Transformer Configuration

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D

D!NNy

Senior Member
Location
San Luis Obispo
Transformer Configuration:
In the attached picture shows the primary voltage of the transformer is "12000 Delta X 20780 Delta".

Questions:
1. I believe all the time voltage provided by SCE or PG&E is phase to Phase unless they mention it as line to ground voltage? am i correct.
2. how would we determine the primary voltage without contacting the PG&E? Does this one has something to do with lead connections?
3. PG&E confirmed that this transformer is fed by a 21kV system? then lead connections should be shown as 3-4?
4. can some one clarify how this lead connections, voltage and transformer configurations connections shown in the picture are related to each other?
5. Can this transformer be used for both 12kv and 21kv primary voltage for different connections?

Thanks in Advance
 

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G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
130530-1247 EDT

DINNy:

Note the input voltages are not a 2 to 1 ratio, but rater 20780/12000 = 1.73 .


So to solve for individual winding voltages let
T1 to T2 = V1
T2 to T3 = V2
T4 to T5 = V3
also by deduction V2 = V3

V1+V2+V3 = 20780, and
V1+V3 = 12000

12000-V3+V3+V3 = 20780
or
V3 = V2 = (20780-12000) = 8780
V1 = 12000-8780 = 3220

As a check
V1+V2+V3 = 3220+8780+8780 = 20780
V1+V3 = 3220+8780 = 12000

Yes the transformer is good for the two input voltages specified by properly connecting the terminals.

.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
D!NNy said:
Transformer Configuration:
In the attached picture shows the primary voltage of the transformer is "12000 Delta X 20780 Delta".

Questions:
1. I believe all the time voltage provided by SCE or PG&E is phase to Phase unless they mention it as line to ground voltage? am i correct.
2. how would we determine the primary voltage without contacting the PG&E? Does this one has something to do with lead connections?
3. PG&E confirmed that this transformer is fed by a 21kV system? then lead connections should be shown as 3-4?
4. can some one clarify how this lead connections, voltage and transformer configurations connections shown in the picture are related to each other?
5. Can this transformer be used for both 12kv and 21kv primary voltage for different connections?

Thanks in Advance
Click to expand...
1) The depicted transformers are delta (line-to-line). Doesn't matter wheter the primary supply is delta or wye.
2) Yes, the connection of terminals is depends on primary voltage (at least for first pictured transformer; can't make out enough detail on second).
3) Yes, 3 connected to 4 (first pictured).
4) The primary is configured delta, and likely not changeable. The lead connections just connect two windings in series in full (3-4) or part of one and the other in parallel (2-4 & 3-5).
5) Yes.
 
D

D!NNy

Senior Member
Location
San Luis Obispo
gar said:
130530-1247 EDT

DINNy:

Note the input voltages are not a 2 to 1 ratio, but rater 20780/12000 = 1.73 .


So to solve for individual winding voltages let
T1 to T2 = V1
T2 to T3 = V2
T4 to T5 = V3
also by deduction V2 = V3

V1+V2+V3 = 20780, and
V1+V3 = 12000

12000-V3+V3+V3 = 20780
or
V3 = V2 = (20780-12000) = 8780
V1 = 12000-8780 = 3220

As a check
V1+V2+V3 = 3220+8780+8780 = 20780
V1+V3 = 3220+8780 = 12000

Yes the transformer is good for the two input voltages specified by properly connecting the terminals.

.
Click to expand...

basically transformer is designed for Delta for 12kV and 21kV system
if the service voltage from the utility is 21kV phase to phase lead connections 3, 4 needs to be shorted and each phase to be terminated to 1,5.
if the service voltage from the utility is 12kV phase to phase lead connections jumped from either 2,4 or 3,5
Since i cant see any terminations from outside the transformer this should be something done in the factory and we cant tell the service voltage by looking at the name plate until utility confirms.
 
D

D!NNy

Senior Member
Location
San Luis Obispo
Smart $ said:
1) The depicted transformers are delta (line-to-line). Doesn't matter wheter the primary supply is delta or wye.
2) Yes, the connection of terminals is depends on primary voltage (at least for first pictured transformer; can't make out enough detail on second).
3) Yes, 3 connected to 4 (first pictured).
4) The primary is configured delta, and likely not changeable. The lead connections just connect two windings in series in full (3-4) or part of one and the other in parallel (2-4 & 3-5).
5) Yes.
Click to expand...

basically transformer is designed for Delta for 12kV and 21kV system
if the service voltage from the utility is 21kV phase to phase lead connections 3, 4 needs to be shorted and each phase to be terminated to 1,5.
if the service voltage from the utility is 12kV phase to phase lead connections jumped from either 2,4 or 3,5
Since i cant see any terminations from outside the transformer this should be something done in the factory and we cant tell the service voltage by looking at the name plate until utility confirms.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
D!NNy said:
basically transformer is designed for Delta for 12kV and 21kV system
if the service voltage from the utility is 21kV phase to phase lead connections 3, 4 needs to be shorted and each phase to be terminated to 1,5.
if the service voltage from the utility is 12kV phase to phase lead connections jumped from either 2,4 or 3,5
Since i cant see any terminations from outside the transformer this should be something done in the factory and we cant tell the service voltage by looking at the name plate until utility confirms.
Click to expand...

