Three phase transformer vector groups

[Surefire01]

I wanted to demonstrate the voltage phase shift between primary and secondary on a three phase transformer so I had one built by a local company. All the winding ends are accessible so I can configure it any way I please.

When connected up as star-star I expected there to be no phase shift but using an oscilloscope I observed a difference of around 1mS between primary and secondary voltages on the same phase. (I say 1mS but it is difficult to be exact and it could be anything between 0.5 to 1mS)

To double check I configured the transformer as delta-delta but still observed the same 1mS difference.

Am I missing something here? surely there should be no phase difference between primary and secondary voltages when configured in this way. Or will there always be a small difference due to the limitations of this transformer.

Thanks

 

[ScottyUK]

Have a look at the simplified electrical model of the transformer, for example:

[image]

http://upload.wikimedia.org/wikipedia/commons/thumb/3/39/TREQCCT.jpg/500px-TREQCCT.jpg

[/image]

The 'T' network is a fairly good representation although there are equivalent [Π] networks and slightly simpler 'L' networks also in common use. The small phase shift is due to the series leakage reactance of the transformer.

 

[waross]

Look at the voltages from phase to phase and from phase to neutral with your 'scope. You may see a distorted waveform. Back in the day of analog voltmeters with d'Arsonval movements this would illustrate that the ratio between average and RMS voltages changed when the wave form was no longer a sign wave. I have seen this effect cause consternation more than once.
Test unloaded. It doesn't take much load to swamp out this effect.
(The d'Arsonval movement responded to average values of voltage which were scaled to RMS values. The readings from line to line and from line to neutral would no longer be in the ratio of 1.73:1)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Surefire01]

Thanks for the explanations chaps, darn that pesky leakage reactance.