MEASUREMENT OF VOLTAGE RATIO & CHECK OF VECTOR GROUP SYMBOL
   
 

PURPOSE OF THE MEASUREMENT
 

The voltage ratio of the transformer is the ratio of voltages (in three-phase transformers line-to-line voltages) at no-load e.g. 11000 V / 400 V.

The purpose of the measurement is to check that the deviation of the voltage ratio from the specified value does not exceed the limit given in the relevant transformer standard (generally ± 0,5%)

The vector group symbol of the transformer is checked at the same time.
   
 
PERFORMANCE AND RESULTS OF THE MEASUREMENT
  The voltage ratio measurements are carried out by means of a voltage ratio measuring bridge the error of the bridge is less than ± 0,1%. The supply voltage is 40, 120, 220 V a.c. The function of the bridge is shown in Fig. 1.1. The voltage on the two windings of the transformer to be checked are comparated between themselves by the measurig bridge which is provided with a decade display unit located in its casing. When the bridge is in balanced we can see the ratio result directly from the numeral display of the bridge.
   
 

Fig. 1.1

Bridge measurement of the voltage ratio
T1 = transformed to be measured
U1 = supply voltage of the bridge
P1 = bridge-meter
 

Since the measuring device is a single-phase bridge the voltage ratio of a pair of windings mounted on the same leg is measured at a time. It is to be observed that the ratio of the line-to-line voltages. The result depends on the connection symbol of the transformer. For each winding connected to the bridge it is important to observe whether the number of turns relates the line-to-line or line-to-neutral voltage.For example the voltage ratio of a 12/0,4 kV Yd-connected transformer is 12000:/400 V = 17,32. The reading obtained rom the bridge is to be compared to this value.

The connection symbol of the transformer is checked in conjunction with the voltage ratio measurement. When the measuring leads from the transformed are connected to the bridge according to the relevant vector diagram in table 1.2 the bridge can be balanced only if the transformer connection is correct.

The voltage ratios are measured for each tapping connection of the transformer in the report the specified tapping voltage ratios are stated as well as the deviations of the measured ratios from these values. The connection symbol is also stated in the test report.
   
 

Table 1.2

Determination of the connection symbol

 

Clock hour figure (left),
Connection symbol (middle),
Vector diagram (right).

 

 

(The complete table is downloadable in our
support area)