How does the sizing of 3 single phase transformers providing 3 phase power relate to a 3 phase transformer. For example if I have 3 - 50 kva single phase transformers wired in delta configuration, does it have the same capacity as a 3 phase 150 kva transformer?
3 single phase transformers vs 3 phase transformer
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JDBrown
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Yes.
Welcome to the Forum.
Welcome to the Forum.
Yes. What's nice is if one transformer goes south you can still operate in an open delto. It would be at a refused capacity such as 50+50= 100x .87= 87kva which is a bit less than the original 150 kva.
If you require aa 120v lighting tap you could used a higher kva transformer with a 120v lighting tap for one of the legs to provide extra capacity that may be required. Should a single 3ph transformer with a delta secondary loss a phase you're SOL. If supplied with a lighting tap only 5% of the transformer kva is available for the lighting tap but what really hurts is that the transformer must be derated By 30%.
So (3) 1ph transformers have many advantage over a single 3ph transformers but it is more expensive.
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broadgage
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Practice here in the UK differs somewhat in that it is almost unknown to utilise 3 single phase transformers for 3 phase service.
A 3 phase service from the smallest available, up to many hundreds of KVA will almost allways be from a 3 phase transformer and not from 3 single phase units.
Single phase pole mounted transformers exist but are normaly only used for single phase service.
A 3 phase service from the smallest available, up to many hundreds of KVA will almost allways be from a 3 phase transformer and not from 3 single phase units.
Single phase pole mounted transformers exist but are normaly only used for single phase service.
kwired
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You have little or no delta secondaries either. Here in rural areas especially, delta transformer banks are used a lot, some because of long distance and limited load makes open delta the more economic choice, some where there is significant 120/240 single phase loads and somewhat limited three phase load they will use a larger 120/240 pot, and either one small pot for the stinger leg or sometimes where there is still primarily single phase load but some significant three phase load there will still be a larger 120/240 pot and two smaller ones to make a full delta system.
This is usually only done up to about 50 KVA units (150 KVA total for a bank). Get much larger than that and you have a lot of transformer weight to support on one pole, or maybe have to build multipole structure to support it. Space and cost are considered and often times a padmount is preferred when you go over 150 KVA. Is a little rare but I have seen delta configurations made from single phase padmounts before, this is usually more because the owner doesn't want overhead lines or structures at the facility but maybe doesn't have enough demand to justify purchase of a larger three phase padmount, and his bank is made of stocked transformers in the POCO system.
Not 100% true for some dry transformer if the lost winding pair can be disconnected. A little different than open delta with two of three single phase tranfsormers in that the core is shared, but a system can limp by on two winding pairs. Of course that's only if the lost pair is not the one having a grounded center tap secondary winding and there are split-phase loads.
I've seen the 5% limitation noted many times over the years and fail to understand it. Also cannot understand what you mean "derated by 30%". Can you elaborate?
But usually a more expensive way to do it.
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It is common for 3ph transformers to be built with 3 legged cores. Since the LV windings are wound on the inside and the connections mode where the leads exit each winding how would the failed winding be isolated? Also, there is the added issue Of the remaining 2 windings sharing a 3-legged core.
Yes, I have some application notes fro a Westinghouse transformer design engineer with states as such from years/ago but when I researched this statement I coulded substantiate it.
What I did find was:
" When utilizing the 120 Volt midtap for single-phase applications, the single-phase load should not exceed 5 percent of the three-phase kVA rating. The three-phase kVA load must be reduced by the same percentage as that added by the singlephase load. Additional loading beyond 5 percent may cause the transformer to overheat and fail. If the single-phase load is in excess of 5 percent, it is recommended that a separate singlephase unit be used to handle the load."
As such this doesn't equal 30%.
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Transformer manufacturers will likely not condone what s necessary to limp by on open delta. Many have wire-type primary voltage tap connections. Simply remove jumper to the affected primary for one side. May have to cut the winding conductor to terminal on other end of winding, and perhaps the same for secondary connections. A lot depends on what one is willing to do to limp by until a replacement can be installed.
Yeah I have read some similar statements... have yet to run across a good explanation.
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I suppose that it may depend in part on whether the "single phase mid-tap" is grounded to create a high-leg delta or is used in conjunction with one end of that same secondary winding being grounded to create just one 120 volt supply point.
