Change of Arc Furnace Transformer Primary from Delta to Wye

[ecamer34]

Hi All,
I have a client who needs to replace their overcurrent protection devices as they are obsolete.
The CTs are mounted externally of the transformer, so line currents are used.
The transformer is an arc furnace transformer, rated 3750 kVA, 13.8 kV primary.
The transformer primary can be reconfigured on the primary between delta (157 A) and wye (90 A). So winding current is rated for 90A.
The secondary is wye, with voltages varying between 400V to 87V through series and parallel bus connections. Let's say that the current rating is 12390 A for now.
The reason for the primary voltage changes is for varying the voltage on the secondary for the arc furnace application.

What I need to know is:
1) Do I need to provide two overcurrent settings for the transformer when the client needs to change the primary? Is this due to the line currents changing?
2) Does changing the primary configuration affect the kVA rating? i.e. for delta, my rating is 3750 kVA, and for wye, 3750/sqrt(3) kVA?

Thanks in advance.

 

[gray.one]

Winding configuration doesn't change kVA ratings.


Sent from my iPhone using Tapatalk

 

[Ingenieur]

from drawing
never mind

 

[Phil Corso]

ECAMER...

Can you detail past Current Variation on the LV side?

Regards, Phil Corso

 

[ecamer34]

ECAMER...

Can you detail past Current Variation on the LV side?

Regards, Phil Corso

Not sure of its present state. Shouldn't matter for this purpose because protection isn't on secondary. See attached file in OP. The transformer secondary does not change.

 

[Smart $]

It seems to me that you are not really changing the primary supply itself, just the transformer primary configuration, i.e. through the board connections and/or tap changer. If you are using CT's to sense overcurrent, all you need is an adjustable breaker rated for the highest possible current... 3750kVA÷13.8kV=272A... times Table 450.3(A) factor. The lower range setting should allow for change to wye.

 

[Phil Corso]

Excamer...

I'm not asking about capacity or protection concerns...

I'm asking about the degree of unbalanced secondary-currents in the furnace! If the unbalance is very severe the primary connected as a "Wye" could result in the phenomenon called "Neutral shift" or "Neutral displacement"! It could impact furnace performance, as well as endanger the xfmr!

Phil