Three Transformers Changing 3-phase to Single Phase?

wwhitney said:
It cannot create a balanced 3-phase load. A balanced 3-phase load of constant current has constant power delivery over once cycle. While a single phase load has time varying power delivery over a cycle, with zero power delivery twice per cycle at the voltage zero crossing.


To do a low zig-zag, you'd need such a transformer with a secondary that has each end of each coil brought out, rather than factory connected in a delta pattern.

Cheers, Wayne
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Yes it would have to be. It's something I would have to check, but won't be going down that road anyway. :)
 
If you ran a transformer with dogleg output I am 95% sure the current flow in the generator would be the same as if you had wired the generator that way.

IOW way out of phase on some windings.

Let's ask the same question we do on people who show up with a 120/208 3 phase generator and want 120/240 single phase.

Why do you actually need 240? Would 208 do just as well? 3 legs of 120 so the load is distributed evenly? If higher voltage is needed how about 125/216 or 127/220 Mexico voltage?
 
Birken Vogt said:
If you ran a transformer with dogleg output I am 95% sure the current flow in the generator would be the same as if you had wired the generator that way.

IOW way out of phase on some windings.

Let's ask the same question we do on people who show up with a 120/208 3 phase generator and want 120/240 single phase.

Why do you actually need 240? Would 208 do just as well? 3 legs of 120 so the load is distributed evenly? If higher voltage is needed how about 125/216 or 127/220 Mexico voltage?
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In this case, there is already a 240/120V breaker panel. They don't want to change that to 3-phase.
 
So with the double delta, I see this as a 120V corner ground delta with only two hot legs being used. Wouldn't the genny become imbalanced?
 
How big is the load compared to the generator?

If it is below a certain percent, the generator will tolerate the imbalance.

If the phase monitoring system is all that fancy, it should also notice that the power factor of 2 phases is very poor, when running single phase dogleg through transformers. It might trip anyway.

Also, have you allowed for the 2/3 derating of the 65 kw genset, or possibly worse?

Stator ohmic heating only allows 2/3 of rated kVA when used on single phase. Rotor iron heating is another matter and the manufacturer must be consulted, it sounds like they are not much help. You might assume 50% of kVA until proven otherwise.
 
As I mentioned, this will feed a manual transfer switch rated at 100 amps. My bid includes installing a 100 amp breaker/disconnect on the output of the transformers, but I am also oversizing the wiring in case they need more power later.
 
TwoBlocked said:
This must be my week for single to 3-phase and visa versa.

Got a different customer that has special 65kw 480V 3-ph gennys that need to provide 240/120 power. ..../.... But no, they have 3 identical 25kw wired up to provide 240/120! Sooo ...

So does my attached drawing look right? And btw, how does this work ???


View attachment 2578537
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Thats interesting does the 25kVA add so it can supply a 75kVA or 313 Amps of single phase 240?
What is the calculated single phase load(s) in kVA and what is the single largest load?
 
TwoBlocked said:
The genny would be unbalanced and trip.
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Kind of forgot this was why you were trying to do a zig zag or other funky connection in the first place. Why they have a 480 volt three phase generator in the first place to supply 120/240 single phase? Is it backup to a 120/240 service? If it is the only source then transformation to 208/120 and balancing the load makes the most sense to me.

generator should be capable of supplying 1/3 of it's rating on single line to neutral output or 2/3 of it's rating between 2 lines and/or the neutral. If something is tripping you apparently are exceeding that 1/3 capacity on at least one phase conductor? Zig zag likely won't give you full generator kVA and you still may have similar issue? May be able to oversize zig zag connected transformers, not sure off top my head how balanced of load they would draw from the supply though.

Otherwise rectify to DC and then invert back to AC?

Some of these methods sound pricey enough to consider selling the generator you have and get one with output you need.
 
tortuga said:
Thats interesting does the 25kVA add so it can supply a 75kVA or 313 Amps of single phase 240?
What is the calculated single phase load(s) in kVA and what is the single largest load?
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Since the genny is rated at 65kw, the transformer would not be at full load of 75 kw. In this case, the largest single load would be a 100 amp breaker panel, IF it was fully loaded, but isn't.
 
If 208 is totaly out of the question off the table, based on my past research with a similar 'no 208' requirement and squeezing the most out of a 480 system without unbalancing the simple and best answer is a Scott-T.
I'd split the single phase load in 1/2 and have two feeders with the same kVA or balance each as best as you can.
 
kwired said:
Kind of forgot this was why you were trying to do a zig zag or other funky connection in the first place. Why they have a 480 volt three phase generator in the first place to supply 120/240 single phase? Is it backup to a 120/240 service? If it is the only source then transformation to 208/120 and balancing the load makes the most sense to me.

generator should be capable of supplying 1/3 of it's rating on single line to neutral output or 2/3 of it's rating between 2 lines and/or the neutral. If something is tripping you apparently are exceeding that 1/3 capacity on at least one phase conductor? Zig zag likely won't give you full generator kVA and you still may have similar issue? May be able to oversize zig zag connected transformers, not sure off top my head how balanced of load they would draw from the supply though.

Otherwise rectify to DC and then invert back to AC?

