Specific cable installation data

This data is used to provide further details about the installation pertaining to specific cable types used in the installation.

An installation may have many cables, but only a few types of cables. Specific installation data have to be entered for every cable type. Once specific installation data is specified for one cable type, the information remains for all cables of this type in the installation.

84 CHAPTER 5 –- STEADY STATE THERMAL ANALYSIS When an execution is duplicated and the cable types are changed, the user should always specify the specific installation data for all the new cable types. The program will not assume any specific installation data.

5.8.3.1 Bonding

The bonding arrangement is a very important factor for ampacity calculations. When the cable sheaths are bonded and grounded at both ends, large circulating currents result, which may considerably decrease the permissible cable ampacity.

For crossbonded and single point bonded systems, only eddy current losses are present (continuous cylindrical sheaths assumed). These losses are much lower than the losses due to the circulating currents in the sheaths when the cables are not crossbonded. For single point bonded systems, standing voltages arise usually at the open end. This voltage can be of concern, particularly for personal safety, and the program calculates it.

The following figure shows the differences between two-point (also referred to as multiple-point bonding or bonded ends) the single-point bonding arrangements and crossbonded sheaths.

Two

Two--Point (multiPoint (multi--point) Bondingpoint) Bonding

Large Circulating Currents No Standing Voltages

Transposition reduces circulating currents

Single Point Bonding

Single Point Bonding

V_STANDING V_STANDING

No Circulating Currents Standing Voltages Inexpensive

CHAPTER 5 –- STEADY STATE THERMAL ANALYSIS 85

Cross Bonding

Cross Bonding

No Circulating Currents No Standing Voltages

Installation Expensive – Reduced Losses

Crossbonding can be applied with equal or unequal section lengths. In the former case, circulating currents are the minimum while in the latter some circulating currents may exist. In the case of unequal section lengths, the program requests the user to use the length of the shortest section as reference and define the remaining two sections (longer and longest) by using the length ratios longer/shortest (AN) and longest/shortest (AM) to quantify the degree of asymmetry and thus calculate the circulating currents in the sheaths accordingly.

Concentric wires shields and sheath reinforcement assemblies follow the bonding option selected for the sheaths. For single-point bonded and cross-bonded systems only sheath and sheath reinforcement eddy currents are considered as losses. Armour wires are always assumed to be bonded and grounded at both ends. Non-magnetic armour is combined with the sheath for circulating current loss computations. Non-magnetic armour wires, in the absence of a sheath should always be modeled as concentric neutral wires.

Since it is not always possible to install cables with one value of spacing along a given route, the program supports unequal spacing of cables. The following relate to the calculation of sheath circulating current losses for 2-point bonded systems when a situation like this occurs. A section is defined as the length along two points of the cable route where shields are solidly bonded. Loss factors have to be calculated based on conductor and external thermal resistance of the closest cable spacing along the section.

a) When spacing along a section is not constant but the various lengths are known, the value for X are derived as per IEC 287 as follows:

n b a n n b b a a

L

L

L

X

L

X

L

X

L

X

+

+

+

+

+

+

=

...

...

La, Lb,..., Ln are lengths of different spacing along a section and Xa, Xb,..., Xn the

reactances per unit length of cable, with appropriate values for the corresponding spacing Sa, Sb,..., Sn.

It is assumed that the cables are in flat formation. Note that the same considerations are also applicable for single core cables arranged in triangular formation. S is the spacing between either one of the outer cables to the middle cable. Here the spacing of the two outer cables is assumed to be equal. If not, enter the GMD (geometric mean distance).

86 CHAPTER 5 –- STEADY STATE THERMAL ANALYSIS b) If the spacing of cables along a section are not known or cannot be really anticipated in the preliminary design stages, the losses will be considered increased by 25%. This is considered to be a typical value.

3-Core cables with metallic tape screens

If the cable has tape screens around each core and no other metallic parts encompassing all three cores, the program will calculate the ampacity assuming that these screens are grounded at one end. If the screens are multipoint grounded, the program will still apply the screening factors but will not calculate any circulating currents in the screens. In the latter case, the results will be somewhat optimistic.

If the cable, in addition to the above-mentioned screens, has another concentric neutral wires or non-touching armour wires around all three cores the program, as before, will not calculate any circulating currents in these metallic assemblies. The calculated ampacities will again be on the optimistic side.

If the cable, in addition to the above-mentioned screens, has another CONTINUOUS metallic part encompassing all three conductors (i.e. sheath, reinforcing tape, armour tapes, etc.) the program will calculate the ampacity correctly for any bonding arrangement.

3-core cables with wire screens (including equalizing tape around the wires)

If the wires screening the cores are not touching and the equalizing tape is thin and not overlapping with a long lay (normally this is the case), the circumferential heat transfer towards the outer cable components is negligible. Thus, one can proceed without applying any screening factor as is mandatory for the case of continuous metallic tapes. The wires can therefore, for ampacity calculation purposes, be neglected under the circumstances. If they need to be represented, the cable can be modeled as SL-type cable. The error this approximation entails is on the optimistic side, i.e. the program will give slightly higher ampacities if there are no other metallic parts surrounding all three cores and the screen wires are multipoint grounded. If, however, the wires are multipoint grounded the program cannot be used, in its present configuration, to analyze this case. Single core cables

In the case there are both metallic tape and wire screen around the cable core, treat the tape screen as sheath and the wire screen as concentric neutral.

