Underground Cable Ampacity

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timm333

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I have a couple of questions about the ampacity calculations of 132 kV underground cables. If the manufacturer's data (inductance, maximum current carrying capacity, etc.) is given to a frequency of 50 Hz, then is there some general formula to convert it to a frequency of 60 Hz? Also If some part of direct-buried cables is in duct-bank, then should the cables be sized for direct-buried or for duct-bank. Thanks for help!
 
R should not change
XL = 2 Pi f L j Ohm
XC = 1/(2 Pi f C j) Ohm
so XL is proportional to f and XC inversely proportional
adjust by the 60/50 (or 50/60) ratio

if some is buried some in duct bank and the parameters change
Either do a composite equivilent ckt or do it in sections
 
I think doing it in sections would be easy. For example if the total cable route length is 600 m, 570 m of which is direct-buried and 30 m is in duct-bank; then we can calculate the cable size for both direct-buried and duct-bank conditions, and chose the larger size of the two.


The main problem is that the cable manufacturers give the ampacity values only for “in direct buried soil” and “in free air”; they don’t give the ampacity values for "in duct-bank". How should the cables be sized for duct-bank? I think that as the soil of direct-buried has more thermal resistivity than the concrete of duct-bank, so the required cable size for direct-buried would be larger than the required cable size for duct-bank. So if we just size the cable for direct-buried, then it should also cover the duct-bank. Is it correct?
 
Look at some NEC tables for similar gauge direct buried vs conduit

ampacity for buried should be higher than duct bank since imo it would provide a better 'heat sink'
just a guess

tables list conduit/buried the same, free air is much higher

although your v is higher look at 310.60.B.1
 
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There are tables for 5-35kv
air, buried, conduit, duct bank
take a look at those
I would use the lowest, the only significant difference is air
 
I have attached here a page from a manufacturer’s data. It has "technical data table" and "ampacity table". The ampacity table gives ampacities at 50 Hz; is it possible to convert these ampacities from 50 Hz to 60 Hz?
The ampacity table gives the ampacities at load factors of 0.7 and 1.0; is it possible to use extrapolation and calculate the ampacities for a load factor of 1.3?
 

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  • Cable Data MH 4.pdf
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Is the "load factor" supposed to be "power factor"? Or is it duty cycle? In either case numbers greater than 1 are meaningless.
Service Factor (of a motor) is something else entirely.
Frequency is only significant for conductors large enough that skin effect is important. If it is, the difference between 50Hz and 60Hz will be a very small change in ampacity.

mobile
 
Load factor (LF) in this case is the ratio of the operating load to the maximum load. For example if a cable is to be sized for a load factor of 0.7, it means that the load connected to this cable would be running only at 70% of its full load capacity.

We have to size the cable for transformer for a load factor of 1.3. The ampacity table that I attached above gives the ampacities of trefoil cables buried in soil for load factors (LF) of 0.7 and 1.0. For example for a cable of 2000 kCMIL, the ampacity is 1377 A at LF = 0.7, and 1126 A at LF = 1.0.

So by extrapolation, the ampacity at LF = 1.3 would be = 1126 – (1377 – 1126) = 875 A.

Will it be Ok to use this approach to calculate the ampacity at LF=1.3?
 
Actually the specification says that the cable for the transformer is to be sized for a load factor of 1.3; perhaps they would sometimes over load the transformer up to 130% in the future.
 
The key is that the ampacity of the wire is not changed by that greater than unity load factor. Instead the required ampacity is increased by 1.3 over the transformer rating.

mobile
 
timm333 said:
Load factor (LF) in this case is the ratio of the operating load to the maximum load. For example if a cable is to be sized for a load factor of 0.7, it means that the load connected to this cable would be running only at 70% of its full load capacity.

We have to size the cable for transformer for a load factor of 1.3. The ampacity table that I attached above gives the ampacities of trefoil cables buried in soil for load factors (LF) of 0.7 and 1.0. For example for a cable of 2000 kCMIL, the ampacity is 1377 A at LF = 0.7, and 1126 A at LF = 1.0.

So by extrapolation, the ampacity at LF = 1.3 would be = 1126 – (1377 – 1126) = 875 A.

