Voltage Drop Formula

[BoltedFault]

I'm a fairly new engineer working for the local power company. I have been tasked with sizing an underground run for a mobile substation. Unfortunately, I didn't have any power courses in college, so I have spent some time researching equations. Can anyone provide a website or an equation to use to calculate Voltage Drop. I have found some equations on-line, but would like to verify I'm doing things correctly. Some information follows:
Size of mobile transformer- 35 MVA
2 conductors per phase
Conductor size- 500 kcmil, triplex
1 run of triplex in 6 inch conduit (second run of triplex in another 6 inch conduit)

I appreciate everyones' response.

Thanks

 

[rovineye]

Here is what I use for the cable voltage drop (in %)

%VD= (KxIxZxLx100)/V

K=Phase Coefficient (2 for single phase circuit or 1.73 for three phase circuit)

I=Full Load Current (Ampere)

Z=Impedance of copper conductor(Ohms/meter)

L=Single length of conductor (Meters)

V=Source voltage at switchboard or transformer secondary (Volts)

 

[Eleceng01]

rovineye,

Try the voltage drop Excel spreadsheet at:

http://www.mikeholt.com/freestuff.php?id=freegeneral

Mike Holt is well respected so I assume it is correct. I have not used it myself though. Hope it helps.

 

[waross]

In regards to the "Mike Holt" spread sheet;
The formula used appears to be based on resistance only.
You should be aware that there may be additional voltage drop due to inductive reactance. Inductive reactance is a function of both conductor size and spacing. It would probably be negligible for short runs of triplex, particularaly as the voltage rises. It is significant in longer runs of overhead lines.
Is this a 34500 volt circuit?
respectfully

 

[BoltedFault]

Thanks for the responses so far. The Mike Holt spreadsheets are informative but do not fully apply to my situation (limited due to the voltage level). The voltage is 34500 (phase-to-phase). The assumed PF is 0.9. I was at the proposed site today and the length of the cable run is appoximately 1500 ft (one way).

 

[wareagle]

The formula I use is I(Rcos theta + Xsin theta). If the distance is only 1500 ft you should not have any problem.

 

[cuky2000]

I hope the enclose sites could help you.

http://nexansenergy.com/egy/equip/uscorflex/cfx18.htm

http://www.skm.com/support/kb/120000.pdf

 

[BoltedFault]

Thanks for the responses and help. The information you all have provided has been helpful and confirmed what I found.