XLPE Cable vs. Bus Duct 4

[enggines]

Hello.
Can anyone make a comment or an opinion with regards to using XLPE Cables on secondary of Power Transformer to Switchgear instead of using Bus Duct. We have a 20/27/33 MVA substation project, switchgear/outdoor type. Our contractor proposed the used of XLPE Cables installed in a concrete encased conduit which, according to them, flooding or water intrusion is very remote.
Thanks in advance. Any reply would be highly appreciated.

 

[bobas]

Not knowing more about the specifics (size, distances, voltages, etc.) I would prefer the bus connection. This is usually done on units that are close to each other. If it is outdoors then the bus connection require special attention (avoid entering the top due to water sealing problems, there will be problems in time).

Using cables (underground encased in concrete) to connect the two units would eliminate the entry problem. One issue with cables is maintainig conductor lengths.

I would recomend that if you go with the conduit/wire have the contractor install at least two full size spare conduits with a pull rope. This will enable you to install a replacement cable in the event of a cable or termination failure.

Lastly, I am a professional engineer who work for many contractors in the past (40 years in electrical designe, construction and maintenance) and I would suspect the cable substitution is most likey driven by economics.

Famous last words in the construction business "you get what you inspect not what you expect"

 

[cuky2000]

For typical substation with a transformer feeding switchgear could be done by both methods using busduct and cable (Underground or overhead).

I believe the busduct is more reliable since is factory rated and tested while the solid dielectric cable has to be engineered for the application, exposed to mechanical damage during installation and more sensitive to insulation degradation and thermal stresses during normal operation.

If the cost difference propose by contractor is not significant, I suggest to continue with the original busduct design.

 

[MarkAthay]

I've had very poor experience with 15 kV class bus duct. Generally in an outdoor application bus duct doesn't like getting dirty, but you can't get it out of service to clean it. We had several throat-connected 22 MVA transformers that we had to convert over to air-insulated bus, not a pretty project. Repeated bus failures drove us to doing that.

XLPE cable by itself is generally not a long-lived cable in medium voltage applications. Moisture is bad news for it, even if you "keep it dry". Use TRXLPE cable or EPR. EPR can handle higher temperatures which is what usually drives the current handling limitations in a duct bank.

Don't worry about getting the cables "exactly the same length" in a duct bank. It's generally a non-issue. 5 conduits run in parallel for 100' will make the cable within 5% of each other or less. The cable ampacity calculations have enough slop in them the way it is.... Since when in this business can we predict something better that within 5%? Not that often.

Mark in Utah

 

[RajT]

I would opt for a cable connection for following rason
Much cheaper in cost
Easy to install
Easy to replace in the event of a failure
Repairs can be carried out quickly by any experienced cable jointer whilst with bus duct its an overseas call to a specialist and mega bucks in cost not to mention the delay.
Historically the cable connection is less likely to suffer from a service fault then bus duct.

 

[dpc]

Does this have to be underground? I would think an above-ground installation, perhaps using cable tray would be preferable, everything else being equal.

You don' mention the voltage, but assuming it is 15 kv class, there will be MANY conductors per phase in a cable installation. This complicate the terminations. Also, for an underground installation, the ampacity of each conductor must be de-rated to account for the mutual heating in the duct bank. This will be significant.

If you don't want to use rigid bus, consider use of cable bus such as manufactured by Calvert. This is a good compromise and is an engineered product.

If you have already bid this project, then the contractor's agenda is not the same as yours. Even if there are substantial cost savings, the contractor will get most of it if you have to execute a change order.

Also, I agree with Mark that use of EPR would be much preferred over standard XLPE in this short run of cable.

There is another old construction industry axiom:

"Everyone has within them the capacity to make a really terrible decision."

Good luck!

 

[Tuas]

The contractor would definitely prefer cables as the costs are cheaper and there's more flexibility in the event if the design changes and site adjustments. Busducts have an inherent problem. They are rigid, as in the switchgear and transformer terminations must be as accurately decided during design, as major site adjustments would mean disastrous. Also, busducts are used for higher ratings, like, 4000A and above, as cable voltage drop would be high. Of course, busducts have a higher premium than cables, but if your application does not invovle high current ratings, cables would do just fine, anyway, cables are also factory tested and complies to international standards.

 

[enggines]

Thanks a lot to all of you guys. If any, this is a 20/27/33 Mva, 69-13.8 kv project, which will carry four (4) Distribution Feeders. Proposed transformer is of conservator type and the site or space is limited, so we opted for switchgear beside the Power Transformer (as against our existing system of overhead). The thing is we want a top cover mounted secondary instead of throat connected. At any rate your initial feedback had been a great help. Thank you.