for Long Spruce, voltage endurance had been restricted to
330 mm bar sections. The experience with voltage
endurance of full stator bars for Limestone is reported in
detail in [26]. The early production (Series 1 & 2) with a
3.1 mm groundwall and 2.6 kV/mm operating stress
performed poorly on voltage endurance with few, if any,
bars meeting the specified 250 hours at 35 kV 120 oC. The
manufacturer elected to add one layer of tape (Series 3, 4 &
5) producing a 3.3 mm groundwall and 2.4 kV/mm
operating stress. This resulted in an improvement in voltage
endurance over Series 1 & 2, but not if the bars first
received 100 thermal cycles from room temperature to
between 135 and 140 oC at the copper with 34 minutes
heating and 46 minutes cooling. Table 10 provides the
comparison.
The solution to the problem required the use of a new resin
rich mica paper tape that allowed for a better flow of the resin
during processing and hence a greatly improved bond
when cured. This was represented at Limestone by Series 6 to
10 with an operating stress of 2.4 kV/mm. The improved
performance is recorded in Table 11.
Since Limestone much the same insulation system has
been provided for six other 13.8 kV hydro generators with
consistently good 35 kV voltage endurance ≥250 hours
(IEEE Std 1553 Schedule B) [27] following 500 thermal
cycles according to IEEE Std 1310 [28]. Three of these
Roebel bar windings replaced asphalt-mica multi-turn coil
windings at Grand Rapids, while the remaining three were
installed in new 94.7 rpm units at Wuskwatim.