Different L-G voltages on a 240v L-L PSU fed from a 3 Phase High Leg Delta secondary.

[MrMoe]

Every individual installation has it's own set of circumstances that may or may not make this system favorable, and you need to factor into those circumstances whether it is a full or open delta and if conditions on primary side are factor in this, particularly the open delta option. If primary situation is somewhat limiting to only using open delta you can increase transformer sizes if the load would dictate so.

Basically there is no simple answer. If all three primary phase lines are available and the majority of loading is 120 volt loads then that is about the only time a wye secondary is pretty much a must.

The only reason for the open delta is to provide three phase power while the replacement transformer comes in to complete a closed delta config. 120V loads make up less than 3% of a single transformer's kVA rating (the bank consists of three equal, well two at the moment, units). As such, your answer is a sigh of relief.

Center-tapped deltas have their purpose and it seems to fit the bill here. I was just somewhat confused as there is so much information out there calling for the avoidance of it, so much so that the POCO's manual goes into great detail as to how to set up a delta-delta (center-tapped) substation yet goes on to finish off the several pages long section with, and I kid you not, (word for word) "This type of connection doesn't serve much purpose, but it does have its application".

Mind you, this is an extremely technical document covering all sorts of things, from pole fixture installations to substation wiring, grounding patterns and mesh requirements, everything down to the last millimeter. I can't find any other instance of a similar comment or description. Yes, there are recommendations as to using this vs that, etc but it just so happens that the delta-delta connection part, despite being clearly included in the document and very well detailed, finished off with something of a "not sure why we're even covering this". This, in great part, is what sent me on this quest to figure out why there is no love for the double D. Imagine my reaction when I see that statement, which, at least to me, despite probably being "technically" correct, sounds so out of place in a manual that is entirely focused on detailed diagrams, schematics and instructions; almost entirely void of comments or observations.

 

[MrMoe]

Did you ask the manufacturer?

You wrote

I don't have the schematics/diagram for the PSU but it's a generic 1800w server PSU; 1800w 200-240V AC to 12V DC (two V+ & 2 V- terminals). Internally the ground from the C14 plug is directly terminated to one the the PCB's mounting screws.

That seems to indicate everything is okay to do it your way.

Indeed. There are no equipment warranties involved. As far as I can tell the manufacturer simply states that the PSU works from 180-300V AC 60Hz and that wiring is up to the user. No mentions of recommended patterns. My initial question was simply based on the fact that I get the AC secondary side of things but I'm definitely a rookie as it pertains to AC-DC PSUs. I just want to understand if feeding the PSU with three wires, two hots and a ground, with line-ground voltage differences between the two hots, would have an impact, if any, on the equipment. I know that some electrical equipment use the ground as a reference point and was wondering if having two differing values would make a difference or if the equipment would reference one of the two, or if it has zero impact, etc. Btw, thanks for your posts!

 

[kwired]

240 volt delta (whether high leg or corner grounded) has a little advantage over 208 if all your loads are motor loads or even if you have significant 240 volt heating loads. Motors, you get a little more "bang for your buck" so to speak as you get a little more VA out similar sized conductors and gear than if operating same thing @ 208 volts. Heating loads you get more VA out of same element than you do if connected to 208.

 

[MrMoe]

240 volt delta (whether high leg or corner grounded) has a little advantage over 208 if all your loads are motor loads or even if you have significant 240 volt heating loads. Motors, you get a little more "bang for your buck" so to speak as you get a little more VA out similar sized conductors and gear than if operating same thing @ 208 volts. Heating loads you get more VA out of same element than you do if connected to 208.

Everything points to this being, as you stated, a let's get more "bang for your buck" scenario. The need for straight-rated breakers for high-leg single-phase loads seems to work against this configuration but in the grand scheme of things, the 15%-ish difference in voltage, being that most of the load is 3 phase, likely more than makes up for it in breaker amp ratings/amounts, cable gauges, etc.

 

[kwired]

Everything points to this being, as you stated, a let's get more "bang for your buck" scenario. The need for straight-rated breakers for high-leg single-phase loads seems to work against this configuration but in the grand scheme of things, the 15%-ish difference in voltage, being that most of the load is 3 phase, likely more than makes up for it in breaker amp ratings/amounts, cable gauges, etc.

Yes a disadvantage is if you need 2 pole straight 240 volt breakers. They are not commonly stocked and are much higher priced than 120/240 breakers. 3 pole breakers are typically straight 240 rated and you don't have to worry about the slash rating with those.

Feeder taps to fused disconnects where feasible wouldn't need to worry about any slash rating either. Plug fuses may be an issue, but cartridge fuses would be rated 240 volts.

 

[MrMoe]

Yes a disadvantage is if you need 2 pole straight 240 volt breakers. They are not commonly stocked and are much higher priced than 120/240 breakers. 3 pole breakers are typically straight 240 rated and you don't have to worry about the slash rating with those.

Feeder taps to fused disconnects where feasible wouldn't need to worry about any slash rating either. Plug fuses may be an issue, but cartridge fuses would be rated 240 volts.

The last project I worked on was really into the "bang for your buck" ordeal and, given that it was mostly 277V single phase loads from a 480/277 3 phase service, fused disconnects were used all around.