At the Dorsey Converter Station there are three 13.8 kV, 300 MVA, 900 rpm synchronous condensers installed in 1990/91 and operating at 2.6 kV/mm.
The contract required minimum voltage endurance as defined by a straight line on
a graph of log electric stress versus log time of 10.7 kV/mm at 10 h and 7.8 kV/mm at 500 h.
While this requirement was met these Roebel stator bars did not meet the voltage endurance requirement of 30 kV 400 hours specified by IEEE Std 1553 Schedule A as is shown by Figure 1 of [30] which also provides 35 kV endurance results on the same population.
The short circuit ratio required by the HVDC conversion equipment was an overriding consideration which prevented a design with stator bars having a lower electric stress.
A mitigating factor may be the operation in hydrogen at 200 kPa (gauge) which tends to suppress partial discharge.
One of these units has twice survived mechanical damage to the end arms of stator bars.
The more recent incident occurred in August 2009 when a piece of triangular shaped file was left inside the air gap following an overhaul.
A ground fault involving C-phase occurred 3 days after the return to service at the location shown in Figure 20.
A total of 3 bars had been damaged in this location at the connection end (CE) with electrical positions as indicated in Table 12.
The largest file fragment found was 25 mm long.
Repairs were made using a mica-epoxy paste at the copper followed by fully cured mica paper tape bonded with brushed on epoxy resin.
Following curing of the resin the stress control coating had to be renewed.
A one minute 60 Hz hipot test was made on the repaired winding as indicated in Table 13.
As B-phase had failed at the patch it was necessary to fully replace this patch.
The subsequent successful 60 Hz hipot was limited to 18 kVrms.
Maximum as left E/M (TVA) probe readings at CE were 2 mA (Slot 72), 11 mA (Slot 76) and 6 mA (Slot 78) for a test voltage of 8 kVrms.
Direct-voltage ramp tests on each phase to -31 kVdc showed evidence of possible delamination for Bphase, but this had disappeared when the test was repeated
in 2014 possibly reflecting a complete cure of the resin.