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12 HYDRO REVIEW / July 2017 www.hydroworld.com
bath through an air gap of 0.3 mm between
the CGB vapor seal and generator shaft.
As all units were running on >95% wicket
gate opening while the plant was operating at
full capacity, the units went into mechanical
over speed condition due to outage of both
power evacuation feeders, which might have
caused damage to the air admission valve
piston rod in Unit 1, causing water leakage
from the water collection tank.
Water mixing occurred in the CGB oil bath
only, but the alarm came in generator LGB
water in oil. Tis was due to wrong variable
tagging in the SCADA system with CGB
water in oil.
Te water drain line from the NRV hous-
ing was originally in reverse slope, causing the
water entry in CC lead and resulting in a rotor
earth fault that caused a forced outage of the
unit. Modifcations performed to avoid water
entry into the CC during annual maintenance
in 2015 to 2016 involved modifying the slope
of the NRV housing drain line so that water
drains through the 125 NB line before entry
into CC lead being on higher elevation level.
Another example occurred during annual
maintenance in 2015 to 2016 on Unit 3. Heavy
water leakage was found from the primary
cooling water pump gland seal. Tis resulted
in pump breakdown within one month of
running during the monsoon period. Te gland
seal was replaced with a mechanical seal with
TC/TC mating contacts with plan 31 (with
Cyclon separator) and it is running smoothly
since April 2016 and resulted in stoppage of
leakage, which enhanced availability of unit.
During annual maintenance in 2016 to
2017, NHPC installed a new isolation valve
in the trunnion greasing line of all three units
(for NRV maintenance during unit running)
and also to avoid the outage due to failure of
MIV/fault in greasing system. Tis shall result
in improvement of unit availability.
During that same period, a new globe
valve was installed to replace the butterfy
valve in the stator cooling water system of
Unit 2. Tis is used for water fow regulation
in the individual stator coolers, to maintain
temperature uniformity in all three units and
also ensure leakproof isolation of coolers for
its dismantling/maintenance. Tis will result
in improvement in unit availability.
Preventive measures taken in
generator step-up transformer
Dissolved gas analysis (DGA) revealed that
the value of ethane is on an increasing trend
in the two transformers. Te other gases
were within limits, but ethane reached 87 on
December 1, 2016 in the r-phase and 80.77 in
the y-phase. Accordingly, these transformers
were planned for overhauling.
Te following major activities were per-
formed during the annual maintenance (2016
to 2017) process:
• Performing electrical test
• Performing DGA, breakdown valve and
PPM testing of oil
• External inspection
• Internal inspection of HV cable bushing
• Internal inspection of main tank
• All connections checked and tightened
After the overhaul, the DGA report
revealed ethane was 3 in the r-phase and
4.732 in the y-phase.