The error arises from the winding resistances. For example we had measured the resistance HV A-B as 4,42 Ohm but you had as 6 Ohm. The temperature values are almost same. The difference between the resistances are nearly % 30 so the difference in the loss is inevitable. Winding resistance measurements must be performed by a winding resistance meter not a resistance meter. We use Tettex 2293 Winding analyzer.  

WINDING RESISTANE MEASUREMENT:- We use Voltage – Current Method for measuring

the winding resistances

WINDING RESISTANCE MEASUREMENT:

R=

WINDING RESISTANCE MEASUREMENT:How do we measure?We use Tettex 2293 Winding Analyzer for measuring. It has 6 channels so it can measure the resistances of high voltage and low voltage at same time.

Notes:The current that pass from the winding should be enough high for reading correct value but also shouldn’t be so high for increasing the winding temperature so changing the resistance value.

In ANSI and IEEE standards, the current that is used in winding resistance measurement should be higher than the 1,2 times of the no-load current and lower than the % 10 of the rated current.

WINDNG RESISTANCE MEASUREMENT:Example:For a 1000kVA 33/0,4 kV transformer:Irated HV = 17,49A Irated HV = 1443A I0 % = 0,9 % so

- The current for measuring the HV winding resistance is between 0,16 – 1,75 A

- The current for measuring the LV winding resistance is between 13 – 144 A