X/R ratio for transformers

[mbrooke]

Question. How does unit size effect X/R?

Picture two scenarios.

1. 115kv to 34.5kv Dyn1 120MVA unit

2. 3 115kv to 34.5kv Dyn1 40MVA units run in parallel

In general, would I have the same X/R for both scenarios or would #2 be higher in resistance while being lower in reactance? If so how does this effect my fault current outcome?

 

[Julius Right]

Transformer reactance[X] depends on leakage magnetic flux and indirect on transformer impedance. X=SQRT(Z^2-R^2).
Transformer resistance[R] depends on copper losses[Pcu] R=sqrt(Pcu/I^2/3).
The transformer performance could be different in order to comply with the specification as short-circuit impedance connected with maximum short-circuit allowable and the losses which are
connected with the losses prices versus copper conductor price.
ANSI Standard C37.010 recommends the usual X/R ratio.
For instance for 120 MVA it has to be about 40 and for 40 MVA about 28.

 

[mbrooke]

Transformer reactance[X] depends on leakage magnetic flux and indirect on transformer impedance. X=SQRT(Z^2-R^2).
Transformer resistance[R] depends on copper losses[Pcu] R=sqrt(Pcu/I^2/3).
The transformer performance could be different in order to comply with the specification as short-circuit impedance connected with maximum short-circuit allowable and the losses which are
connected with the losses prices versus copper conductor price.
ANSI Standard C37.010 recommends the usual X/R ratio.
For instance for 120 MVA it has to be about 40 and for 40 MVA about 28.

Thanks

But for a typical order, what would be more X/R?

Question, would Pcu not be P=(I^2xR) x 3.14?

 

[rian0201]

Why not measure the actual impedance?


Sent from my iPad using Tapatalk

 

[mbrooke]

Why not measure the actual impedance?


Sent from my iPad using Tapatalk

I do not have the smaller units made yet let alone hooked up.

 

[Ingenieur]

mfgs will have data for previously built units
for these sizes I'd be surprised if there was a signiicant difference

http://arcadvisor.com/files/GET3550Fapp41-43.pdf

 

[Julius Right]

mfgs will have data for previously built units
for these sizes I'd be surprised if there was a signiicant difference

http://arcadvisor.com/files/GET3550Fapp41-43.pdf

That is quite the same quotation from ANSI C37.010 I said about.

 

[Besoeker]

For what it's worth, I generally use a four to one ratio.
It has worked for me.

 

[Ingenieur]

That is quite the same quotation from ANSI C37.010 I said about.

He's got a common wound 120 MVA 3 ph
and 3 each 40 MVA 1 ph configured as a 120 MVA 3 ph bank
a rough approximation 3 ph to 1 ph equivilent
the 3 phase ~ 41
the 3 1 phase sqrt3 x 26 ~ 45

the x/r is phase ang
the delta between 41 and 45 is 0.12 deg, moot

R has to be similar since i is the same
Xl (L) must be close since same power is transformed

 

[mbrooke]

He's got a common wound 120 MVA 3 ph
and 3 each 40 MVA 1 ph configured as a 120 MVA 3 ph bank
a rough approximation 3 ph to 1 ph equivilent
the 3 phase ~ 41
the 3 1 phase sqrt3 x 26 ~ 45

the x/r is phase ang
the delta between 41 and 45 is 0.12 deg, moot

R has to be similar since i is the same
Xl (L) must be close since same power is transformed

Close, 3, three phase 40MVA. That way on unit can be taken out of service and the remaining two still carry 80MVA of load.