VGB POWERTECH 7 (2020) - International Journal for Generation and Storage of Electricity and Heat

A journey through 100 years VGB | Hydropower | VGB POWERTECH · Issue 9 (2007)
Maintaining Know-how
20 Member companies from 9 European countries
15,000 MW Run-of-river
30,000 MW Storage or pumped storage
Figure 3. The large European hydro power
operators practise a systematic
exchange of experience under
the roof of VGB.
During an extensive planned outage in a
pumped storage power plant after 15 years of
service, it was also time to overhaul the generator
poles. One of the tasks was to renew the
field coil frame which is to ensure the gliding
of the coil on the pole core. When tendering
for the scope of services to be provided, this
item was tendered on a functional basis, relying
on the expertise of the companies involved.
This was to pave the way for an open innovative
solution for the problem incurred.
Some weeks after finishing the planned outage,
the field coil frame moved out axially.
Figure 4 shows the problem area with a
movement of approx. 25 mm. The field coil
frame seated on the pole core is shown in
Figure 5.
Due to dynamic stresses, the field coil frame
moved out by up to approx. 38 mm. It was
thus indispensable to dismantle the generating
set again, which resulted in several
weeks of unavailability. With the support of
external experts, the structural design was
completely changed. This example was especially
interesting because another shutdown
due to faulty work in the workshop would
have been unavoidable if it had not been possible
to remedy the defects in the workshop
of the external contractor during several onsite
checks.
The second example also relates to a pumped
storage plant. Problems arose in connection
with standard components, namely fastening
screws that were to be built in during a mechanical
inspection outage. These screws were
to fasten the split rings and seal rings of
the pump turbine. Upon delivery, however,
the screws did not have the usual standardised
properties nor had their quality been assured.
During final assembly, a great number
of brittle fractures occurred at the transition
of the screw head to the screw shaft (Figur
e 6 ). These fractures were due to a manufacturing
fault. The screw material, a chromium-nickel
alloy, had reached an inadmissibly
high degree of hardness.
Moreover, the hexagon socket of various
screws was not located centrally (Figure
7).
If the fault had not occurred so early, dynamic
and shock-like operating stresses could
have led to machine damage which would
have caused the generating set to be shut
down for at least six months because of long
component delivery times in particular. This
would have caused fatal losses in revenue in
view of the installed capacity of the plant
concerned. As a consequence of this experience,
it was determined that components that
are material in terms of stress and function
must be subjected to adequate additional quality
assurance.
Figure 4. Damage at a field coil frame. Axially moving out
of the field coil frame.
Figure 5. Field coil frame seated on the pole core.
Figure 6. Brittle fracture of a screw head.
Figure 7. Position of the hexagon socket out of the centre.
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