It looks like strand to strand test is an optional quality test for new coils. I'm not sure exactly how they do it (there is a photo). Perhaps they check insulation resistance between strands (before making the end connections which would short the stands together). I don't know what test voltage would be appropriate (250vdc?) or what acceptance criteria is applied.
Interested to hear more comments from others.
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In single turn roebel bars, where multiple insulated conductors are twisted and turned over the entire slot region, it is required to apply about 110 V AC between the conductors (each conductor has to be tested with all other conductors of the coil) to check for strand insulation failures created by the roebel manufacturing process. Any inter-strand shorts will cause localised high eddy current loss leading ultimately to the coil failure due to excessive heat. This is a routine test to be done on all roebel bars.
electricpete has a very good research about the topic..it is all about quality control on new winding.
STRAND TEST is intend to test insulation resistance failure between wire conductors especially on pre-formed coils (high voltage coils) where it revails or detects the inter-turn shorts on single coil. This test is also verified with almost the same result using SURGE COMPARISON TEST where we used to test the coil prior on putting it on in the slots.
I have seen this test done on hydroelectric coils during the manufacturing process and confirmed in the field. It is to unsure you done have a strand to strand short.
Surge comparison test (for multiturn stator coils and completed stator windings) detects faults relating to turn to turn, phase to phase, wrong winding connections etc. This test is done for individual coils and windings at manfacturer/repair shop and also at site.
Strand test (entirely different from surge test) is applicable only to single turn, multi conductor roebel bars. At sites, where the winding is already completed, this test cannot be done.
As was mentioned, the strand-to-strand insulation test is made on each strand of the single turn bars with the 360 degree transposition (or other transposition). The purpose for the test is to verify the integrity of the insulation between the strands. The test voltage is not less than 120 VAC. Each strand (in some specifications)is required to be also continuety tested. These tests are performed twice, once prior to shaping and the second test after shaping the bar to assure that no damage to the strands insulation. This is a Factory Test Only.
In large hydro coils that do not have the strands brazed together, is it possible to do a strand test in the field?
I have seen this done after coils have been stored for a long period of time. At least I thought I had. What was it that I was watching when the test tech hooked up and tested each strand against the other strands. Maybe I have the name of the test wrong. The coil supplier (a large one that I'm sure you all know) told me that it was a strand insulation test.
What you describe is indeed a strand test. But normally, it is a factory test, where you reject the bars that fail the test. At sites, it may be done on spare coils (that are to be used for a partial rewind) to ensure no strand shorts. However, it is moot, whether bars which passed factory strand test will develop shorts in storage at site.
