However if a core clamping finger should fracture the
results can be very serious as happened for Kettle Unit 4 on
11 August 2006. In this case one of the fingers at the top
fractured at the edge of the clamping plate and entered the
air gap. Figure 12 shows the absence of the finger and the
immediately adjacent resultant damage. At the site of the
most serious core damage fused laminations were ground
out, and thin sheets of mica were inserted as shown in
Figure 13. Following this penetrating epoxy was applied
and an EL CID (Electromagnetic Core Imperfection
Detector) test [17] yielded satisfactory results. A total of
three fibre optic temperature sensors were placed in
damaged stepped laminations adjacent to clamping fingers
and a fourth in a similar undamaged location. An ultrasonic
test was made on all 1200 clamping fingers. Six fingers
were found to be cracked at the top and one was loose at
the bottom of the core. Cracked fingers were reinforced by
stainless steel plates on either side. Ultrasonic tests were
also performed on the clamping fingers of Units 1, 2 and 3.
As for the stator winding, 69 coils were patched with minor
damage in the end arm area and with 10 of the patches being
close to the core. 40 new stator coils were installed requiring
3 lifts. Two defective coils remote from the lift areas were
bypassed and clamp-on CTs were installed at the neutral of
these two circuits.
As the faulted unit had been deluged with water, hot air
drying was done for 7 days with insulation resistance
measured every 12 hours. Also there was heating of selected
coils by circulating direct current. In addition to visual
inspection, the results of polarization index measurements
and defect detection by ramp tests were used to determine
those coils to be replaced or bypassed. The reconnected stator
was further dried by operating the generator with a 3-phase
short at the line end at 100% rated current for 72 hours with a
winding RTD temperature of 86 oC [18].