Previously 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.