Will it be Ok to use this approach to calculate the ampacity at LF=1.3?
Click to expand...

no, it is not linear
sqrt(0.7) x 1377 = 1152 (close to 1126)
sqrt(1/1.3) x 1126 = 988
probably some exp term
I would not assume you can run at a derated ampacity at 1.3 x rated ampacity
if it is not on the data sheet do not make assumptions
 
GoldDigger said:
Is the "load factor" supposed to be "power factor"? Or is it duty cycle?...
Click to expand...

timm333 said:
Load factor (LF) in this case is the ratio of the operating load to the maximum load. For example if a cable is to be sized for a load factor of 0.7, it means that the load connected to this cable would be running only at 70% of its full load capacity. ...
Click to expand...
Source: http://www.okonite.com/engineering/ampacity-correction-factors.html
Okonite said:
By definition the load factor is the ratio of the average load over a designated period of time to the peak load occurring in that period. For variable continuous loading the base period is 24 hours. These apply for cables in conventional underground duct installations since there is a time lag between the temperature rise of the cable and the temperature rise of the duct structure and surrounding earth. This heat-time-lag characteristic permits assigning higher current ratings for cables in ducts which do not carry full load continuously. For in-air installations 100% load factor is used. These ratings are used to any load factor due to the relatively low thermal capacity of the surrounding air.
Click to expand...
 
The switchgear which feeds the transformer also has other loads on it. For cable sizing: the load factor is 1.3 for the transformer and 1.0 for all other loads.

How should the incoming feeder to this switchgear be sized, should it be sized as:

130% of the operating load current of the transformer + 100% of the operating load current of all other loads combined?
 
timm333 said:
The switchgear which feeds the transformer also has other loads on it. For cable sizing: the load factor is 1.3 for the transformer and 1.0 for all other loads.

How should the incoming feeder to this switchgear be sized, should it be sized as:

130% of the operating load current of the transformer + 100% of the operating load current of all other loads combined?
Click to expand...

you are talking specialized work here, 132kv class
what size and voltages are the xfmr
fan cooled? etc
what is the max operating point? 130% of the MVA rating?

I'd size the conductors for the next size up from 130% of the transformer rating (using the 100% load factor ampacity)
I like sleeping at night lol

save them a buck now, quickly forgotten upon project completion
cost them a buck AFTER project completion, they will never forget, and most likely will come after you

is there a PE/Engineer involved in this design?
 
The transformer is 132-13.8 kV, 300 MVA without fan-cooled, and 500 MVA with fan-cooled. I'm on the client side. someone from the engineering company will perform the calculations and take liability, but it will be at least one year from now. So I was doing the calculations for personal/internal reference only, my calculation does not have to be 100% accurate. Sizing the incoming feeder for "130% of outgoing transformer feeder plus 100% of all other outgoing feeders" looks like a good option.
 
timm333 said:
The transformer is 132-13.8 kV, 300 MVA without fan-cooled, and 500 MVA with fan-cooled. I'm on the client side. someone from the engineering company will perform the calculations and take liability, but it will be at least one year from now. So I was doing the calculations for personal/internal reference only, my calculation does not have to be 100% accurate. Sizing the incoming feeder for "130% of outgoing transformer feeder plus 100% of all other outgoing feeders" looks like a good option.
Click to expand...

what is the 'operating load' of the xfmr?
traditionally the feeder is sized for the xfmr mva
are you saying it needs to handle 1.3 x 500 mva? 1.3 x 300, 500, etc. ?

the swgr feed would be the transformer ampacity from above + other loads as you stated

is this utility, private generation or plant work?
 
This is plant work. By operating load I meant to say the actual anticipated load on the transformer (rather than the fan cooled rating of the transformer.) But as you have mentioned, the transformer has to be sized for its MVA which in this case would be the fan cooled MVA which is 500 MVA. So, lets say, if the non-transformer load is 200 MVA, then the incoming switchgear feeder will have to be sized for:

1.3 x 500 + 1.0 x 200 = 850 MVA.
 
The electric utility load factor m = 0.7 is based on a load curve that is usually in power supply company networks. You may have 100% or 70% as a quotient of the area under the load curve.
Load factor 0.7.jpg


See ABB Switchgear Manual vol.11 part 13.2.2 Current carrying capacity.
As already said above, the skin and proximity effect will be increased for 60 Hz.
and so the conductor resistance. However, the difference between directly buried and in duct bank ampacity could be much more. It depends also on concrete thermal resistance. For instance, NEC considers it 55 K.cm/W, but IEC 60287-2-1 recommends 100 [RHO]. The increased dielectric losses[due to increased rated voltage] will decrease also the ampacity.
If the shield, the metallic sheath- as aluminum foil or lead- or the armor is grounded at both ends then circulating grounding currents will increase the losses and reduce the ampacity.
By-the-way, NEC tables 310.60 are good for single-core directly buried underground but no single-core cable one cable per duct table is available.
 
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