Dielectric Discharge (Dd)Test - AVO MEGGER BM25 Guide De L'utilisateur

Application of Test Techniques

Dielectric Discharge (DD)Test

The Dielectric Discharge (DD) Test is a diagnostic insulation test
that allows ageing and detorioration of insulation to be assessed.
The result is dependent on the discharge characteristic, so the
internal condition of the insulation is tested, largely independent
of any surface contamination. The charge that is stored in the
insulation material under test, is measured 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
20
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.
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
=
I
1min
V x C
Insulation Condition
Bad
Poor
Questionable
O.K.