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open delta transformer voltage instability

open delta transformer voltage instability

open delta transformer voltage instability

Power pole has two hot lines(?2400v?) and neutral coming to it. These go to two transformers' primaries. The two Secondaries have one hot and one neutral wire coming out of each of them. The neutrals are tied together. The 3-phase voltages on the secondary are measured:
A-B, hot to neutral, 480v;
B-C, neutral to hot, 480v;
A-C, hot-to-hot, 480v!!!
This is an open delta. How can there be 480 across the two hots? What are the phase relationships? We are getting real wild voltage swings and spikes, even coming into a downstream, 3 phase transformer, with a  480 delta primary, and a 208/120 wye secondary. The control circuit comes off of one of the 120 line-to-neutral legs, and jumps around so much that relays and contactors are bouncing in and out by themselves. I'm ready to cal Scully and Mulder .

RE: open delta transformer voltage instability

From your description, it sounds as though the primary 2400V (?) distribution system is itself a open delta, with two phases (say A & C) operating at 1385V to ground and the third phase (say B) operating grounded.  When you connect your two transformers between hot phases and neutral (ground), you end up with an open delta secondary system, as you note, with 480V between any two leads.  The phase voltages should be an equilateral triangle, with 120 degrees between each set of phases (A-B, B-C, C-A).

As regards your voltage swings, you didn't say anything about grounding of the 480V system. With an open delta system, one leg (the common) needs to be grounded to ensure that the secondary doesn't float.  Also, is the secondary neutral of the 208/120V system properly grounded?

RE: open delta transformer voltage instability

Of course, I really meant to say "...with two phases (say A & C) operating at 2400V to ground, ..."

RE: open delta transformer voltage instability

Papasully, what you have is a combination of two not-too-mainstream variations of 3ø three-wire delta service.  The transformer configuration described is grounded open-wye primary / corner-grounded, open-delta secondary and is recognized in a number of electrical publications.  The secondary-side arrangement is sometimes referred to as “grounded-B” service.

Would you please clarify and detail what the meaning of, “…real wild voltage swings and spikes…” ?  For a corner-grounded connection, the nominal 480V line-to-line and line-to-ground voltages should be fairly stable [in the range of 504-456V, but with a phase-to-phase imbalance of not more than ~10V or 2%] and that sounds like what you are describing.  Specifically, how and when are you observing the described problems?  

Are you measuring varying voltage anywhere on the 480 circuits when control relays are chattering?  Sounds like a intermittently high-resistance (poor-quality) connection, that, depending on measurements to determine the exact location [i.e., high or low side of the 480Δ-208Y xfmr], could cause control-relay chatter.  

{A comment about terminology—it can be easy to mistake a circuit ‘neutral’ connection with an “X2” or “X3” terminal on the individual transformers.  At any rate, the grounded conductor of one phase (“corner”) is not a always a neutral; it is a grounded-circuit conductor, with NEC requiring the white color.  Specifically the only time a 3-phase circuit conductor is both would be where secondary windings are in a symmetrical [wye] arrangement.}

References: http://saskpower.apogee.net/foe/ftdttd.asp

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