Problem on Wye-Wye Bank 2

[cwhit03]

Had a field connection done wrong the other day and wanted some input on the scenario. It was a wye-wye bank.

Transformer # 1: H1=A, H2=N ===> X1=a, X2=n
Transformer # 2: H1=B, H2=N ===> X1=b, X2=n
Transformer # 3: H1=C, H2=N ===> X1=n, X2=c (Incorrect On Sec. Side)

The field guy swapped the secondary connections on the third transformer. So we were getting the 120V out of the bank, but no 208V. I was running into problems when determining the phasor diagram. With the above scenario, what kind of phase angles will I get on the secondary.

 

[davidbeach]

You'd have c midway between a and b. So, 0[°], -60[°], and -120[°].

 

[rbulsara]

May be the orignial tags were correct. By swapping, the "c" 120V is 180deg backwards, that would give you what davidbeach said, instead of 0,-120 and -240. What voltages you were getting prior to swapping?

 

[cwhit03]

The tags were definitely correct. We were single phasing all customers. According to my foreman we were not getting the appropriate 3 phase voltage. I am in the process of getting the p-p voltage data. I will post when I get them.

 

[cwhit03]

Sorry I did not cleary state in my original post. The correct way to install was supposed to be:

X1=a, X2=n
X1=b, X2=n
X1=c, X2=n

The Journeyman put it on as x1=n, x2=c. I understand the 180 degrees and it being backwards. So the phase angles would be 0, -60, -120. Can explain in a little more detail so I fully understand. I appreciate your time in this matter.

 

[rbulsara]

See if this helps. Fig 1 is for a correct 3 phase phasors. Fig 2 has one (c) phase inverted which is equivalent to Fig. 3.

http://server4.pictiger.com/img/1257100/other/phasor-1.jpg

 

[cwhit03]

Ok now I see the phasor diagram and I understand the voltage vectors. Assuming the 0, -60, and -120 phase shifts, what would the phase to phase voltages be for A to B, B to C, and AtoC

 

[waross]

Voltages A to B = 208 V, B to C = 120 V, C to A = 120 V.
All 120 volt, single phase equipment should work but there will be heavier neutral loading. The exact neutral currents will depend somewhat on the power factors of the individual loads.
respectfully

 

[cwhit03]

And A to N , B to N , C to N would still remain 120V based on the internal connections of the transformers. THe swapped connection on the third transformer is still 120V across the x1 and x2 postion, just the polarity is opposite.

So to reiterate, with the secondary bushings as follows:

(Wye-Wye Bank)

X1=a, X2=n
X1=b, X2=n
X1=n, X2=c

Is the following data correct?

a to b = 208V (0 degree angular displacement) ; a to N = 120V
b to c = 120 V (-60 degree angular displacement) ; b to N = 120V
c to a = 120 V (-120 degree angular displacement) ; c to N = 120V

 

[davidbeach]

Yes, your voltages listed would be correct.

 

[apowerengr]

This mistake is often called a "crow's foot" based on how the phasor diagram looks /|\ instead of |
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