Underground Cabling Heat Calculation

[AAS.Suja]

1. Calculate the temperature rise in a system consisting of six conductors carrying 1500 amperes each at 132kV, insulated with XLPE, and spanning a cable length of 9 meters. The system is enclosed within a chamber with the following dimensions: length 10 meters, width 10 meters, depth 3000mm. The ambient temperature is 30 degrees Celsius, and the chamber is one surface is exposed to air, with concrete walls that is 0.5 meters thick other five surface is connected with soil. What is the expected temperature rise within this system?
2. To maintain the cable temperature at 35 degrees Celsius, how much power must be supplied through ventilation to dissipate the excess heat generated within the system?
3. Under stable conditions, what would be the equilibrium temperature of the cables, given the heat generation, ventilation power, and the surrounding environmental conditions as described above?

 

[ron]

You will need to perform Neher McGrath calculations, then determine the heat release in wattage and determine ho to get rid of that heat.

Pretty challenging questions.

 

[AAS.Suja]

You will need to perform Neher McGrath calculations, then determine the heat release in wattage and determine ho to get rid of that heat.

Pretty challenging questions.

Thankyou.If you have more suggestion to do this help me

 

[David Castor]

Check out the AmpCalc software. It will do the Neher-McGrath calculation if you have adequate data.

 

[AAS.Suja]

Check out the AmpCalc software. It will do the Neher-McGrath calculation if you have adequate data.

Thanks and is it freely available sir?

 

[Julius Right]

According to IEEE 835/1994 page 1174
6 cables in cable duct copper conductor each cable 1250 mcm [633.4 mm^2-630 mm^2 as per IEC standard], XLPE insulated 90oC,Earth resistivity 90 [Rho] 751 A permissible.
Page 2405:
138 kV 1/c Self Contained Liquid Filled Aluminum Sheathed Power Cable. 85ºC - Copper Conductor - Hollow Core Concentric Strand 40ºC Air Ambient , no sun, no ventilation, sheath[shield] open.
750 mcm[380 mm^2-400 mm^2 IEC] 872 A
In this case cable power losses=6*Rcopper*872^2
Rcopper[90oC,50Hz]= 5.6444E-05 ohm/m then power loss=
6*5.6444E-05*9[m]*872^2/1000= 2.317630269 kW
Ventilation flow required to evacuate these losses and maintain 35 oC air temperature when the inlet air temperature it is 30 oC:
V=(I1-I2)/cp/(Tmax-Ti) m^3/h
I1=Ploss in kcal/h
I2=kcal/h evacuated by conduction through walls [for us I2=0]
Tmax=35, Ti=30
cp=air specific temperature [0.31 kcal/m^3/oC]
V=2.317*0.238846/0.31/(35-30)=0.35713 m^3/h

 

[Julius Right]

Sorry I have to multiply instead of dividing
I1=2.317*859.845=1992.8 kcal/h. Then V=1285.68 m^3/h

​

 

[zbang]

Is this a test question? It would be good for the OP to research Neher McGrath, do some of the calculations, and come back with more specific questions.

"I can explain it to you, but I can't understand it for you."

 

[David Castor]

AmpCalc is not free.

 

[Julius Right]

Approximate, the required power in order to produce 1285.7 m^3/h Pv=V/η/102 where η=total efficiency [ventilation and motor]≈0.8 Then Pv=15.75 kW

 

[Julius Right]

Calculating the power evacuated through the upper surface through the air, it seems to me, the all the heat produced within the chamber may be evacuated through this concrete ceiling.
The power evacuated through the upper surface of 0.5 m concrete and 10*10=100 m^2 surface when the chamber air temperature it is 35oC and the environmental air temperature is 30 it is:
W=A*(θ1-θ2)/δ*λ where:
W=evacuated heat in W
A-cross section area m^2
δ=0.5 wall thickness m
λ=1.5-2.5 thermal conductivity of the wall [W/m/oK]
W=100*5/0.5*2.5=2500 W
However, you have to check what kind of concrete it is there.
See, for instance:
https://www.gf.uns.ac.rs/~zbornik/doc/NS2018.23.pdf

 

[AAS.Suja]

You will need to perform Neher McGrath calculations, then determine the heat release in wattage and determine ho to get rid of that heat.

Pretty challenging questions.

Can you please assist me to solve this problem using Neher McGrath. Since I am fresher finding some difficulties to do this.

 

[AAS.Suja]

Is this a test question? It would be good for the OP to research Neher McGrath, do some of the calculations, and come back with more specific questions.

"I can explain it to you, but I can't understand it for you."

This is an practical problem that I am currently encountering.Can you please assist me to short it out this problem?

