%.
    • For multiple non-touching cables we also investigated the inpact of considering equal losses. In case 5-2 the impact was significant (1.23%), in case 1-2 noticable (0.19%) and case 6 with very large spacing of 10 m irrelevant.
  • The differences of 0.0001% in case studies 0-3 with single-side bonding (ssb) and 0.00008% in case study 1-1 are due to tiny rounding differences as a result of how the position of the cables are stored and processed. Our approach allows user defined positioning of the cables in the ground and not only touching or equally spaced trefoil and flat arrangements.
  • A difference of 0.0001% in case study 8 remains without further analysis.

At the moment, the only differences which still exist and cannot be fully explained, are in case study 0-5 with cables in an unfilled trough. We assume the differences to be related to losses in screen/sheath. We consider this not of importance because the difference was only 0.2-0.3% in all cases and because single-core cables are typically not bonded on both ends. We may further investigate this at some later stage. It should be noted also that TB 880 recommends to round the ampacities to the nearest 1 A, 5 A or 10 A value anyhow, as explained in Guidance Point 2 .