Tettex MIDAS 2881, Operating Instructions Manual

Page: 139 / 164

Applications Guide 129
The short-circuit impedance (Z
short-circuit
) is defined as the corresponding impedance to the voltage
(U
cc
) that must be connected to one pair of terminals of a transformer with another pair of terminals
shorted, which causes rated current (I
rated
) to flow on the two sides of the transformer.
Z
short-circuit
= U
cc
/ I
rated
Using the „Current Booster 5287“ in combination with the MIDAS is required to perform this
measurement. The current booster is an additional device connected to the MIDAS power output that
provides an output current up to 10A (15A intermitted) at low voltage (10V or 100V). The test
connection for measuring short-circuit impedance is outlined below.
Measurement of short circuit impedance measured in UST A mode
Measurement Procedure
If this measurement setup is used to calculate load losses the winding resistance measurements and
the winding temperature measurement shall be carried out before the actual short-circuit impedance
measurements are taken.
If there are built-in current transformers, they must be shorted during the test to avoid saturation of
their iron cores and over-voltages at the secondary terminals.
The bushing taps must be earthed.
The duration of the test should be as short as possible to avoid any significant heating of the
windings. As a rule of thumb 30 seconds are recommended.
The measurement current should be as close as possible to the rated current and not lower than half
of rated current. For verifying the test results a second measurement with approx. 10% lower current
can be executed. After extrapolation, the two measurements should agree.
Measuring Data Interpretation
The measurement of short-circuit inductance of transformers doesn’t just give information about the
stray inductance of the coil itself. Percentage changes in inductivity, compared to earlier
measurements or manufacturer’s specifications, can indicate changes in geometry.
Changes of between 2..4% in Z
short-circuit
generally mean a change in coil geometry or coil
compression. Values between 4..6% and over indicated damage to the winding.