Engineer by mistake, Poet by heart, Author by nature...!!!
Definition
The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage
reactance expressed as a percentage of the rated voltage.(at either side)
The percentage impedance can then be calculated as follows:
Z% = Impedance Voltage x 100
Rated Voltage
Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the
short-circuited secondary would have an impedance of 9%.
Changing the Impedance Value
The most economical arrangement of core and windings leads to a 'natural' value of impedance determined by the
leakage flux. The leakage flux is a function of winding ampere turns and the area and length of the leakage flux
path. These can be varied at the design stage by changing the volts per turn and the geometric relationship of the
windings.
The Effect of Higher and Lower Impedances
The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current
that will flow under fault conditions.
It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions. By way of
example, consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary
side is:
2 MVA x 100/5 = 40 MVA
and from this figure the equivalent primary and secondary fault currents can be calculated.
A transformer with lower impedance will lead to a higher fault level (and vice versa)
The percentage impedance of a transformer a crucial parameter when operating transformers in parallel
The normal method of expressing transformer impedance is as a percentage voltage drop in the
transformer at full-load current. For example, an impedance of 10% means that the voltagedrop at full-load current is
10% of the open-circuit voltage, or, alternatively,neglecting any other impedance in the system, at 10 times full-load
current, thevoltage drop in the transformer is equal to the total system voltage.
suppose I tell you that a 1-phase transformer has X=110 ohms and negligible R, next you will ask me what is the KVA
rating, say 990 KVA, then you will ask me about the voltages, say, 33/11 kV.
Then you will ask me as to which side this X refers to, say 33 kV side.
Now, after all this clarifications, you find the current on 33 kV side as 30 A and on 11 kV side 90 A.
Then X referred to 11 kV side = [(11/33)^2]110= 12.22 ohms
Then you find voltage drop on 33 kV side is 110x30=3300 V (i.e., 10% of 33 kV)
Then you find voltage drop on 11 kV side is 12.22x90=1100 V (i.e., 10% of 11 kV)
Now see the other scenario X= 10 %
It means the voltage drop is 10% of the voltage of the HV or LV side.
