X/R variation with transformer rating?
X/R variation with transformer rating?
(OP)
Hi
Suppose a given transformer has a given X/R rating for a given MVA value. Is this X/R value the same at a different rating value for the same trfr?
As an example I have a 20/25MVA trfr where 20MVA is the ONAN rating and 25MVA the ONAF rating.
I would tend to think that the resistance of the primary and secondary windings do not change appreciably due to the increased current flow? One can allow for slightly increased resistance due to the conductor heating up due to increased current flow but this is offset by the increased forced fan cooling.
Secondly I cannot see the leakage reactance changing due to increased current flow so I tend to then the X/R remains the same.
Any comments?
Suppose a given transformer has a given X/R rating for a given MVA value. Is this X/R value the same at a different rating value for the same trfr?
As an example I have a 20/25MVA trfr where 20MVA is the ONAN rating and 25MVA the ONAF rating.
I would tend to think that the resistance of the primary and secondary windings do not change appreciably due to the increased current flow? One can allow for slightly increased resistance due to the conductor heating up due to increased current flow but this is offset by the increased forced fan cooling.
Secondly I cannot see the leakage reactance changing due to increased current flow so I tend to then the X/R remains the same.
Any comments?






RE: X/R variation with transformer rating?
RE: X/R variation with transformer rating?
The impedance is tested with the transformer at operating temperature.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: X/R variation with transformer rating?
Regards.
RE: X/R variation with transformer rating?
Now, if the resistance is increased the voltage drop will increase and the e.m.f will decrease, so the magnetic flux density. That means less saturation and then less field losses then less reactance. In turn, the voltage drop will decrease and so on.
An equilibrium occurs somewhere but it could be a slight offset-less X/R.
Pure theoretically.
RE: X/R variation with transformer rating?
I certainly wouldn't worry about any X/R differences for normal fault current calculations. If you're doing some esoteric analysis that requires a real accurate X/R ratio, then you might worry.
RE: X/R variation with transformer rating?
RE: X/R variation with transformer rating?