Hi Jim,There may be a fly in the ointment when trying to use a delta configuration: can the secondary winding-core insulation system support a delta connection? Your existing floating wye configuration places virtually no voltage stress between the floating common connection and the grounded core. If your transformer was designed to specifically run ONLY in wye mode, the designers may have skimped on the insulation between the winding and core. You may have the three-phase equivalent of three one-eared pigs. You'll need to closely inspect the core-winding insulation thickness and clearances to see if you can reliably run with a delta configuration on the secondary side.
If you reconfigure to delta and then ground the negative DC rail, the maximum winding-to-core voltage stress will be the full phase-phase peak voltage (~13 kV). Whether your transformer can withstand this stress depends on how your transformer is constructed.
If you instead let both DC outputs float, the delta-connected secondary windings will also float, and the voltage across any pair of phases will be approximately centered on around ground. The maximum winding-core voltage stress will be about half that for the grounded DC rail case above. However, toe better insure balance, you might need to add HV resistors and three small-value HV capacitors, with one end connected to the phase output and the other end to the core, to create a wye-like "soft ground". And, you'll definitely want to add two HV bypass capacitors (one from each DC rail to your RF ground) to bypass stray RF currents or accidental streamer hits away from your transformer.
Bert Jim Mora wrote: <snip>
*** I have a question in the diode configuration design stage. My Wye input neutral will see ground as will the transformer / diode tank case of course. Can the negative side of a delta side 6p rectifier also share this ground or do I need a two wire raw DC output? If so, grounded fault currents would more likely trigger primary breaker protection, true? Thanks, and its cool we are on the same kind of track Stephan. Smoke and misery enjoys company ;-^) Jim Mora
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