Voltage unbalance on Y-Y trafo 2

[unclebob]

Hi all.

We have a situation where a Y-Y 2500 KVA transformer gives weird voltage readings on the secondary side.
Just after the first energization, we take voltage readings, without any load connected.

All phase to phase voltages are OK at approx 600 V. But we have A-N =347 V , B-N = 320 V and C-N = 347 V.

H0 is not grounded, but X0 is. All transformation ratios and winding resistances are OK on all taps.

Transformer:
2500 / 3325 KVA
HV= 13.8 / 7920 V
LV= 600 / 347 V
Imp = 5.6 %
5 taps

How can we explain this?

 

[jghrist]

A Y-Y transformer is not a ground source. Without the H0 connection to a primary system neutral, and connection between H0 and X0, the secondary neutral voltage neutral is not determined except through secondary system capacitance. This would be the same as voltages on an ungrounded secondary.

 

[waross]

Possibly a slight unbalance on the primary voltages.
Or unbalanced capacitance on the secondary causing a neutral shift on the primary neutral.

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

 

[ViCH]

Try to check star connection on secondary winding. Is it loosen or oxydated.. And check AN, BN, CN winding resistances..

 

[unclebob]

Customer wants to have balanced voltages even at 0 demand... God knows why. I'm pretty sure adding some balanced loads will negate/correct the issue. As for now, I think that some CPT's at secondary for control and heating (connected between A and B phases) cause this slight imbalance. Substation is outdoor type, and heating is surely working (1250 W total). Will try taking voltage measurements with control power removed tomorrow.

 

[ViCH]

Too much unbalance, problem must be inside TF, if primary voltage is balanced. Did You check secondary star connections?

Viktor
Electrical Technician II

 

[jghrist]

You need a ground source if you want balanced Ø=grd voltages on the secondary. Is the primary system a grounded system? Why not ground H0? If secondary fault current is too high, use impedance grounding on secondary.

 

[ViCH]

unclebob, any findings or results?

Viktor
Electrical Technician II

 

[che12345]

Dear Mr. unclebob

1. You posted on [9th Jan 2020] the problem that you are facing and expected (advice free-of-charge) from this community. Numerous experts had taken their invaluable time trying to solve [your] problem; FOC.
2. " ViCH (Electrical)28 Feb 20 06:14
unclebob, any findings or results? "
3. The community would certainly be thankful and benefited if you had answered what Mr VICH had asked.
4. It should be an obligation of the person who posted the problem to report back [with a brief description] on how the problem is being resolved; for the benefit of the community.
Che Kuan Yau (Singapore)

 

[Palletjack]

I don’t see that as unusual.
Perhaps the primary voltages aren’t exactly right.

Is HO internally grounded or is there an external HO ground point?
three, four or five leg core?

 

[waross]

Roll the primary connections to the transformer.
If the unbalance follows the roll, refer the client to the customer service department of the utility.

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

 

[unclebob]

Customer has decided to replace the Y-Y with a D-Y. The Y-Y was used with a D-Y to use the 15 degree shift to mitigate harmonics. Load is 80% lighting (LEDs), so much harmonics generation was anticipated. Because of possible 3rd order harmonics, the HO on the Y-Y could not be grounded, moreover primary 13.8 kV is low impedance grounded with a 400 A resistor. Now, 2 D-Y's are feeding the plant. It's new, so not much loads are fed and harmonics content is still too low to be of concern.

I was not on that project, sorry for the late answers.

 

[jghrist]

So has the change in transformers corrected the voltage problems? The OP did not mention that the Y-Y was used with a D-Y. Used in what way? In series?