Mining Transformers

Were they perhaps "taps" to adjust the output voltage for variations in the input voltage. (Presumably no tab changer, but just some extra windings that could be manually connected and used to boost or (hmm, unboost) the voltage, either at the high or low voltage side?

Nice idea and thanks for the suggestion, but there were no external connections to these windings. To change anything internally would be major as the tanks were Ex.

I'm beginning to wonder if they could be for fault limiting as they were for mine use.

This sounds like the transformers may have been designed for extremely high voltage work. In the Tesla community, that is a goal...cheap easy HV (usually on a much smaller scale, obviously). I got this from a HV/Tesla site:

There is a solution, however. Assume a transformer with 3 windings: the 110V primary, the HV secondary, and another 110V tertiary. Also assume that the insulation of the transformer is such that the two 110V windings can take the full secondary voltage. If this is the case, then you can feed a second transformer from the first, essentially using the first transformer as a isolation transformer. The primary of the second is fed from the tertiary of the first. You can, of course, extend this ad infinitum, feeding a third transformer from the tertiary winding of the second, etc.

http://home.earthlink.net/~jimlux/hv/xfrmr1.htm

If the tertiary winding is delta connected then it is there to carry the third harmonic of the exciting currents which in turn minimizes the Zero Sequence currents through the grounding system.

Now that makes sense. Thanks RAM J

In you op you stated they were not brought out. Wouldn't the delta ZSF have external taps to effect a neutral to the loads?

TonyS,

What is the vector group of the transformer!

If there is not a single delta (either on primary or secondary) that confirms that the unloaded / hidden delta tertiary is for circulating the third harmonics and thus prevent them from flowing in the input power cables.

The harmonics in the cables cause disturbance in the telecom circuits, additional heating, additional voltage drop etc.

If you read my original post you will see this is a recollection from the 70's. I found references to it in old logbooks. It's stuck in my mind since then, mainly because I got involved in a project that had to be proved wouldn't work!

The transformers are long time scrapped so I don't know the vector grouping, there're not in my log.

Harmonics in telecoms equipment is hardly going to bother mining equipment. Whereas it would with sensitive fault detection. They hadn't got the fancy relays in the 30's we have today.

Dear Mr. TonyS,

Generally, it is practice to provide a Delta- Star transformer to suppress the flow of zero sequence component of secondary side earth fault current on the upstream side,thereby limiting the operation of sensitive earth fault protection only at low voltage side of the transformer.<p>

For some economy of design the transformer would have been designed with Star -star transformation . then it becomes a must to provide tertiary delta to circulate the zero sequence current of the earth fault in the secondary side earth fault.<p>

As this was designed for dedicated purpose and to discourage anybody to use this as source for connecting any load, this circuit would have been terminated in side .