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8
User manual - Annex III
High Voltage Instrument Transformers | DDB/DFK/DDN/DFN
The measurement of the insulation resistance is done to check
the integrity of the transformer insulation and to ensure that it
has not been internally damaged during transportation.
This test depends largely on the design of the transformer, the
general confi guration of the test, the instruments used, etc.
The values obtained may not be representative of the actual
state of the insulation.
If this test is performed, it is recommended to contact the
manufacturer.
For fi eld tests, the use of the megohmmeter (Megger) is very
common, and the tests that are usually done are:
› Neutral against secondary and ground.
› Each secondary against ground.
› Among the secondaries.
In the case of new equipment, the value obtained does not
provide any information, unless a short circuit is detected.
Generally, values above 200 M
Ω
should be expected. However,
the comparison of long-term values - such as in the case of
tan
δ, or of power factor - can help determine if a device is still in good
working conditions
.
Do not test at voltages above 3 kV AC or 1 kV DC.
Do not test for more than one minute. It is not recommended
to repeat this test often as the internal insulation could be
damaged.
INSULATION RESISTANCE MEASUREMENT
CAPACITIVE VOLTAGE TRANSFORMERS/
Neutral against Secon Ground N/S+G
› The line cable of the measuring equipment must be connected
to the terminal N/H2* disconnecting the grounding link of the
primary winding.
› The ground cable of the measuring instrument must be
connected to the short-circuited secondary terminals together
with the ground terminals on the secondary terminal block as
well as the tank.
Secondaries against Ground S/G
› Primary terminal A/H1 must be disconnected and the neutral
connection of the (N/H2*) primary winding must be grounded.
› The line cable of the measuring equipment must be connected
to the short-circuited secondary terminals.
› The grounding cable of the measuring equipment must be
connected to the tank.
Between secondaries S/S
› This test is only applicable to transformers having more than
1 secondary.
› Primary terminal A/H1 must be disconnected and the neutral
connection of the (N/H2*) primary winding must be grounded.
› The secondaries to be assessed will be connected; one to
the line cable and the other(s) to the ground cable of the
measuring equipment.
› The test is repeated with each secondary.
* The N or H2 terminal can be located inside the secondary terminal box on the side of the box.