For single point bonded sheaths, the wire screens can be neglected, the tape screen should be combined with the sheath and the combination represented as sheath.

If there is metallic screen, armour wires and the metallic parts of the cable are single point bonded the program will not calculate eddy current losses in the screens. The screens in this case can be represented as sheaths.

In the case that all metallic parts of the cable are multi-point bonded and grounded, the present version of CYMCAP will not calculate circulating losses in the screens. The only remedy will be to combine the electrical resistance of the sheath, screens, tapes and concentric neutral. This, however, is a tedious and delicate process.

CHAPTER 5 –- STEADY STATE THERMAL ANALYSIS 87 5.8.3.2 Barring certain bonding options

Bonding arrangements need to be defined for every cable type within the installation. Depending on the geometrical disposition of the cable layouts the application may bar certain bonding arrangements as a precaution to invalid data entry. For instance, triplexed formations will not be permitted to be assigned bonding options pertinent to flat arranged cables. Non-accessible bonding options are shown with the Pad-lock option, the same symbol used for invalid selections in the cable library.

5.8.3.3 Cables touching

The program supports the following options: a. Single conductor (core) cable

b. Single conductor (core) cables touching

If trefoil formations are selected in the installation data, the program will automatically assign the cables as "touching".

5.8.3.4 Cable transposition

The transposition of single conductor cables reduces the circulating currents in the sheaths when cables are bonded at both ends and they are arranged in flat formation. Both options are supported:

• Cables are regularly transposed • Cables not transposed

This consideration is relevant only when the single-core cables are specified as being two point bonded. Furthermore, the specification of transposition bears no relevance for the case one single conductor per cable is specified. Single core cables in triangular formation are assumed transposed. The notion of transposition is only applicable to three-phase circuits composed of 1 single core cable per phase.

5.8.3.5 Duct bank/duct materials and construction

When cables are installed in ducts, CYMCAP supports the calculation of the external thermal resistance as a function of the duct construction. The following choices are supported: ρD

(duct thermal resistivity °C-m/W) can be user supplied or selected from the list below.

Material

ρ

Metallic conduit (non-magnetic) 0.0 Metallic conduit (magnetic) 0.0

Fiber duct in air 4.8

Fiber duct in concrete 4.8 Asbestos duct in air 2.0 Asbestos duct in concrete 2.0

PVC duct in air 7.0

PVC duct in concrete 7.0 Polyethylene duct in air 3.5 Polyethylene duct in concrete 3.5

Earthenware duct 1.2

High pressure gas filled pipe type 0.0 High pressure oil filled pipe type 0.0

88 CHAPTER 5 –- STEADY STATE THERMAL ANALYSIS Remarks:

• The duct/duct bank material, along with its dimensions, is used to determine some constants necessary for the computation of the external thermal resistance of the cable. When the option User supplied RDH is selected, the user has some flexibility in providing the thermal resistivity of the duct material. The duct construction, however, MUST BE one of the 12 listed above. For example, if the case at hand exhibits asbestos ducts in air and the thermal resistivity of the asbestos variant used is different than the one tabulated in entry 4 above, the user can supply the required asbestos thermal resistivity by selecting its RDH, but the entry asbestos ducts in air must be selected.

• For the case where plastic ducts are considered (PVC and polyethylene), CYMCAP, in the absence of officially tabulated experimental values, will consider the same constants as in the case of asbestos ducts for the calculation of the thermal resistance of the air in the duct.

5.8.3.6 Fraction of return current for single phase cables

By “fraction of reference” CYMCAP defines the return current in the concentric wires assembly, sheath or shield to that matter for circuits composed of 1 single-phase cable only. This is defined in p.u. of the conductor current and cannot exceed 1. A value of 1 for the fraction of reference means that all the current returns through sheath, shield or concentric neutral. A value of 0 means that no return current exists. The value for the fraction of reference is important for ampacity calculations. The higher the fraction of reference the lower the ampacity due to losses associated with the circulation of the return current.

Notes:

• This quantity is only pertinent for single conductor cables and must be entered if the default value is not desired. Three-core cables are always assumed to carry symmetrical loads in the three conductors and no return current exists.

• When three single conductor cables are modeled with the same circuit number, the program assumes that no return current exists in any of the phases and will set the fraction of reference to 0 independently of what the user specifies. This is consistent with the implicit assumption adopted for ampacity calculations, which stipulates that all cables in one circuit (cables having the same circuit number) will have to carry the same current.

5.8.3.7 Pipe material and dimensions

For pipe-type cables, the pipe material is used to calculate the multiplier for the so-called "in-pipe-effect". This multiplier is used to take into account losses due to proximity and eddy currents because of the presence of the pipe.

The following choices are supported:

a. User supplies pipe material. In this case the user has to enter the coefficient PIPFAC for the "in-pipe-effect".

b. Stainless steel pipe, PIPFAC=1.0

c. Steel pipe, PIPFAC=1.7

CHAPTER 5 –- STEADY STATE THERMAL ANALYSIS 89 The pipe dimensions support not only the regular pipe dimensions, but also a provision for coating is made. The user can specify any coating material. If custom made, (user supplies material), the thermal resistivity has to be given to the program, although normally the selection can be made from the same list of the materials used for the jacket. In this case, the program has embedded thermal resistivities for the listed materials. When specifying the dimensions, the overall pipe diameter has to be larger than the outer pipe diameter in order to model the pipe coating. If no pipe coating material is present, the overall pipe diameter has to be entered equal to the outer pipe diameter.