MEASUREMENT OF WINDING RESISTANCE
   
 
PURPOSE OF THE MEASUREMENT
 

The resistance between all pair of phase terminals of each transformer winding are measured using direct current. The measurement is performed for each connection of connectable windings and for each tapping connection. Furthermore the corresponding winding temperature is measured.

The measured resistances are needed in connection with the load losses measurement when the load losses are corrected to correspond to the reference temperature. The resistance measurement will also show whether the winding joints are in order and the windings correctly conected.
   
 

APPARATUS AND MEASURING CIRCUIT
The measurement is performed by means of volt-ammeter method.
 


Fig. 2.1

Resistance measurement using
volt-ammeter method.
 

The principle of the measurement is as follows: the voltage drops caused by the direct curret I across the resistances Rx is read by means of a voltmeter (Fig. 2.1).

The ratio U/I represent the value of the read resistance.

In order to obtain the value of phase it is necessary to consider the connection of winding under test. If the winding is start connection the read value of resistance has to divided by two. If the winding is star connection the read value of resistance has to be divided by 2. If the winding is delta connection the read value of resistance has to be moltiply by 1.5.

The temperature is measured by means of Pt 100 with an accuracy of +-0.1 °C. Direct curret is obtained from a battery.
   
 
PERFORMANCE OF THE MEASUREMENT
 

Before the measurement start the transformer is standing for at least 3 hour filled oil and without excitation. During this period the temperature differences of the transformer will equalize and the winding temperature will become equal to the oil temperature. The average winding temperature is obtained by determining the average oil temperature. The average oil temperature is obtained by measuring the top oil temperature in an oil-filled thermometer pocket situated in the cover, and the bottom oil temperature in the drain valve, and taking the average of these two.
   
 
To shorten the time for the current to become steady so high a measuring current is used that the core will be saturated and the inductance will be low. The measuring current is usually 5...10 times the no-load current of the winding. However, the current should be less than 10% of the rated current of the winding, otherwise the temperature rise of the winding caused by the measuring current will give to measuring errors.
   
 
TEST RESULT
 

The resistance values and the average temperature are calculated. In the report the terminals, between which the resistances are measured, the connection, the tapping position and the average temperature of the windings during the measurement are stated.