No... both pair are jumpered, 2 to 4 AND 3 to 5.

The jumpering for one configuration is likely done in the factory, changed in the field if need be.
 
D

D!NNy

Senior Member
Location
San Luis Obispo
Smart $ said:

No... both pair are jumpered, 2 to 4 AND 3 to 5.

The jumpering for one configuration is likely done in the factory, changed in the field if need be.
Click to expand...

By Jumpering both part of winding between 2 to 4 and 3 to 5 they are parallel to each other.
As V1+V2 =12kV
V1+V3 =12kV
As long as we do the jumpering for one of them we should be fine but then then we are not using the part of the winding.
And also terminating only 2 to 4 compared to both 2 to 4, 3 to 5 will have different impedance correct (lower Impedance)?

So transformer should have the different impedance in case 21kv configuration Vs 12 kV configuration (could be minor difference)?.


Thanks
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
D!NNy said:
By Jumpering both part of winding between 2 to 4 and 3 to 5 they are parallel to each other.
As V1+V2 =12kV
V1+V3 =12kV
As long as we do the jumpering for one of them we should be fine but then then we are not using the part of the winding.
And also terminating only 2 to 4 compared to both 2 to 4, 3 to 5 will have different impedance correct (lower Impedance)?

So transformer should have the different impedance in case 21kv configuration Vs 12 kV configuration (could be minor difference)?.


Thanks
Click to expand...
It is a little more complicated than that, since the ratio of the two voltages is not exactly 2:1.
If it were 12kV or 24kV, the simple series/parallel jumping would work, just as for a 240/480 volt motor.
But instead the 12kV connection parallels only part of the 21kV series winding set so that instead of being reduced to 10.5kV the needed applied voltage is instead reduced only to 12kV. This means that the portions of the two windings that are not paralleled will carry almost twice the current in that configuration that they do in the series configuration and will probably be wound with thicker wire to allow that.

To complete the parallel circuit, you have to put in both jumpers, as Smart $ stated, otherwise you will end up putting twice the rated current through one of the windings. The voltage will be right, but the transformer will not work as designed.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
GoldDigger said:
It is a little more complicated than that, since the ratio of the two voltages is not exactly 2:1.
If it were 12kV or 24kV, the simple series/parallel jumping would work, just as for a 240/480 volt motor.
But instead the 12kV connection parallels only part of the 21kV series winding set so that instead of being reduced to 10.5kV the needed applied voltage is instead reduced only to 12kV. This means that the portions of the two windings that are not paralleled will carry almost twice the current in that configuration that they do in the series configuration and will probably be wound with thicker wire to allow that.

To complete the parallel circuit, you have to put in both jumpers, as Smart $ stated, otherwise you will end up putting twice the rated current through one of the windings. The voltage will be right, but the transformer will not work as designed.
Click to expand...
I concur.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
1305021-1859 EDT

I conclude from the original post that D!NNy has a transformer sitting on the floor unconnected to anything, and he does not know how it was internally jumpered. Therefore he is unwilling to install the transformer until he knows its wiring. Connecting a low voltage source to the primary that is much lower than the input rating of the transformer presents no hazard. Use 24 V or 5 V if you want instead of 240.

There might be much higher leakage flux at very low input voltages so the ratios might be somewhat off. Also ratios are probably adjusted to provide desired output voltage with a fully loaded secondary.

In the high voltage connection the primary to secondary ratio is 75 to 1, thus with 240 input to the primary delta the wye output from a leg to neutral is 3.2 V. If the internal connection is wired for 12000 input then the ratio is 43.3 to 1 and the output voltage is 5.5 V. Even if the secondary read 4.3 (half way between the two ideal values) it would still be a good suggestion of the 20780 V selection.

It would be interesting to know the output with 240 V. Actually need to know both input and output voltage.

.
 
Last edited:
beanland

beanland

Senior Member
Location
Vancouver, WA
Reconnectable & Scott T

Reconnectable & Scott T

The first transformer is reconnectable as either 12kV or 21kV using taps on the windings. These are very common and may even have a dual-voltage switch.

The second is a rare beast, a Scott T 3-phase transformer. A Scott T uses two windings and two cores to transformer 3-phases. It is basically a tapped 2-phase (90 degree) transformer.
 
D

D!NNy

Senior Member
Location
San Luis Obispo
beanland said:
The first transformer is reconnectable as either 12kV or 21kV using taps on the windings. These are very common and may even have a dual-voltage switch.

The second is a rare beast, a Scott T 3-phase transformer. A Scott T uses two windings and two cores to transformer 3-phases. It is basically a tapped 2-phase (90 degree) transformer.
Click to expand...

Can you provide me more information about the Scott Transformer?
 
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