Cable Correction factor Application 2

[cuky2000]

Can anyone help to understand the above statement?

Other manufacturers do not have issues applying correction factors for variation of soil thermal resistivity, phase spacing, earth temperature, etc. for single shielding bonding or both ends grounding arrangement.

See the enclosed excerpt for more details

 

[Rejeckted]

In Single End Bonding there is no circulating current. Hence Cable Ampacity is not reduced due to circulating current flowing in earthed sheath.(Because its only earthed on one end, hence no closed path)

Cable Derating in Ampacity is due to many factors like Formation, Earth Resistivity, Ambient Temperature, Depth of Laying et cetera.

Just check: Ampacity X (Derating Factors) > Load Current (Current Required for Application)


This means even after derating you application will work correctly

 

[cuky2000]

Hi Rejeckted,

It is true that Single End Bonding there is no circulating current but still the cable resistance will generate heat (Joule effect) that needs to be released to the ambient.

It is understood that the ampacity table is based on defined soil thermal resistivity (usually 1[sup]o[/sup]C.cm/W), temperature (20[sup]o[/sup]C), burying depth, etc. If those conditions are different than the site-specific, it does appear that the base current in the table needs to be adjusted by the correction factor regardless of the shielding bonding method.

I checked in a few manufacturer data and only the ABB info does not use derating for single bonding.

Do I miss something here?

 

[jghrist]

I think that you would still need rating factors, but they would be different from those without single point bonding.

 

[7anoter4]

It is not clear if the cross bonded [equal to single end grounded] ampacity in table 1 and 2 does not take in consideration the eddy current for 35[95] sqr.mm shield. However, the columns entitle "both ends" present the ampacity considering circulating current for shield of 35 sqr.mm [or respective 95 sqr.mm] and the rating factor on the Table 5 and 6 should be employed if the shield cross area is different.
I think, if you don't intend to ground both ends [or you intend to use cross bonding] the standard 35 sqr.mm [or respective 95 sqr.mm] is sufficient.
I have no time to check it [according to IEC 60287 the eddy current calculation it is more sophisticated]

 

[7anoter4]

Calculated for 3*500 sqr.mm copper, 66 kV, trefoil, 35 sqr.mm shield the eddy current λ"=0.07088 and the ampacity for cross bonded cables is 787 [in table 2 it is 785 A].
For non-cross bonded grounded both ends I got λ'=0.114815 for circulating current at 35 sqr.mm shield and ampacity of 774 A .If I use λ" also I got ampacity 754 A. In table 2 it is show 760 A on "both ends" column.So, I think ABB used both factors in table building.
The problem remains using more than 35 sqr.mm shield.
For cross bonded grounded the short-circuit current will be the same as circulating current -practically negligible- since it is already grounded both ends and the induced voltage will be [ideally],0.
Not cross bonded cable the short-circuit current -if accidentally both ends will be grounded- will be different.
For 500 sqr.mm copper XLPE insulated maximum allowed short circuit current is 71.2kA [from 90oC up to 250oC in one second]. The induced voltage in shield will be 4.08 V/m and the impedance of the shield is Z=58.9E-05+j5.83E-05 so Ishldsc=6.89 kA.
The allowed current for 35 sqr.mm shield is only 5.8 kA for 1 sec. Then you have to rise the shield cross section.
However, the actual short-circuit current has to be less. If the maximum apparent short-circuit power is 3000 MVA at 45 kV[See IEC 60076-5 Table 2] Isc=3000/sqrt(3)/45=38.49 kA.

 

[cuky2000]

Below is the published allowable current from 3 different cable manufacturers for a 2,000 kcmil XLPE cable in a double circuit configuration in an underground duct bank. The allowable current per circuit for a double-ended shield bonded is much lower than the single-bonded as expected.

If the site condition has a soil resistivity of 50[sup]o[/sup]C.cm/W and an average soil temperature of 20[sup]o[/sup]C how the corrections factors will apply for a single and double-ended bonded?
[sub]NOTE: At the bottom of the graph are some published correction factors to explore how to apply in the above example.[/sub]

 

[kartracer087]

Usually double bonded sheaths without cross bonding are used in distribution cables. Often the shielding (concentric netural wires, lead or copper sheath) of the cable is used as a system return for distribution loads connected between phase and ground such as single phase transformers. Naturally in this arrangement the eddy current losses are much higher which results in heating of the cable. Single point bonding greatly reduces this effect, but causes the shield (sheath) voltage to rise which diminishes its effectiveness. Typically single point bonding is for cables of short length. Cross bonding is similar to double bonded sheaths with the exception that the sheaths are split into segments and then crossed bonded between phases in transpositions. At one end for example you might have ABC connected together and to ground. Then at a manhole or splice pit, you join the incoming A phase cable sheath to the outgoing C phase cable sheath and the incoming B phase cable sheath to the outgoing A phase cable sheath and the incoming C phase cable sheath to the outgoing B phase cable sheath. You do this at each successive manhole or splice pit until all three cables have been fully transposed ABC evenly. This greatly reduces the circulating current in the shield which reduces the losses and allows for higher ampacities. Cross bonding is the preferred bonding method of longer solid dielectric (XLPE and EPR) transmission cables. Keep in mind that the shielding sizes of transmission cables varies greatly in terms of cross sectional area. It is something you can actually spec based on how much fault current you have in your system and the ability to withstand that fault current without degrading the shield. You'll notice companies like ABB and Southwire have different shielding styles availble such as wire shielded and corrugated. The wire sizes and corrugated thicknesses can be specified as required (I think Southwire uses a 30 mil corrugated sheath in their ampacity calculations). For ampacity calculations, reference the Neher-Mcgrath paper from 1957 (still applicable even though its old) or IEC-60287 standards. The methods of calculating ampacity are well defined in those papers and can be prepared using spreadsheets.

 

[7anoter4]

In both cases the current can be increased by 30%

 

[cuky2000]

Hi 7anoter4,

I do concur with you that both cases, single or double bonding, need correction factors.

If the load factor (LF=65%) instead of 80% or 100% as is based on the table, do you think there is an additional increase in ampacity rating?

[sub]NOTE: The low LF is because of the estimated load profile for an offshore wind project.[/sub]

 

[7anoter4]

I have still a problem .According to IEC 60287-2-1 Table 1 – Thermal resistivities of materials the concrete resistivity is 1[100] and that increases very much the thermal resistance T4 -from duct through concrete duct bank to the native surrounding earth. IEEE 835 consider this resistivity only 60.If I take ρc=1 then the ampacity decreases to only 1.1.

 

[cuky2000]

Hi 7anoter4,

The water table on the selected site is low and the concrete is expected to be web at all times. Also, there is an opportunity to use engineered backfill with thermal resistivity targetted as low as 50[sup]o[/sup]Ccm/W. The enclosed link provides an alternate source with the concrete thermal resistivity that we hope helps to harmonize the potential conflict with the IEC & IEEE data.

The precise value will not be known until detail calc and site data are validated. However, the preliminary expectation is that the ampacity published by the cable manufacturers might increase if the site has better thermal resistivity, soil backfilled and load factor is lower than the base value regardless of applicable to both bonding methods.

[sub]NOTE: We are thinking of one end bonding with sheath voltage limiter (SVL) since provides a better ampacity rating and is a short run. In addition, a copper cable will be run in the duct bank to[/sub]