 

[AAS.Suja]

Calculating the power evacuated through the upper surface through the air, it seems to me, the all the heat produced within the chamber may be evacuated through this concrete ceiling.
The power evacuated through the upper surface of 0.5 m concrete and 10*10=100 m^2 surface when the chamber air temperature it is 35oC and the environmental air temperature is 30 it is:
W=A*(θ1-θ2)/δ*λ where:
W=evacuated heat in W
A-cross section area m^2
δ=0.5 wall thickness m
λ=1.5-2.5 thermal conductivity of the wall [W/m/oK]
W=100*5/0.5*2.5=2500 W
However, you have to check what kind of concrete it is there.
See, for instance:
https://www.gf.uns.ac.rs/~zbornik/doc/NS2018.23.pdf

Thankyou very much for your efforts and precious help sir. Could you please help me further.

Problem is like this.
The six cable is going in a chamber and it is insulated with XLPE. Chamber is Rectangular cubic chamber and its thickness is 0.5m.Only one surface is touch with air and other all surfaces are touched with soil. So I need to know what is the maximum ampere can flow thorough chamber considering insulation property. And if that current passing through that chamber how much temperature will be rise in chamber and what would be the stable temperatures in the chamber as a result of conduction and convection.
if the stable temperature is above 35 degree how much air ventilation needed to remove excess heat.

Expecting quick help since I need to catch the deadline of my work

 

[Julius Right]

First I apologize I was wrong I thought it is about of 1500 A per phase. Now
I saw it is per each conductor.
According to DOHA catalogue, for 132 /76 kV in trefoil, 1200 mm^2 copper at 40oC air ampacity is 1604 A.
The ac resistance at 90oC copper conductor it is
Rac [90oC,50 Hz]= 0.030662 ohm/km
Ploss=6*0.030662/1000*9*1500^2=3725.4 W
The heat evacuated through 0.5 thick concrete 100 m^2
W/A=(θ1-θ2)/δ*λ then λ=W/A* δ/ (θ1-θ2)= 3.7254*1000/100* 0.5/ 10=1.8627
That means in this case [if λ=1.86] then the air temperature in chamber will be 40oC. No need ventilation.

 

[AAS.Suja]

First I apologize I was wrong I thought it is about of 1500 A per phase. Now
I saw it is per each conductor.
According to DOHA catalogue, for 132 /76 kV in trefoil, 1200 mm^2 copper at 40oC air ampacity is 1604 A.
The ac resistance at 90oC copper conductor it is
Rac [90oC,50 Hz]= 0.030662 ohm/km
Ploss=6*0.030662/1000*9*1500^2=3725.4 W
The heat evacuated through 0.5 thick concrete 100 m^2
W/A=(θ1-θ2)/δ*λ then λ=W/A* δ/ (θ1-θ2)= 3.7254*1000/100* 0.5/ 10=1.8627
That means in this case [if λ=1.86] then the air temperature in chamber will be 40oC. No need ventilation.

Thanks Sir

 

[Julius Right]

I think I exaggerated with ventilator motor power since the air
flow may be produced also with a 3 kW motor -depends on static pressure of the
ventilator and on the resistance of the exhaust air path.

 

[Julius Right]

In order to take a close case, I found a mistake in the ventilation power formula-the translation from IEC to Imperial units .
So, in IEC formula it has to be:
Pv=V*pt/102/η
where V in m^3/sec; pt in mmH2O
pt=total air pressure =pstatic+pdynamic
pstatic usually maximum =0.072 psi=0.072/1.4223=0.0506 mH2O then 50.6 mmH2O.
pdynamic depends on load pressure and pressure drop through evacuation pipe.
Let's it is a steel pipe of 6" 10 m with two 90 degrees curves =10 mmH2O
And another 30 mmH2O for different static pressure pt=90 mmH2O.
Pv=90*0.3969/102/0.8=0.438 kW
A 1 kW induction motor will be enough it seems to me.
A more exact calculation has to be done when all the data are knower.

 

[Julius Right]

If you need a calculation of temperature rise of cable conductor you need the details of the cable cross section area as insulation thickness, shields, and sheath. But better is to get the manufacturer recommendations.
The calculation is very simple for this case and the manufacturer catalogue or a standard as IEEE 835/1994 it is all you need.

 

[Jpflex]

1. Calculate the temperature rise in a system consisting of six conductors carrying 1500 amperes each at 132kV, insulated with XLPE, and spanning a cable length of 9 meters. The system is enclosed within a chamber with the following dimensions: length 10 meters, width 10 meters, depth 3000mm. The ambient temperature is 30 degrees Celsius, and the chamber is one surface is exposed to air, with concrete walls that is 0.5 meters thick other five surface is connected with soil. What is the expected temperature rise within this system?
2. To maintain the cable temperature at 35 degrees Celsius, how much power must be supplied through ventilation to dissipate the excess heat generated within the system?
3. Under stable conditions, what would be the equilibrium temperature of the cables, given the heat generation, ventilation power, and the surrounding environmental conditions as described above?

How are you using conductor insulation XLPE? my boss ordered triplex XL # 2 AWG aluminum to be used as an underground feeder from service to a sub panel to a building.

Everybody commented on my last post that this could not be used because it was not listed in NEC table for conductors or cables?