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TR A N S FOR M E R BUS H I NG S T Y PE G S B 8 0 0
I N S TA L L AT I O N A N D M A I N T EN A N C E G U I D E
2.14 Waiting time before energizing
Different waiting times are required, depending on
the transformer oil filling process, in order to avoid
flashover caused by air bubbles. Choose a suitable
method below.
Vacuum-filled transformer
No waiting time is necessary regarding the bushing.
De-gassed, oil-filled transformer
During mounting, use a clean and dry paintbrush to release
surface bubbles. Wait 6 hours before energizing.
Gas-saturated oil-filled transformer
During mounting, use a clean and dry paintbrush to release
surface bubbles. Wait 24 hours before energizing.
De-gassed, oil-filled transformer with reduced oil-level
After restoring the oil-level, wait 24 hours before energizing.
For all alternatives except “vacuum-filled transformer”, when
used according to IEC 60137, the oil should be allowed to
enter the center tube to at least 1/3 of the height to the
external part, and when used for IEEE C57.19.01 the oil level
shall be at least at flange height. This is done by releasing
the outer terminal sealing system and allowing air to escape.
2.15 Recommended tests before energizing
The following tests should be performed to check the
insulation, sealing and current path of the bushing. The tests
should be carried out after mounting, but before connecting
the outer terminal of the bushing to the rest of the
switchyard power circuit.
1. Tightness test between transformer and bushing flange
2. Tightness test of outer terminal
3. Measurement of capacitance and tan
d
4. Check of resistance
2.15.1 Tightness test between transformer and bushing
flange
Several different methods may be used for this test but we
refer to instructions provided by the company responsible
for field erection. As an example, the tightness of the
sealing between the transformer and bushing flange can be
checked when the transformer is oil-filled by using chalk or
paper strips.
2.15.2 Tightness test of outer terminal
Since the top terminal is often situated above the oil level of
the transformer expansion system, a leak at this point is
extremely serious because water could enter directly into
the transformer insulation. It is therefore recommended to
perform a tightness test after assembly, preferably both
with under vacuum and over-pressure. Several different
methods may be used but we refer to instructions provided
by the company responsible for field erection. One possible
method is the tracer gas method:
5. Fill tracer gas into the center tube before mounting the
outer terminal. The oil level of the transformer must be
above the bottom end of the bushing but below the
bushing flange.
6. Increase the pressure in the center tube by increasing the
oil level as much as possible.
7. Search for leaking gas at the gasket with a gas detector
(sniffer).