ABB Power Grids Ltd.
GKSL 680205.021 RE
Rev. 2, 26.09.2019
10
7.5 Grounding the flange
Danger:
The flange of the bushing must always be ground by a safe earth connection (line) after the
installation!
The bushings are provided with one or two threads M12 at the flange in order to ground the flange.
After tightening the bolts fixing the bushing to the transformer tank, the flange should be earthed.
This prevents electrical discharges between bushing flange and transformer tank during service.
Apply a flexible cable between the M12 earthing hole in the bushing flange and a corresponding connection
point in the transformer. The tightening force is 40 Nm.
7.6 Evacuating the transformer
Evacuating the transformer with the bushing installed is permitted. If the transformer oil is filled in under
vacuum, a subsequent deaeration at the bushing or the transformer dome is usually not necessary.
The high voltage
Un/
3 should be applied to the bushing not earlier 12 hours after filling the transformer
with oil.
8 Measurements after installation
After installation of the bushing on the transformer
С
1
и
tg
1
measurements are recommended to compare
with values in a acceptance test report. A significant difference in the value of the capacity C1 from that
specified in the acceptance test report (more than 5%) may indicate damage (transport or during installation)
and the commissioning should not be put into operation.
9 Recommended maintenance and supervision
Cleaning of insulator surface
Measurement of capacitance and tan
Thermovision (infraded camera) check for local overheating on connectors
10
Testing
10.1
General
Measurement of C
1
and tg
δ
1
are carried out after the bushing was installed on the transformer or during a
regular check of the transformer.
The frequency of such measurements must be in accordance with local requirements.
Notice!
For diagnostic of the bushing insulation is used C
1
and tg
δ
1.
The recommended voltage for measuring
C
1
and tg
δ
1
– 10 kV and C
2
and tg
δ
2
– 1 kV.
The measurement of the resistance of the test tap must be used mega ohmmeter with the voltage not more
then 1000 V DC.
It is not recommended to measure C
2
and tg
δ
2
as the result of their measurement depends on environment
contamination and humidity.
Moreover during operation condition, the outer layer is earthed. Consequently, the insulation between
the outer layer and the mounting flange are not subjected to an electrical field and thereby do not cause
any dielectric heat losses.
