3.2 MICA PAPER, EPOXY RESIN VPI PROCESS
A 2-pole turbo alternator at Brandon rated 13.8 kV, 118MVA (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.