3.2 MICA PAPER, EPOXY RESIN VPI PROCESS
A 2-pole turbo alternator at Brandon rated 13.8 kV, 118
MVA (Table 8) experienced an out-of-step synchronizing
incident in 1985. Following this it was observed that there
had been a significant increase in partial discharge (PD) as
monitored on-line by 80 pF directional couplers.
Subsequently visual inspection showed a number of broken
lashings in the end arm area which were replaced. When
the on-line PD measurements were repeated the values had
reduced to those prior to the incident for the same voltage
and hydrogen pressure [19]. While the slot section of the
stator bars are insulated with mica paper bonded with
epoxy resin by the VPI process the end arms are insulated
with asphalt-mica which provided the flexibility necessary
to survive the out-of-step synchronization.
The outcome was less favorable in the case of a 17 kV
160 MVA synchronous condenser with 8 salient poles
which was synchronized out-of-step. Following the incident
in January 2003 the unit was returned to service until April
when maintenance level overpotential tests were scheduled.
These tests consisted of direct-voltage ramp tests up to a
maximum of -31 kVdc or alternatively 60 Hz voltage up to
a maximum of 22.8 kVrms. A total of nine front bars failed
in C-phase and one in A-phase. Four bars failed from 22 to
26 kVdc and six bars failed from 13 to 22 kVrms 60 Hz.
The failures occurred just outside the slot near the bar-tobar
blocking. Most broken lashings were in C-phase. Figure
14 shows the location of one of the failures together with a
number of broken lashings.