Star Delta Conversion: Why Phase Shift?

[vinayjain]

Why there is a phase shift of 30 degree (positive or negative) if we convert our winding from star to delta or vice versa?

 

[gsal]

Can you be more specific as to what exactly it is you are talking about?

Are you talking about a small 3-phase generator?

Have you tried a phasor diagram and perform the vectorial additions that happen when you do such wye and delta connections?

Please explain what you have tried and why you think the 30 phase shift shouldn't be there or how much shift you were expecting or something!...need to know where you are coming from.

 

[vinayjain]

I tried this on a transformer of 40MVA, 35/10 kV with higher side as delta connected and low voltage side as star connected and it gives a minimum of 30 degree phase shift and my concern is that why do we get a phase shift of minimum 30 degree.

 

[gsal]

Like I said...all you need to do is vector additions.

For example, say you have delta connection in the primary and your voltages are
V_AB=1<0
V_BC=1<-120
V_CA=1<-240

And you do a wye connection on the secondary and connected
the secondary winding of V_CA between a and neutral, that is V_an = V_CA
the secondary winding of V_AB between b and neutral, that is V_bn = V_AB
the secondary winding of V_BC between c and neutral, that is V_cn = V_BC

Now, you turn around and want to measure the line-to-line voltages on the secondary side...what are they?

Vab = Van + Vnb = Van - Vbn = 1.732<150
Vbc = Vbn + Vnc = Vbn - Vcn = 1.732<30
Vca = Vcn + Vna = Vcn - Van = 1.732<-90

So, not only is there a phase shift, but there is a ratio in magnitude for transformers with 1:1 turns...1.732 from delta to wye and 1/1.732 from wye to delta.

 

[vinayjain]

thank you