120, 208 and 240 volts From Distribution Transformer Bank?

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A client inquired as the following connection.
Connect three 120/240v distribution transformers in 120/208 volt configuration. The secondary windings of two transformers would be paralleled for 120 volts with one end of each paralleled winding connected to ground and the other ends to phase (A and B). The third transformer would be left in a 120/240v configuration. One end would be connected to phase C. The center tap would be connected to ground. The other of the winding would be used along with phase C to obtain single phase 240 volts. The third transformer would be larger than the first two to compensate for the smaller capacity of the winding in the three phase configuration. The purpose is to feed a three phase 120/208v load and a 120/240v residence from one transformer bank, Is this feasible?
 

winnie

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Springfield, MA, USA
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Electric motor research
Feasable, yes. But also something of a nightmare because it would be quite non-standard. If you wanted, you could even make a 'hexaphase' service by joining the center taps of your 3 120/240V transformers, and you could then pick any of two 208V services or 3 240V services. The physics will work...but in practice I'd stay away from it.

Essentially what is being asked for is an unbalanced 4 phase system (4 distinct hot legs with irregular phase distribution). Presumably there will be a mix of balanced 3 phase (208V) loading and balanced single phase (240V) loading. To load all of the transformers reasonably, you will need to oversize the 'C' phase transformer.

To avoid the massive confusion of a single distribution panel supplying both types of load, you would want to have _separate_ panels, one for single phase and one for 3 phase. (Could you imagine the nightmare of a 4 phase panel where some breaker locations give 240V L-L, others give 208V L-L, and still others give 120V L-L????)

At this point, IMHO it would make _much_ more sense to have two separate systems (208V/120V 3 phase and 120V/240V single phase), feeding two separate distribution systems. Or, if it makes sense because of metering requirements, then I would suggest a normal 208/120V three phase service, and then a transformer to provide a single phase 120/240V single phase supply.

-Jon
 

don_resqcapt19

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If you really have 208Y/120 and 120/240, you would have two connection points to ground and short out a winding.
 

mbrooke

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Technician
A 3 phase 4 wire delta would be more practical or buck boosts.

It is doable, however I do not know how severe voltage imbalance will be. To me this sounds something resembling a 6 phase diametrical connection.

Keep in mind that many residential dwellings are on 2 legs of 208Y/120 and do fine.
 

mivey

Senior Member
One end would be connected to phase C. The center tap would be connected to ground. The other of the winding would be used along with phase C to obtain single phase 240 volts.
That is where it falls apart because you have two 120 voltages paralleled but they are out of phase so it creates a fault. add2: like Don said

You can still have three transformers but you can't mix them. Two transformers can work in an open-wye configuration to get you the 208Y/120 three phase and the third transformer can supply the 120/240 single-phase in a separate service. add: like Jon said.
 

GoldDigger

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That is where it falls apart because you have two 120 voltages paralleled but they are out of phase so it creates a fault. add2: like Don said

You can still have three transformers but you can't mix them. Two transformers can work in an open-wye configuration to get you the 208Y/120 three phase and the third transformer can supply the 120/240 single-phase in a separate service. add: like Jon said.
When you say "open wye" are you referring to a two phase only source that cannot support a three phase load? I cannot figure out any way, analogous to an open delta, which could produce all three phases with only two single phase pots.
 

mivey

Senior Member
When you say "open wye" are you referring to a two phase only source that cannot support a three phase load? I cannot figure out any way, analogous to an open delta, which could produce all three phases with only two single phase pots.
One 3-bushing secondary and one 4-bushing secondary with the windings separated (center bushings isolated). Bank capacity is 75% of total nameplate capacity.

Primaries:
V_A = V<0d
V_B = V<-120d

Secondaries:
Transformer A has a 3-bushing transformer with the center-tap grounded. We have two voltages across the two winding halves but sharing a common connection.

Transformer B has a 4-bushing transformer with the two center bushings separated. Only one of the center bushings is grounded and we have with isolated voltages across the two winding halves.

