Tettex TTR 2795, Operating Instructions Manual

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Connection and Setup 19
4.9 Transformer Look Up Tables
4.9.1 Definitions
The term “Turns Ratio” (TR) is defined as the theoretical voltage ratio, the term “Voltage Ratio” (VR) as the ratio of the
rated voltages (“boilerplate voltages”).
TR =
VR =
As a rule, the voltage ratio of three-phase transformers can be measured using a single-phase supply, as long as the
distribution of magnetic flux in the core is taken into consideration. Only windings, winding segments and winding
combinations which have the same magnetic flux applied, can be compared with one another. The measuring circuit
can be derived from the phasor diagram of the test transformer's vector group. The two voltages which are to be
compared must be in phase and have the same orientation.
But –depending on the winding configuration and phase rotation of the transformer - it’s not possible to have always
phase voltages which are being compared. The TTR 2795 therefore internally corrects the measurement with the
“TR/(V
A
/V
a
)” factor and displays automatically the correct ratio value.
The TR/(V
A
/V
a
) factor determines how the turns ratio is calculated from the ratio of the
energizing voltage to the output voltage of the winding. The ratio of the measured voltages
are multiplied by this factor to give the turns ratio for that winding/tap.
Due to similar reasons the voltage ratio VR and the turns ratio TR is –depending on the winding configuration and
phase rotation of the transformer – not always equal. The TTR 2795 therefore internally corrects the entered rated
voltages (VR) with the “VR/TR” factor and displays automatically the correct ratio deviation which is calculated out of
the rated voltages (boilerplate voltages) as reference and the actually measured (and corrected) ratio value.
The VR/TR factor determines the multiplier applied to the measured voltage ratio (VR) to get
the turns ratio (TR) of the transformer. When the boilerplate voltages for the transformer are
set up, the calculated voltage ratio is divided by this factor to give the nominal turns ratio for
that winding/tap.
_Windings High Voltage side (HV turns)_
Windings Low Voltage side (LV turns)
Phase to Phase Voltage (HighVoltage side)_
Phase to Phase Voltage (Low Voltage side)
TR Turns Ratio
VR
Voltage Ratio