If the latter case is considered, look at the extra current in that part of the winding from a single phase 120V load:
240V delta will have a total kVA rating of three times 240xAx3 where A is the current in a single delta winding of the secondary. (Not the current in the phase line, where root 3 would come into play.)
The power from a single-sided 120V load will be 120xA where A is the current in the winding supplying the 120V load, not considering for the moment the parallel current source path through the remainder of the delta. Since that path will have five times the internal resistance, that is a fair approximation.
Notice that for a given load current, A, the power to the 120 volt single phase load will be 1/6 of the power the same winding current could deliver when working to supply a balanced three phase load.
Presto: If you reduce the three phase current by the amount of the 120V load, you end up reducing the available power by six times that amount.
So 5% of the rated kVA going to the 120V load cost you 30% of the available three phase power.
That calculation works for me as a good first approximation, and I would not be surprised if the exact calculation comes out the same.
It all happens because the winding conductor for the tapped winding is no larger than the winding conductors in the rest of the windings.
To the extent that the localized winding current rather than the overall heat dissipation is the limiting factor for the transformer, I think that you have your answer.
kwired
Electron manager
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- NE Nebraska
I don't know about that, there are many pole top multiphase transformer banks built out of separate single phase transformers all over the place around here. If it were less expensive to do it another way they likely wouldn't do it the way they do. Cost of raw material necessary is one thing, availability and sale price is another factor. Using single phase transformers to build the bank may use more copper and steel but they are also typically using transformers that they purchase in larger quantities, keep more in stock, etc. possibly making them cost less when all is done. This is also possibly seen from the manufacturer side as certain units are made in larger quantities that they may actually sell for less than a so called "special order" unit that otherwise uses less material to produce. Re-tooling or set up of production lines can be a reason to sell the not so common or one of a kind type units at higher prices.
Most of these transformers I am talking about are common and also used for single phase services, some may even be able to be reconfigured for different voltages meaning even less items to keep in stock for new builds or as replacements, you simply pull the right kVA unit, and configure for desired output voltage and send to the installation site. But some of the utility guys maybe need to verify if that is done, but I think it is possible with some units.
Number of limbs was my point.
Many who purchase a 240/120 3ph/4w transformer assume that there is more 120/240 1ph3w Kva available than there actually is and the sacrifice that there is for the 240v 3ph3w kva. You have to wonder if those who sell these transformers have the where with all to explain that to the buyers. I know I didn't at first.
Ahhhh... I don't think so. The scenarios, as I recall, had nothing to do with a corner grounded, or corner referenced 120V lighting tap...
If we consider a high leg delta system, you could take the center tapped winding up to rated output with balanced 120V loads, and load the other phases with 240V 1? loads. That'd be a completely balanced system with no 3? loads...????
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But if the only load options are 120V lighting loads and balanced three phase loads, then you cannot balance the system.
If you feed your 120V loads equally from both sides of the grounded center tap, then I would calculate the penalty (lost 3 phase load capacity) to be 15% rather than 30%.
In any case, a big problem in interpreting the quoted statement is that there is no wiring diagram nor even a complete verbal description of what the commentator is assuming to get the 30% figure.
15%, 30% of what???
If you have balanced 120V lighting load across A-C and only 3? other loads, the 3? load capacity is diminished by 66.7% of the lighting load kVA times 3... or lighting load times 2 (two-thirds of lighting load current is handled by winding A-C, while one-third is handled by the other two windings). For example, a 75kVA 240/120V 3? 4W xfmr with 12kVA of balanced 120V lighting load... the 3? capacity is diminished by 24kVA.
mbrooke
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Yes,
But if you loose a winding the whole unit must be replaced. Also avoid wye primaries in 3 phase units since an open phase can induce inductive heating in the tank from non canceling core flux.
But if you loose a winding the whole unit must be replaced. Also avoid wye primaries in 3 phase units since an open phase can induce inductive heating in the tank from non canceling core flux.
kwired
Electron manager
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- NE Nebraska
All problems with single unit with a single core I believe. The thread topic is about 3 separate single phase units connected to make a three phase secondary.
As far as balancing such a bank - sometimes the 120/240 pot is a different size than the others, this will make balancing a mute point and overloading of each unit must be looked at differently than if they were all same size. A single unit with single core likely has all three coils the same size though.
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