B) Input connection:
- N. 1 cable for low voltage measurement connection, shielded, 4
m long, for the 10 V measurement, plus N. 2 Clamps to connect
the voltage measurement.
The test is performed as follows.
- First of all, move the tap changer all the way to the starting
position: this will be recorded as test number one.
- Select the base time of the recording, as a function of the
nominal selection time.
- Start the test and generate a current of one or two amps (if
possible). While you are generating, current, voltage and winding
plus tap contact resistance are displayed.
- Set the resistance trigger level to a value 1.5 to two times
greater than the displayed resistance.
- When you have defined the trigger level, the instrument stays
in the "trig mode", and waits until a resistance greater than the
trigger level is measured. Once triggered, the instrument shows
the resistance diagram on the display. Note that the vertical axis
is autoranging; so, for the measurement of the limiting resistor,
take into account the measurement of the winding resistance.
- Save the result, then, before performing the next test, change
the test number high left on the screen.
- Continue the same way until the stop position of the tap
changer has been reached.
- The Load Tap Changer fault is easily detected: if, at any
moment, the contact is lost, the resistance grows to infinity.
The resistance waveform changes as a function of the parasitic
transformer impedance and of the selection time. In the example,
the default of 50 ms per division covers a total time of 300 ms,
and there is no effect of the parasitic impedance.
The parasitic impedance is expressed by the parameter X/R, that
is one of the characteristic parameters of the transformer. The L/R
time constant of our test is related to the X/R parameter by the
following relationship:
L/R = (R/X) / 314
The resistance waveform of the example can be observed only if
L/R is small (less than 10%) of the selection time; otherwise, the
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