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High Voltage Instrument Transformers | CA/UT/KA
User Manual - Annex III
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:
› Primary vs. secondary + Ground for current.
› N/P2 against secondary and ground for voltage.
› 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 MΩ should be expected. However,
the comparison of long-term values, as in the case of tanδ, or
power factor, can help determine whether a device is still in
good working order.
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
› Fig. 5 › Fig. 6 › Fig. 7
Primary vs. Secondary + Ground P/S+G (Fig. 5)
› The line cable of the measuring equipment must be connected
to the P1 and P2 primary terminals, which are short-circuited
with each other.
› The ground wire of the measuring equipment must be
connected to the short-circuited secondary wires, by the
ground terminal, and directly to the base of the equipment.
› If the guard wire is used, it must be connected to the porcelain
insulator.
Secondaries vs Ground S/G (Fig. 6)
› The P1 and P2 Primary terminals must be disconnected.
› The line cable of the measuring equipment must be connected
to the short-circuited secondary terminals.
› The ground cable of the measuring equipment must be
connected to the base of the equipment.
Between secondaries S/S (Fig. 7)
› This test is only applicable to transformers having more than
1 secondary.
› The P1 and P2 Primary terminals must be disconnected.
› The secondaries to be assessed will be connected; one to
the line cable and the other(s) to the ground cable of the
measurement equipment.
› The test is repeated with each secondary.
CURRENT TRANSFORMERS/
SUPPLY
ELECTRODE
GUARD
ELECTRODE
GROUND
ELECTRODE
GROUND
ELECTRODE
GROUND
ELECTRODE
SUPPLY
ELECTRODE
GROUND
ELECTRODE
SUPPLY
ELECTRODE
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