Structures for UG parallel circuits

Angulo

Member
Location
Texas
Occupation
Engineer
Good morning,
I need some advise on how to chose the right amount of substructures for parallel UG MV (34.5kV) circuits.
I have 5 3-phase circuits that run parallel to each other with an 8' separation between them. That is, each circuit runs in its own separate trench.
My question is the following, When they reach a point where they have to enter a substructure (Manhole or Vault), can tey merge into one or must they have their own separate structure along the entire way (2.5 miles)?
All 5 are going to the same substation from a solar farm and at no point do they branch out.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
I’ve been consulted by engineers many times over the years, it’s not they didn’t know what they where doing, it was sometimes they needed advice on a more practical way of doing things.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If the reason for the trench separation is to provide redundancy by avoiding a single point of failure, running them through a common manhole may violate that intention.
 

Angulo

Member
Location
Texas
Occupation
Engineer
Late response here, but the purpose of the 8ft is to avoid the effect of mutual heating and at that distance the effect is almost negligible.
This was a design that the client simply wanted a price for without much detail.
Doing my own model in CYMCAP I've proposed a concrete encased duct bank that is much more compact and does enter a single structure.
The other layout (8' separation) would cause the conductors to overheat at the choke point of entering a single structure.
This was my first project ever, so thank you all for your responses.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Wow.. 8’ separation..
what kind of currents you running and what’s the geometry of the laid cables?
Without going into Nether-McGrath calcs here it seems like with a little different geometry the distances could be much less.
(Although CYMCAP should model with different geometry’s)

🤔
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Wow.. 8’ separation..
what kind of currents you running and what’s the geometry of the laid cables?
Without going into Nether-McGrath calcs here it seems like with a little different geometry the distances could be much less.
(Although CYMCAP should model with different geometry’s)

🤔
Thats what I was thinking, but then I’m not versed on high voltage,
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
In my opinion, there are some issues here: 1.the ampacity 2.the phase currents unbalance 3.the voltage drop
NEC 310.60 recommend 2 ft distance [det.8] for buried two triplex cables, then for 5 triplex we have to increase the distance.
If the cable duct bank length it is more than 10 fts [310.15 (A) (2) Exceptions] then the entire length of cable ampacity will be the cable in duct bank ampacity.
If ,for instance, the power plant station transformer will be 50 MVA, the total current at 33 kV will be 50/sqrt(3)/33=0.875 kA and per one cable will be 875/5=175 A.
If we take NEC det. 3 copper conductor then we need 4/0 in duct bank [XLPE insulated 90o]
In trench, as per det.8 , #2 will be enough, but NEC limit the minimum at 1/0.
In order to reduce the currents unbalance and the reactance the cables will run in trefoil formation.
However, the voltage drop for 5 parallel cables [copper conductor] it is about 0.8% [34.5 kV]
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
The distance between cable in trench depends also on thermal resistance of the soil. NEC considers RHO=90[K.cm/W]. However there are very arid soils where RHO may be even 250.For instance IEC 60364-5-52 considers RHO=250.
Another issue may be when the partial drying-out of the soil occurs. In this case the overall cable surface temperature is limited [usual 50oC].
 
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