VOLTECH AT3600 USER MANUAL TRANSFORMER TESTS
TESTS AND TEST CONDITIONS VPN 98 - 024 PAGE 7.1.51.
Therefore, the full test sequence is a follows:
Preliminary Impulse: The value of V
P
is measured, and the starting conditions
changed to compensate for the charge sharing effects.
Impulse #1: The value of V
P
is again checked. If it is as specified for the
test, this becomes the first impulse of the sequence, and the
transient is analysed. (If not it is treated as a second
preliminary impulse, and impulse #1 is repeated.)
...: Repeated impulses, up to the number programmed for the
test.
Impulse #n: The value of V
P
is re-checked, and the transient analysed on
each impulse of the sequence.
TRANSIENT ANALYSIS
During the decay phase after the impulse has been fired, the AT3600 measures both
the voltage amplitude along the transient, and the time of decay.
A good transformer will have a clean and sustained transient, with a long decay
period. A transformer with a shorted turn will have a heavily damped response, with
a shorter decay period.
The calculation performed is to calculate the ‘area’ underneath the graphical plot of
the decaying transient. (For the calculation used, both negative peaks and positive
peaks add to the total area.) The area, measured in Volts-seconds, is much smaller
for the faulty winding with a shorted turn.
SPECIFYING THE TEST LIMITS
It is very difficult to predict the Volts-seconds ‘area’ under the curve from theoretical
calculations.
The recommended method is to use the Measure Mode (see Chapters 3 & 6) to obtain
some values. The procedure is as follows:
Measure the area on a known good transformer; let this result be area A
G
.
Wrap an additional single turn round the core, short the two ends together, and
re-measure the area; let this result be area A
F
.
Set the limits as follows:
Max Area = 3A
G
/2
Min Area = (A
G
+ A
F
)/2
Remember that these limits are taken from only one transformer, and may need to be
revised after more have been tested.
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