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Info History:
The transformer is in the factory hall for changing switches.
Under the order number 490220, this was the place where a repair was
carried out in 2000. Existing gas-in-oil analyses from the years 2006
– 2008 do not show any indications of disturbances in the operating
behavior
Measurements and excavated action parts
Results: see Annex B
At the converter transformer 1 (StTr 1) according to Figure 1 in
Appendix A, the press bar and core have already stopped at 500V, at 9V
test voltage the insulation resistance is only 68kOhm. In addition,
several closures of clamping screws/press iron were measured on both
StTr's.
Diagnosis of the active parts
The removed active parts of the converter transformers including
chokes (Figs. 11-14) and the control transformer (Figs. 2 – 3) were
inspected with customer representatives on 18.03.2009.
The damage to the step selector contacts (Figs. 4 – 6 ), especially
to the slip rings, was striking. According to customer information,
the switches were stepped about 5.5 million times, contrary to the
number of a good 335000 that can be read from the motor drive!
Figure 7 shows the load switch sets heavily contaminated with soot.
The control cabinet (Fig. 16) does not meet the current occupational
safety requirements (clamping stone not covered).
Oil Analysis, DGA, D-Polymiersation Degree Determination of Paper
Insulation
The results of six D polymerization degree determinations (Appendix C)
indicate severe paper aging with values 267 to 429. The windings and
dissipators are thermally worn! The DGA on entry into the plant
confirms the results achieved in previous years with low corrosive gas
concentrations.
According to the result of the insulating oil test (also in Appendix
C), the oil can be reused. It is recommended to become a passivator
because of the potential presence of corrosive sulfur.
Recommendations for repair
In addition to the commissioned works – including replacement of the
complete tap changers,
thorough cleaning of the active parts, the boiler – the following is
recommended:
- Renewal of all windings, departments.
- Attempt to remove the connections on the active parts (whereby the
closure on the StTr1 press beam / core without G51264438 rewinding
hardly seems to make sense because of too much effort and because of
the condition of the transformer / paper aging, if its cause is not
only surface dirt).
- After cleaning, the feed-through plates, which are conspicuous by
signs of hot spots, can continue to be used
- Retrofitting of the control cabinet in accordance with the
applicable safety regulations; Renewal of the protection and control
devices, their wiring.
- Renewal of corrosion protection
After installation, the transformer must be subjected to an output
test in accordance with DIN VDE 0532. It is proposed that the voltage
tests be carried out at 70% of the original values, not at 75% as
required by the standard in force in 1980, the year of manufacture of
the transformer. This is because of the described poor condition of
the windings (if a winding is not replaced). The 70% level is drawn
from an older version of DIN VDE 0532, after which repeat tests had to
be carried out on transformers built up to about 1973.
Germany
90552 Röthenbach 