Some of these methods sound pricey enough to consider selling the generator you have and get one with output you need.
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It's gotta be this generator. It's a gas turbine with air bearings and so has no lubrication oil. It's super "green" and very quiet, too. The customer has a number of them and it seems a going thing in the gas industry. They learned by experience that these gennys will trip out on imbalance if the typical two leg 480 to 240/120 transformers are used. It may be more sensitive to imbalance. I don't know at what load they had problems. At one site I visited, it was using just two legs to a 480 to 240/120 transformer, but was drawing less than 10 amps 240v at full load. They are going to put, (now get this!) a Stirling engine genny with hydrogen gas as the "medium" if that's what you call it, rated at 5kw to replace the 65kw turbine, which then will be moved and require the transformer skid we've been discussing.

Oh, I love my job, but mostly because I got a boss I actually like, respect, and admire. :)
 
tortuga said:
If 208 is totaly out of the question off the table, based on my past research with a similar 'no 208' requirement and squeezing the most out of a 480 system without unbalancing the simple and best answer is a Scott-T.
I'd split the single phase load in 1/2 and have two feeders with the same kVA or balance each as best as you can.
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When I first heard about this, I suspected a Scott transformation with only two secondary legs being used. I have no idea how balanced that would be, but there would only be 1/2 the output, I believe. Only time I've seen a Scott transformation was for an electric oven to produce a high current on two separate heating circuits.
 
TwoBlocked said:
When I first heard about this, I suspected a Scott transformation with only two secondary legs being used. I
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If the largest single phase load was 100A or 24kW it should be possible to split the load into two single phase feeders.
I have never seen the setup you posted, and can't comment on how it works, but I have seen a few newer scott-t setups.
 
tortuga said:
If the largest single phase load was 100A or 24kW it should be possible to split the load into two single phase feeders.
I have never seen the setup you posted, and can't comment on how it works, but I have seen a few newer scott-t setups.
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Possible, but not practical. It would involve installing a second breaker panel for the other half of the Scott transformation.
 
IMHO something is going on here that is not fully understood. One transformer (T2) creates 120V and is used directly. Transformers T1 and T3 create the second 120V leg as an 'open jaw'. Because of the connections, the current flowing in T1 and T3 secondaries _must_ be perfectly in phase. By transformer action, the current in the T1 and T3 primaries should be mostly in phase, with the exception of a bit of magnetizing current.

I expect very low current to flow on the Bn 480V lead, even with high 120/240V loading. I expect the actual loading of the generator to be nearly as unbalanced as if a simple 480V:120/240V single phase transformer were connected Ye-On.

Yet we have the key piece of data that with the single phase transformer this fancy generator trips, but with the low dogleg transformer in place the generator doesn't trip.

My _guess_ is that the 480V delta transformer primary passes sufficient current on the Bn lead that the generator control system decides 'all is well', but that the actual generator balance is not significantly changed. But that is just a guess :)
 
winnie said:
IMHO something is going on here that is not fully understood. One transformer (T2) creates 120V and is used directly. Transformers T1 and T3 create the second 120V leg as an 'open jaw'. Because of the connections, the current flowing in T1 and T3 secondaries _must_ be perfectly in phase. By transformer action, the current in the T1 and T3 primaries should be mostly in phase, with the exception of a bit of magnetizing current.

I expect very low current to flow on the Bn 480V lead, even with high 120/240V loading. I expect the actual loading of the generator to be nearly as unbalanced as if a simple 480V:120/240V single phase transformer were connected Ye-On.
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Case 1: the 480 delta generator supplies a 480V:120/240V single phase transformer on 480V phases A-B. If the currents on the 120/240V side are I1 and I2 (neutral current is I1-I2), then the 480V primary current is (I1+I2)/4. If the generator windings are equal impedance, then the generator sees (I1+I2)/6 current on its A-B winding, and (I1+I2)/12 current on both the B-C and C-A windings.

Case 2: the 480 delta generator supplies (3) 480V : 120V single phase transformers in a low zig-zag. Say the current I1 is on the leg provided by one transformer on A-B, and the current I2 is provided by the zig-zag on B-C and C-A. The current on the generator A-B coil is I1 / 4, while the current on the B-C and C-A coils are both I2/4.

So if the 120/240 load is balanced of current I = I1 = I2, in case 1 we have I/3 on one coil and I/6 on the other two, while in case 2 we have I/4 on each coil.

Cheers, Wayne
 
wwhitney said:
Case 1: the 480 delta generator supplies a 480V:120/240V single phase transformer on 480V phases A-B. If the currents on the 120/240V side are I1 and I2 (neutral current is I1-I2), then the 480V primary current is (I1+I2)/4. If the generator windings are equal impedance, then the generator sees (I1+I2)/6 current on its A-B winding, and (I1+I2)/12 current on both the B-C and C-A windings.

Case 2: the 480 delta generator supplies (3) 480V : 120V single phase transformers in a low zig-zag. Say the current I1 is on the leg provided by one transformer on A-B, and the current I2 is provided by the zig-zag on B-C and C-A. The current on the generator A-B coil is I1 / 4, while the current on the B-C and C-A coils are both I2/4.

So if the 120/240 load is balanced of current I = I1 = I2, in case 1 we have I/3 on one coil and I/6 on the other two, while in case 2 we have I/4 on each coil.

Cheers, Wayne
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So Wayne, to be clear, are you saying that the generator will experience a balanced load with a low zigzag transformer, assuming that the secondary load is balanced?
 
You folks have been VERY helpful. After building the transformer skid (if we get the bid) I would like to test it in the shop with the 208V 3-phase we ave available. Of course the secondary voltage will be less than half of compared to energizing with 480V. Given the info on the attached nameplate, is there a way to know what the unloaded inrush current might be? Hopefully I can test this without tripping the main breaker!

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