So with the following voltages from the outer to center bushings:

Va1 = v<0d with center bushing grounded
Va2 = v<180d with center bushing grounded
Vb1 = v<-120d with center bushing grounded
Vb2 = v<60d with center bushing connected to outer bushing of Va2 so we can use the series combination of Va2 and Vb2

we can get the following 3-phase voltages:

Van = Va1 = v<0d
Vbn = Vb1 = v<-120d
Vcn = Va2 + Vb2 = v<180d + v<60d = v<120
 

kwired

Electron manager
Location
NE Nebraska
A client inquired as the following connection.
Connect three 120/240v distribution transformers in 120/208 volt configuration. The secondary windings of two transformers would be paralleled for 120 volts with one end of each paralleled winding connected to ground and the other ends to phase (A and B). The third transformer would be left in a 120/240v configuration. One end would be connected to phase C. The center tap would be connected to ground. The other of the winding would be used along with phase C to obtain single phase 240 volts. The third transformer would be larger than the first two to compensate for the smaller capacity of the winding in the three phase configuration. The purpose is to feed a three phase 120/208v load and a 120/240v residence from one transformer bank, Is this feasible?


Is this what you are trying to describe



That could supply both 208/120 three phase as well as 120/240 single phase.

Depending on loading conditions the 120/240 transformer may need to be higher capacity then the other two as you are only using half it's winding for the three phase portion of the system, plus you also need to add the load from the 120/240 single phase portion of the system to the common coil on the "C" leg to come up with loading on that individual transformer.

Volts from 4th ungrounded conductor to A or B should be 120 volts but at a 60 degree phase angle and neither conductor of a circuit connected here is a grounded conductor, don't try to use this 4th ungrounded as part of supply to a three phase load.

ETA

Actually may not quite work as described because the two coils on the vertical line are on the same core, I'll let some of those more educated in such things determine just what voltages and out of sequence problems may be here.
 

meternerd

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Athol, ID
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retired water & electric utility electrician, meter/relay tech
Is this what you are trying to describe



That could supply both 208/120 three phase as well as 120/240 single phase.

Depending on loading conditions the 120/240 transformer may need to be higher capacity then the other two as you are only using half it's winding for the three phase portion of the system, plus you also need to add the load from the 120/240 single phase portion of the system to the common coil on the "C" leg to come up with loading on that individual transformer.

Volts from 4th ungrounded conductor to A or B should be 120 volts but at a 60 degree phase angle and neither conductor of a circuit connected here is a grounded conductor, don't try to use this 4th ungrounded as part of supply to a three phase load.

ETA

Actually may not quite work as described because the two coils on the vertical line are on the same core, I'll let some of those more educated in such things determine just what voltages and out of sequence problems may be here.

It's pretty common on overhead services. Usually because you have a 120/208 3 ph apartment complex and next door is a single family residence requiring 120/240 single phase. The 120/240 is up-sized to accommodate the increased load.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
In the diagram posted, I will re-label the terminal labeled '4th ungrounded' as C'.

From all phase terminals (A, B, C, C') to neutral you have 120V.

From C to C' you have 240V.

From A to B, B to C, and C to A you have 208V.

From A to C' or B to C' you have 120V.

For a given (fixed) L-N voltage, the L-L voltage depends on the phase angle between the terminals. It varies from 0V at 0 degrees to 2*(L-N) at 180 degrees. The equation is simply 2 * sin(angle/2).

With a phase angle of 60 degrees, the L-L voltage is equal to the L-N voltage.

Another way of thinking of this (as GoldDigger already described): A-C'-N forms an equilateral triangle in the phase diagram, just like an ordinary delta connection. All of the voltages between any two terminals will be the same.

-Jon
 

mbrooke

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Location
United States
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Technician
That's awesome info, I never would have guessed.


One a side note, wouldn't the voltage regulation for 3 phase loads be ugly?
 

GoldDigger

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That's awesome info, I never would have guessed.


One a side note, wouldn't the voltage regulation for 3 phase loads be ugly?
Not necessarily. As long as you wire your three phase loads to A, B, and C, regulation will be like any other three phase wye with additional single phase loads.
Any single phase loads involving 4th will not cause any more regulation problems than single phase loads from A, B, or C to N.
The pot driving the single phase 120-0-120 that included 4th may have to be larger to handle the extra load.
 

kwired

Electron manager
Location
NE Nebraska
The pot driving the single phase 120-0-120 that included 4th may have to be larger to handle the extra load.

They do the same thing with delta high leg systems where the pot with the grounded conducor when there is significant 120/240 single phase loads to be served, true whether it is open or closed delta.
 
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