[BC] AC Voltage Drop

[RichardBJohnson at comcast.net]

Methinks that all utility companies know that open-delta, i.e., two transformers where there should be three, will work for motor loads only. Their "engineering" may have the crew install it so they can save the cost of the third leg, which they phantom at your site. Once they see that you don't accept it, they'll install the correct number of legs and pegs (pole transformers).

In the days of "12-pulse" 3-phase power supplies for high-power transmitters, we encountered many new sites in which the utility companies hung only two pole-pegs. When confronted, they always fixed their problem. The third phase will exist naturally from two phases, spaced correctly. if and only if equal current is drawn from all three phases simultaneously. Full-wave rectifier loads conduct in pairs around the unit circle, therefore the phantom third phase will not be properly developed. Mathematically, it will end up being sqrt(3) less in magnitude than necessary for balance, i.e., phase-to-phase voltage / 1.73. A 240-volt phantom leg will end up at about 139 volts. Twelve-pulse supplies demand that the phase-to-phase voltage be the same (or close).

Before I worked at Sparta, twelve-pulse power supplies were commonplace, but expensive. The HV transformers required secondary windings connected in what was called, the "zig-zag" configuration. This produced two 3-phase (120 degree) secondary outputs with one lagging the other by 60 degrees, i.e., inter-phased. The result of rectifying these two circuits in series produced 720 Hz as the lowest frequency ripple component. This made filtering relatively easy, plus resulted in a very low impedance supply. If this was connected to an open-delta circuit, the lowest frequency ripple component would be 120 Hz, not well-filtered by the existing HV supply components.

Since 12-pulse (or 12-phase) transformers were expensive, I decided to cheapen their construction because I figured out that the exact same inter-phase effect could be achieved by using a delta and a wye secondary, designed for the same phase-to-phase voltages. Our transformer vendor declined to make be a sample, claiming that I didn't know elementary electricity and such a simple scheme couldn't possibly work. Therefore, I contacted an old friend Perley Noone, at NWL (Northelfer Winding Labs http://www.nwl.com/contents/view/2 ), to make me a transformer. The industry was suddenly "transformed!" From that time on, and even forever more, 12-pulse rectifying transformers are configured with delta/wye secondaries. Cetec should have gotten a patent, but in those days we though it was just "neat" engineering and we liked to show it off!

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Phil Alexander" <dynotherm at earthlink.net>

The big problem with open delta is a sometimes high harmonic content.
(Depends on loads on primary side as well as secondary, and VAR
correction on line plus regulator setting etc.)