3 Phase Transformer Connections

[an error occurred while processing this directive] 3 Phase Transformer Connections

WARNING:
DO NOT ATTEMPT TO PLAY WITH TRANSFORMERS; MAJOR INJURIES, OR DEATH MAY RESULT!!!
ONLY QUALIFIED PERSONELL SHALL INSTALL! USE THIS INFORMATION AT YOUR OWN RISK!

If you are looking to refresh your 3 phase transformer connections, this is a good page for you.

Primary Wye Primary Delta Emergency Connections

Secondary Wye Secondary Delta Secondary Delta w/Center Tap

Full Pri. Wye Connections Full Pri. Delta Connections

This is what you will start off with, 3 fresh transformers...
Click to enlarge...

The High Voltage rating (found on data plate) of the transformers will determine whether or not you use a primary Delta () or Wye connection.
If your system voltages are as follows, you may use either connection method (still have to match HV ratings...):

2400/4160V 3

2400V (phase) will use Wye primary

4160V (line) will use Delta () primary

7200/12470V 3

7200V (phase) will use Wye primary

12470V (line) will use Delta () primary

7620/13200V 3

7620V (phase) will use Wye primary

13200V (line) will use Delta() primary

This is by no means a complete list...
If your system voltage are as follows, you may ONLY use a Delta primary ():

2400V

4160V

12000V

12470V

13200V

69000V

And up and up until around 750KV (750,000V) or so.

Hint: if your system voltage is only a single number, it's delta ()

Next up is the Low Voltage rating (also on data plate). This all depends on what your customer needs.
Lower voltage transformers (12470V and less for sure) can either have 120V or 240V secondary windings.
You can connect the windings either in series (, voltage additive) or parallel (Y, voltage constant).
If you connect the windings in series, you will get this:
120V windings:

series: 120V between X1/X3 and X2; 240V between X1 and X3.

parallel: 120V between X3 and X2; X1 not used in my examples.

240V windings:

series: 240V between X1/X3 and X2; 480V between X1 and X3.

parallel: 240V between X3 and X2; X1 not used.

That sounds fairly simple, right? Add in that this is 3, and you have a multiplier 1.73 to factor in.
The factor messes it all up when you do parallel connections. What happens is the voltage is like this:
120V windings:

phase voltage: 120V

line voltage: 208V

277V windings: (common for lighting systems)

phase voltage: 277V

line voltage: 480V

You have to mach this all up with your customer's needs. If all they need is line voltage, use a Delta secondary.

You can (should) add a bonding jumper between system and customer neutrals on Wye/Wye to help reduce harmonics. Harmonics are extra cycles added into the cycles already present and can damage sensitive electronics. You can hear harmonics as noise produced by transformers and such, wasted energy. The bonding jumper helps neutralize this some more.

You may "float" (not hook up) the system neutral on Wye/Delta connections. A neutral is unnecessary on a straight up Delta, so there is no need for it here. The transformers still do their job as they should without it hooked up, and it saves materials and time.

There is another thing that needs mentioning. If you use a Delta secondary and hook up a Center Tap for a neutral, one of the phases is going to be a "wild leg" (a.k.a. "stinger"). This means that between one phase and neutral, there will be a weird amount of voltage present due to the path of travel going through 3 secondary windings instead of 2 like the other 2 phases.
If you have 120V secondaries, you would think the voltage would be 360V between that phase (A in these examples) and neutral (remember, voltage is additive in a series circuit). This doesn't happen due to the fact that one of the coils is in a different transformer with the cycle off by 90 degrees. This causes the voltage to drop to around 180-210V, usually 196V. This is an unusable voltage, it will damage/destroy your equipment (ex. a 120V 1 motor will spin WAY too fast and burn up). The "wild leg" or "stinger" is marked at all visible locations by orange (i think...) tape or paint per NEC requirements. Reason you would add a Center Tap is for a place that has a large power load (lots of motors, etc) and a very small lighting load. A good example is a pump house or a mechanical room.

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Primary Wye
Here's the connections for a primary Wye configuration.
Will use Phase system voltage (7200V on 7200/12470V system).
Click to enlarge...

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Primary Delta
Here's the connections for a primary Delta Configuration.
Will use Line system voltage (12470V on 7200/12470V system).
Click to enlarge...

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Secondary Wye
This is a secondary Wye configuration.
Will put out 120/208V -or- 277/480V
Click to enlarge...

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Secondary Delta
This is a secondary Delta config.
Puts out 240V
Click to enlarge...

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Secondary Delta w/Center Tap
Here is a secondary Delta with a center tap that creates a neutral (ex. a pump station w/1 light)
Puts out 120/240, also has stinger between A and N.
Click to enlarge...

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Complete Wye/Wye Scroll down for more
Here is a completed Wye/Wye connection
There is a bonding jumper between system and secondary neutrals to help reduce harmonics.
Click to enlarge...

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Complete Wye/Delta Scroll down for more
This is a Wye/Delta config.
Normally only used if the HV rating on your transformers only meets phase system voltage.
You may "float" the primary neutral on power-only Wye/Delta configs.
Click to enlarge...

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Complete Wye/Delta W/Center Tap
Here's a Wye/Delta with a center tap.
Again, Wye/Delta normally not used.
Since there is a neutral being created, don't float the primary neutral.
A stinger is created between secondary A and N.
Click to enlarge...

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Complete Delta/Wye Scroll down for more
This is a common 3 phase set up, Delta/Wye.
Nothing special here, used if the high voltage rating of transformers matches line system volts.
Click to enlarge...

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Complete Delta/Delta Scroll down for more
This is a power-only setup, Delta/Delta.
Used if a heavy 3 phase load to be applied and *no* lights.
Feeds large factories and such that have a seprate light input.
Click to enlarge...

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Complete Delta/Delta w/Center Tap
Delta/Delta with a center tap.
Used if a large power load, and a small light load is needed, like a pump house.
Stinger is between A and N.
Click to enlarge...

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Emergency Connections Scroll down for more
What do you do in an emergency if this happens...
Click to enlarge...

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Delta-Open-Delta
You can "fake" 3 phase like this Delta-Open-Delta setup.
Used if transformer HV rating matches system line volts.
Shown with optional center tap, which if used, creates stinger between A and N here.
Click to enlarge...

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Wye-Open-Delta
Here is Wye-Open-Delta
Used if transformer HV rating matches system phase volts.
Also shown with optional center tap, stinger between A and N.
Click to enlarge...

All this was done in Windows , yay!
Email me at kaneo666@yahoo.com if you would like to use these pictures, thanks.
Last Modified: Monday, 16-Aug-2004 18:25:25 EDT.
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Primary Wye	Primary Delta	Emergency Connections
Secondary Wye	Secondary Delta	Secondary Delta w/Center Tap
Full Pri. Wye Connections	Full Pri. Delta Connections