open delta transformer voltage instability
open delta transformer voltage instability
(OP)
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 .
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
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
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
http://www.tpub.com/doeelecscience/electricalscience2184.htm
home.att.net/~benmiller/elecsys.htm
http://www.keenjunk.com/_junkyard/jp0005_5.htm
http://www.powerclinic.com/tex10.htm