Transformer impedance and different loads
Transformer impedance and different loads
(OP)
Hi all,
Can someone please help me understand nameplate transformer impedance values better, and how it can be used for varying loads.
I realize that the nameplate transformer impedance (let's call it Z%) is by definition the voltage (in pu) that causes full load current circulation in the secondary, if the secondary is short circuited.
I also understand that this Z% can then be taken as the transformer voltage drop in pu (let's call it Vdrop%), while operating at rated current, or in other words at rated load.
(My understanding is that this is because this is the voltage that is basically being dropped over the transformer impedances due to iron/copper/core losses while there is rated current circulating as set up in the experiment, as this was the voltage needed to overcome these impedances while there's no load in the circuit)
My question is, how can we know what the Z%, or in other words the Vdrop% is, if we are operating at a current different than the rated current?
Thank you in advance
Can someone please help me understand nameplate transformer impedance values better, and how it can be used for varying loads.
I realize that the nameplate transformer impedance (let's call it Z%) is by definition the voltage (in pu) that causes full load current circulation in the secondary, if the secondary is short circuited.
I also understand that this Z% can then be taken as the transformer voltage drop in pu (let's call it Vdrop%), while operating at rated current, or in other words at rated load.
(My understanding is that this is because this is the voltage that is basically being dropped over the transformer impedances due to iron/copper/core losses while there is rated current circulating as set up in the experiment, as this was the voltage needed to overcome these impedances while there's no load in the circuit)
My question is, how can we know what the Z%, or in other words the Vdrop% is, if we are operating at a current different than the rated current?
Thank you in advance






RE: Transformer impedance and different loads
RE: Transformer impedance and different loads
Regards
Marmite
RE: Transformer impedance and different loads
I will reword my question as follows:
The only impedance information available on most transformer nameplates is a Z%.
Let's say the transformer nameplate has the following information:
kVA = 1000kVA
Vpri = 4160V
Vsec = 480V
f = 60Hz
3phase
Z% = 10%
If we know the value of our operating current, how can we calculate the voltage drop over the transformer?
i.e. what would be the voltage drop at:
a. 100% of FLA
b. 50% of FLA
c. 25% of FLA
Thank you
RE: Transformer impedance and different loads
RE: Transformer impedance and different loads
I'm still confused with the '%' unit for the impedance, and it is my understanding that is based on rated current.
Assuming an X/R ratio of 1 (for simplicity, let's assume Z% = X%), how would you calculate voltage drop at different loads (i.e. 100% FLA, 50% FLA, etc.)
RE: Transformer impedance and different loads
RE: Transformer impedance and different loads
Regards
Marmite
RE: Transformer impedance and different loads
RE: Transformer impedance and different loads
The reason why the z impedance is expressed as % is because it is defined (in IEC 60076-1) as z[%] = 100 (Z/Zref) where Z is the ohm value of the impedance, Zref = U^2/Sr and U is the rated voltage and Sr is the rated power of the unit (this is the Per-Unit system representation, see here if you are not familiar with this representation.
RE: Transformer impedance and different loads
Based on the J&P document Marmite attached, I can see that there is a load value dependence aside from power factor, as the simplified model in the document is expressed as:
percentage regulation = a[Vr cos(phi) + Vx sin(pi)]
Where a is the load factor (expressed as a multiple/fraction of FLA)
So prc, I think the further-simplified equation you cited would be:
voltage regulation = a [%Z sin (phi)]
Where as explained above, a is the loading factor.
Would you agree?
RE: Transformer impedance and different loads
RE: Transformer impedance and different loads
RE: Transformer impedance and different loads
RE: Transformer impedance and different loads