Google searches:

"Two phase to ground" : 768 hits
"Double phase to ground" : 282 hits
"biphase line-to-ground" : 0 hits

References:

"Voltage recovery after unbalanced and balanced voltage dips ... dips due to two-phase-to-ground and three-phase faults, the re- ... voltage recovery after two-phase-to-ground, three-phase, and ...
http://ieeexplore.ieee.org/iel5/61/27676/01234695.pdf


"The first dip is due to a two-phase-to-ground fault. The second dip is due to a single-phase-to-ground fault. ..."
http://grouper.ieee.org/groups/sag/IEEEP1564_00_12.ppt


"and the fault alternated between a single-phase-to-ground fault and a two-phase-to-ground. fault, apparently as a result of a failed shield wire bouncing on ..."
http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-...




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Note added at 6 hrs (2007-04-22 05:02:11 GMT)
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You could also hyphenate it, as "two-phase-to-ground-fault". Many of the reference text that I found have it hyphenated, although some do not.


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Note added at 6 hrs (2007-04-22 05:10:16 GMT)
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Sorry, I only just noticed that you asked for the complete sentence. I would say something like:

"Two-phase-to-ground fault in the 500 kV line, and subsequent tripping of the fault circuit".

You could use "disconnection" instead of "tripping". I would use "tripping" if the context makes it clear that the fault detection and disconnection occur automatically, by automatic circuit breakers, contactors, circuit interrupters, or something like that. Otherwise I'd use "disconnection", as that covers both manual and automatic. However, I would certainly hope that a 500 kV line has automatic devices on it!