As we know that Transformer Open Circuit Test is conducted on LV side and that Short Circuit Test on HV side. To understand the reason behind this, we will consider a Single Phase Transformer of rating 3300 / 220 Volt, 33 kVA.
Thus the voltage at the LV side of this Transformer is 220 Volt. Therefore, for Open Circuit Test on LV side the range of Voltmeter will be 220 V.
Now, Full Load Current = (33×1000) / 220 = 150 A
So, excitation current = 4% of Full Load Current
= 0.06×150 = 6 A
Here note that Excitation Current is taken as 4% of the Full Load Current as the range of excitation current is 2-6%.
Also, the range of Wattmeter will be 220 V and 6A.
We see that the rating of instruments required for the testing are standard and easily available. Furthermore, using standard instruments, more accurate results can be obtained.
If the Open Circuit Test is conducted on HV side then a source of 3300 Volt may not be readily available. At the same time the instrument ranges required will be 3300 V, 0.4 A [4% of(33×1000) / 3300 = 4% of 10 = 0.4 A] and 3300 V & 0.4A which are not within the range of ordinary instruments and hence result obtained may not be accurate. Also, it is not safe to work on HV side from safety point of view.
Range of Voltmeter = 5% of Rated Voltage
= 0.05×3300
= 165 V
Note that voltage required to circulate rated current is around 2-12% of the rated voltage, that is why we have considered 5% for selection of range of voltmeter.
Range of Ammeter = Rated Current
= 33×1000 / 3300
= 10 A
Range of Wattmeter = 165 V , 10A
Thus we observe that the range of instruments required to perform the test fall within the range of standard instruments which are easily available and accurate.
Range of Voltmeter = 5% of Rated Voltage
= 0.05×220
= 11 V
Range of Ammeter = Rated Current
= 33×1000 / 220
= 150 A
Range of Wattmeter = 11 V , 150A
Instruments of such range and Auto Transformer capable of handling 150 A may not be readily available and at the same time result may not be much accurate.
