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Case Study : Use of Complimentary Technology
In another station, a 500 MW unit, with step up
transformers of the 20 kV to 400kV range, (single phase OFWF) type
were subjected to high electrical stresses due to failure of current
transformers in the substation and were suspect. The anxiety of
station management further increased when one of the transformers
failed. The remaining transformers were surveyed for a condition
determination to ascertain their reliability.
The survey using acoustic fault detection
indicated ultrasonic activity at many places. The
correlation with DGA indicated no abnormality. It was recommended to
continue monitoring the locations for any increase in the magnitude
of activity. Any increase was expected to result in increased
evolution of gases along with the deterioration of the oil, which
would be reflected in the DGA results at that time. In the same
transformer vibration measurements and spectra were taken at all the
corners. At one of the corners near the On-Load Tap Changer (OLTC),
high overall vibration levels (5.57 mm/sec and 7.11 mm/sec )
were observed. Also in the spectra, 6X peaks were dominant
compared to 2X peaks which are normal. (X=50 Hz.) These 6X levels
indicated an intermediate level of severity. It was recommended to
investigate further and inspect for any physical damage. It was also
intended to verify the exact positions of jack (coil holding to
wall) so that vibration measurement points could be located near it
in future surveys.
During the next inspection, the plant engineers noted the loose
nuts at the jack. The tightening of the nuts restored the core to
the original condition and the overall vibration levels reduced to
minor levels. This confirmed the looseness of coil, which was
vibrating during operation. This kind of looseness damages the coil
over a period (slow damage progression) and also damages the
insulation. When the failure occurs it is always catastrophic
requiring total rebuild of the transformer. This kind of failure, if
experienced, would cost the utility generation loss of more than 25
days (about 171 million kWh). The cost of repair and transportation
to & from the manufacturer’s works would be additional (about
Rs15 million, $341K US
Dollars). | 