Megger BM25 Guia Del Usuario página 20

APPLICATION OF TEST TECHNIQUES
during the discharge phase.
The charge that is stored during the insulation test is automatically
discharged at the end of the test when the discharge resistors are switched
across the terminals. The rate of discharge depends only on the discharge
resistors and the amount of stored charge from the insulation.
The capacitive current quickly decays from a high value with a relatively
short time constant (a few seconds). The absorption (or re-absorption
during a discharge) current starts from a lower level but has a much
longer time constant (up to several minutes). This is caused by ions and
dipoles re-aligning themselves within the insulation. When an electric field
is applied some ions are able to move, and some dipoles align themselves
within the field. These effects reverse themselves slowly when the test
voltage is removed, caused by particles returning to their natural random
state.
The DD test measures the discharge currents 1 minute after an insulation
test has been completed. At this time the capacitive current has usually
become insignificant compared with the re-absorption current. The level
of re-absorption after this time shows the condition of the insulation
material, providing that the insulation material has been fully charged for
full absorption to take place (typically 10 to 30 minutes). A high re-
absorption current shows that the insulation has been contaminated,
usually by moisture. A low current usually shows that the insulation is
clean and has not absorbed much moisture.
The dielectric discharge test measures the discharge current 60 seconds
after the insulation test is completed. This is converted to a figure of merit
which gives a figure for the quality of the insulation, independent of the
test voltage. This value is temperature dependent so it is important to test
at a reference temperature, or record the value.
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The DD value is defined as:-
Current flowing after 1 minute (mA)
Test Voltage (V) x Capacitance (µF)
The DD result will only be produced if the capacitance is in the range 0,2
µF to 10 µF and reverse measured current does not exceed 10 µA.
The maximum DD value that can be measured (assuming the default test
voltage of 500 V) is typically 20, if the capacitance of the test sample is 1µF,
but decreasing to 2 for a capacitance of 10 µF. If the maximum value has
been exceeded the result will show a '>' symbol in front of the number
(e.g. ' >20').
DD Test Result
>7
>4
2 - 4
<2
=
I1min
V x C
Insulation Condition
Bad
Poor
Questionable
O.K.