3 x Single Phase OR 1 x 3-phase Power Transformer

[doehl1]

I have the opportunity to install qty (3) 333kVA single phase transformers (pole-mount, oil filled) or qty (1) 1000kVA 3-phase transformer (pad-mount, oil filled).

Not considering any other factors (installation, space, etc.) which set up would be most electrically efficient at full load? Is there a rule of thumb that one arrangement is x percent more efficient than
the other?

The 3-phase is expected to ~95-98% efficient. Would single phase arrangement be worse than that? By how much?

 

[bacon4life]

Per US DOE standards, those two sizes happen to have identical requirements to be 99.43% efficient. I am puzzled by the requirements that for small units the 3 phase unit has a higher efficiency requirement and for larger banks the single phase units have a higher efficiency requirement. For a fixed amount of materials, I had assumed a 3 phase unit would be lower losses.

The cabling difference can have an impact as well. Overhead wires are often smaller. Overhead vs underground secondaries usually are at least 15 meters different in length.

We no longer install 3-333 kVA overhead transformers because a typical distribution pole is not strong enough to hold up a transformer bank that big.

 

[cranky108]

Bacon, you can also use a platform.

Space could be a big issue. But, adding forth transformer can give an added since of backup for a transformer failure.

I don't know if you care about zero sequence, that will likely be different between the two applications.

 

[dpc]

I doubt there's much efficiency difference - core losses might be worth considering - more so than full load. But main issue is that those are large cans to cluster mount and you'll need to make sure your poles can support the weight.

 

[prc]

Three phase versus single phase transformer selection discussions were there from the early days of transformer engineering. Three phase units were developed in the last decade of 19th century (due to lower cost and higher efficiency as core loss get reduced though load loss will be same when a P kVA three phase unit is compared with 3x P/3 kVA single phase units.)Europe immediately switched over to three phase units but US preferred single phase banks till the middle of last century.

Today single phase units are used only for HVDC and UHV units (800 KV &above due to difficulty in maintaining the phase to phase clearances),single phase distribution lines or where there is stringent transport limitations eg. hydro power stations) or rail track side feeding transformers (out put of single phase 25 kV)

The attached note prepared by self may be useful for the comparison purposes

 

[mc5w]

The scuttlebutt elsewhere on this forum is that 3-phase transformers that have a closed delta winding on a 3-legged core can have a large amount circulating zero sequence in the delta winding when there a heavy single phase load unbalance. This is particularly true of delta-delta transformers even if the secondary is 3-wire delta. The scuttlebutt is that delta-delta transformers that have a 3 legged core actually run cooler in some instances if the windings of one phase are disconnected both primary and secondary. There are alos similar problems with a wye-wye transformer on a 3-legged core also allow power from all 3 phases to contribute to a secondary single phase to ground or neutral fault. This all because a 3-legged core has very little zero sequence impedance because the return pathway for zero sequence flux is through the air and the case of the transformer.

Most power companies figured out that many years ago and specify that 3-phase transformers have fewer problems when they have a 5-legged, 6-legged, or 7-legged core that provides a ferromagnetic pathway for zero sequence flux.

Another consideration is that if you derate the table values in Annex B of National Electrical Code for a conductor temperature of 90 Celsius and an earth surface temperature of 50 Celsius which will be the case the middle or end of the summer here in Ohio you get these values for single phase and 3-phase duct banks:

3x500 KCM copper for 600 amps single phase or 3-phase using isolated phase and protective earth neutral configuration i.e. all 3 A phase in 1 PVC or fiberglass conduit, all the B phase in the next conduit, etc. This is also the easiest way to have all of the wires of the same phase the same length. I have seen parallel wiring installations that have highly unequal lengths of parallel wires because the registered professional engineer, the electrical contractor, and the electrical inspector did not pay attention in high school geometry class. Whether you you need to count this a 4 ducts for 4-wire systems depends on how much of the capacity of the PEN wires is being used. If the loading of the Protective Earth Neutral does not exceed the installed PEN wires the heat contribution of the PEN can usually be neglected. You will need to reduce wire size to 4/0 to 350 KCM copper before going into the 200-amp circuit breakers of fusible switches. Or, use a main panel with the 3 service switches in it and put in 800 amp main lugs to accommodate the 500 KCM.

Also a good idea to backfill with red colored concrete to 6 inches below the surface as that will be more thermally conductive than most soil. For this it is entirely practical to use three 200-amp service switches which will also be more economical than a 600-amp particularly since you need to divide into 200 amp feeders to get reasonable series short circuit ratings between main and branch breakers. Also, 200-amp time delay class J fuses limit arc flash hazard to negligible levels as proven by the Forklift Truck Short Circuit Test.

4x500 KCM copper for 800 amps using 2 wires per conduit in an isolated phase and PEN configuration.` Keep min mind that will will have 2 wires per duct unless neutral calculations indicatye that the PEN can be done 3x500 KCM. Keep in mind that the 2 ducts of each phase need to be the same length. Just build a mockup using concrete tight boxes, some ENT a.k.a smuft tube, and some 12 guage wires to see if you have done your geometry homework right.

6x500KCM fort 1200 amps unless the PEN can be done as 4x500 KCM in 2 ducts in an isolated phase and neutral configuration.

9x500KCM for 1600-amps isolated phase and neutral using 3 ducts each phase. However, a 1600-amp service switch will be 3 times the cost of a 1200-amp. In other words when you go over certain size boundaries i.e. 255 and 1200 amps for circuits breakers and 200 amps and 1200 amps for fuses switchgear prices will TRIPLE or QUADRUPLE.