35kV cable parallel installation spacing

Status
Not open for further replies.
A

alal

Member
Location
KY
I am trying to use two conductors per phase of the 1000kcmil Okoguard Okoseal Type MV-105, 35kV Shielded Power cable. The cables will be in a tray in air. What spacing should I keep between the two conductors in each phase and what spacing between three phases?
I know I should keep a diameter space between each cable if there's only one cable per phase.
Thanks.
 
Alal,

1) Are the two-circuit sets, each having 2-conductors per phase, carrying the same load? In other words are the two circuits in parallel?

2) If not in parallel, are the currents per conductor phase about the same?

3) Are circuit (1-way) lengths so long that phase Roll-Over is a concern?

Regards, Phil Corso
 
Phil,

Sorry I am not being very clear.

1) The circuit is a three phase transmission line, but in order to improve the voltage drop, I need to parallel cables to reduce the total resistance. So I came up with two cables bundled for each phase (each cable is a single conductor cable)

2) The current will be shared equally

3) I am not sure about what is phase roll over, but the length is about 20 miles

Thanks.
Alal
 
You should have 2 ckts of phases A,B and C bundled. What is the system voltage? Must be one hell of a load to have any VD.
Are you installing cable in a tray for 20 miles?
 
It is a 34.5kV system. The system is designed to have a 2% voltage drop.

The cable may just lay on the concrete for 20 miles.

bob said:
You should have 2 ckts of phases A,B and C bundled. What is the system voltage? Must be one hell of a load to have any VD.
Are you installing cable in a tray for 20 miles?
Click to expand...
 
The bundling of the phase conductors cancels the magnetic fields around the conductors and reduces the impedance. 20 miles is a
long way. Why not an overhead transmission line. Capacitors install at the end of the cable will raise the voltage level. However you
might end up with a feroresonance problem.
 
Three phases bundled together, i.e. not parallel bundled together, you need to keep at least at 2.15 X O.D of cable as a spacing.

I would recommend using a 3 core, armored cable. You only need 1-diameter between cables and the armor is the raceway, or string poles and use the in - air ampacity of the cable, you may be able to downsize.
 
Why cable for 20 miles? An overhead line will be ten times cheaper. Why in tray? In underground will be cheaper. The sun will heat the cables so you have to cover the tray and to derate the cable ampacity
The cable bundle has to be well bounded in order to overcome the short-circuit force. A sort of cable clamp has to be fitted on it all about 3-5 feet:weeping:
 
Yes, I tried parallel the two circuits and SKM shows worse harmonics results, bundling is better (at least with the spacing distance I am using). I may end up using capacitors as well, right now is only the design phase.


bob said:
The bundling of the phase conductors cancels the magnetic fields around the conductors and reduces the impedance. 20 miles is a
long way. Why not an overhead transmission line. Capacitors install at the end of the cable will raise the voltage level. However you
might end up with a feroresonance problem.
Click to expand...
 
Thanks. I will look at 3 core cable.

kingpb said:
Three phases bundled together, i.e. not parallel bundled together, you need to keep at least at 2.15 X O.D of cable as a spacing.

I would recommend using a 3 core, armored cable. You only need 1-diameter between cables and the armor is the raceway, or string poles and use the in - air ampacity of the cable, you may be able to downsize.
Click to expand...
 
It is in the design phase right now. Maybe I need to change the design based on the cost. Thanks for the comment on short circuit force.

Julius Right said:
Why cable for 20 miles? An overhead line will be ten times cheaper. Why in tray? In underground will be cheaper. The sun will heat the cables so you have to cover the tray and to derate the cable ampacity
The cable bundle has to be well bounded in order to overcome the short-circuit force. A sort of cable clamp has to be fitted on it all about 3-5 feet:weeping:
Click to expand...
 
Alal,

There is far more than just spacing to contend with. My use of the term, phase-rollover, is more properly called 'Transposition'!

If I've interpreted you correctly you propose to install a flat-array of six, evenly-spaced, single-core, 1,000 kcmil, conductors. Two typical arragements could be A1-B1-C1-A2-B2-C2 and A1-A2-B1-B2-C1-C1! Unfortunately, especially for the length cited, this practice will result in asymmetrical impedances, unbalanced sheath-currents, unbalanced core heating, and an appreciable increase of cable resistance (ohm/mile)!

The practice of Transpostion periodically changes the position of the cores, just as in done for lengthy overhead lines! Also, you must consider sheath-grounding, sheath cross-bonding, periodic cables connection!

As pointed out by others, the trefoil arrangement will provide a much more efficient method than the proposed method. That said, what is the power/pf of the load carried? Also, are you investigating alternatives?

Phil Corso
 
Last edited:
I agree it is a great idea but the three-core cable-if there is such a manufacturer- will be about 5 inches diameter.
[same as 2.15*2.27 inches single-core overall diameter].
You have to protect the cable against water and debris anyway, raising it one inch above the bottom of the trench.
In case of three single-core cable at 2.15 times one overall diameter distance the current imbalance will be negligible anyway-even laying cables parallel and not twisted-as in three-core cable.
 
Julius,

It is certainly do-able! And it has been done! Many times! In fact, FPL (FLorida Power & Light) is considering a 90-mile cable between the Bahamas and Pompano, Fl! It's sized to carry 90 MW! Granted... it's different than OP's, i.e., it's oil-filled!

However, I do agree that the OP's proposal is not the most economical solution! And, if it's not just an EE exercise, I strongly suggest he contact some manufacturers!

Regards, Phil Corso
 
bob said:
Is there a substation connected to this 35 kv cable?
Click to expand...

Alal responded to my note by saying there were 3 substations connected to this 35 kv cable. In the original post, Alal said
It is a 34.5kV system. The system is designed to have a 2% voltage drop. The cable may just lay on the concrete for 20 miles.
Click to expand...

If you are trying to correct a 2% VD, and if the substations are equipped with regulators, the VD could be corrected by the regulators. If there are not
regulators, I would think it would be less costly to install 3 regulators rather that the 20 miles of new cable or 20 miles of OH transmission line.
Regulators can correct +/- 10%.

https://www.google.com/search?q=ele...oK6uN8QGyioCwCg&ved=0CFIQsAQ&biw=1310&bih=528

Alal, please respond.
 
Last edited:
Status
Not open for further replies.
Top