Variation of spacing between sheath bonding points
For single-core cable circuits with sheaths solidly bonded at both ends and possibly at intermediate points, the circulating currents and the resulting losses increase as the spacing increases, and it is advisable to use as close a spacing as possible. The optimum spacing is achieved by considering both losses and mutual heating between cables.
Posted on July 3, 2016
The IEC 60287-1-1 chapter 2.3.4 has recommendations for the calculation of sheath circulating current losses when it is not possible to install cables with a constant value of spacing over the length of one electrical section.
- Where spacing along a section is not constant but the various values are known (as well as the length of the sub-sections), the value for the reactance $X_s$ which is used to calculate the loss factor for sheath and screen $λ_1$ shall be derived using the given formula. The formula contains the lengths with different spacings along an electrical section and the reactances per unit length of cable calculated with the relevant formulae and appropriate values of spacings.
- Where in any section the spacing between cables and its variation along the route are not known and cannot be anticipated, the losses in that section, calculated from the design spacing, shall be arbitrarily increased by 25 %, a value found to be appropriate for lead-sheathed high voltage cables. A different increase may be used by agreement if it is considered that 25 % is not appropriate for a particular installation.
- Where the section includes a spread-out end, the allowance in point 2 may not be sufficient and it is recommended that an estimate of the probable spacing be made and the loss calculated by the procedure given in point 1 above.
The recommendations give values for loss factors which apply to the whole of a section, but the appropriate values of conductor resistance and external thermal resistance must be calculated on the basis of the closest cable spacing at any place along the section. The optimum spacing is achieved by considering both losses and mutual heating between cables.
Implementation in Cableizer
Spacing variation for double-sided bonded single-core cable systems acc. to IEC 60287-1-1, chap. 2.3.4 has been implemented in Cableizer. You have the following possibilities based on the three IEC recommendations:
- You can enter three section lengths and variations of spacing with respect to the "normal" spacing. For example, if you have a spacing of 5 cm, entering 1.5 [p.u.] for section 2 will lead to a spacing of 7.5 cm in that sub-section.
- You can select "unknown spacing" which adds 25% to the screen/sheath losses and you don't have to enter lengths or variations of spacing.
- which leads to point 1 above (paragraph a from the IEC) and requires no special input.
This variation of spacing has only an influence on the screen/sheath losses and does not consider higher or lower mutual heating due to changing distances between cables which has a larger impact on ratings. That is why we limited the input of variation to +/- 50% of center - center spacing.
- The functionality is only available for systems with 2 or 3 single-core cables with double-sided earthing.
- The laying arrangement can be flat horizontal or flat vertical, but it is the same for all sub-sections.
- In Cableizer, spacing is entered as space between the cables and not center-center distance which is calculated by the software. So for example: if spacing between cables is set to 5 cm, then a variation of 1.2 leads to a spacing center - center of 1.2 x cable (or duct) diameter +1.2 x 5 cm.
- The software checks if the distance is less than the touching distance and if the input is 0.5 <= s <= 1.5.
- We implemented this feature to be consistent with inputs for cross-bonding, meaning the same section lengths are used for variation of spacing for cables bonded on both sides even though the meaning of the sections are technically not the same. If you want to apply the variation of spacing for two sections only, you have to combine the first and second, or second and third section.
- For two-phase systems (like railway systems with 16.7 Hz) there are only two sections which again is consistent with cross-bonding of such systems.