ECN Forum Forums Technical Discussion Electrical Theory and Applications Grounded Delta Transformers

We ran into an issue recently regarding grounding of a delta, and had quite the heated argument regarding center-grounding of a delta connection. The switchboard was 3W delta, which is unusual for us. Motors and distribution transformers clearly don't care if the delta drifts, but the issue was that TVSS measures voltage from the lines to ground, as does the static switch of the delta-fed UPS on the switchboard, and both could malfunction if the delta became unbalanced and any of the phases drifted high or low with relation to ground. The generator was wired in WYE, so it was easy to ground the neutral tap to center the delta (no neutral connected, just the grounding jumper), but doing that causes some other issues...

Grounding one leg of a delta secondary eliminates the circulating current inside the transformer. This helps to reduce heat and losses internally. There can be a small amount of current that moves inside the transformer due to imbalanced load that will continually circulate thru the transformer if it is not grounded.

Wow, that's a great point, I'd have never thought of that! It's completely counterintuitive, but makes perfect sense now that I think about it- grounding a corner causes a wider rotation, which places the triplens into destructive interference. Nice tool to have in the toolbox the next time we're having harmonic problems at a transformer

...Although now that I'm thinking about this more, I'm getting confused again. Gonna have to chew on it a while longer.

To the original question, the advantage of the corner grounded delta as opposed to a totally ungrounded system is voltage stability to ground and better lightning resistance. In an ungrounded system transient voltages between any phase and ground are essentially indeterminate and limited only by the breakdown voltage of the insulation. Consider motors or transformers where the phases are enclosed in and wound on grounded metal equipment. More of a problem on older systems (and insulation materials) it can still present issues today. Corner grounding doesn't buy you anything operationally.

Did some electrical/safety training in a small food-processing plant a couple of decades ago. It had an original {LA Dept of Water&Power} 800A 240V 3ø 3w corner-grounded ∆ for motors and forklift-battery chargers, with odds’n’ends 120V dry-type transformers for lighting and convenience outlets.

Later, a 400A 120/240V 1ø 3w service was added. This gave the plant three separate sets of grounded-circuit conductors [all correctly white] to keep track of. At the end of training, I had the chief maintenance mechanic sort of clear on what was what, {none there were originally hired as ‘electricians’} with the agreement that he would call me if he needed further explanation.

Big John, on the west coast, [CA/OR/WA] there are thousands of utility-owned 4-wire open-delta-secondary [two-transformer] banks in service—primarily for smaller farms (house, barn and water pump), isolated pump motors, dairy barns and small, individual retail firms. The ANSI-standard voltage designation for both delta (three transformer) and open delta (two transformer) is 240/120-volt 4-wire, [3ø is implied in the designation] where 120/240 3-wire is the common residential 1ø designation.

The tradeoff between the two configurations for electric utilities is purely economic—open-delta banks are usually limited to loads of ±20 horsepower.

For open-delta services, the primary-winding configuration may be either open-delta or open wye. (A slang term for open-wye primary is “v-phase,” and must also include a primary-side neutral conductor connected to both transformers.) An easy way to differentiate 2-transformer, crossarm-mounted-bank primaries is by the number of fused cutouts—two means open-wye—three indicates open-delta.