Corner Delta Grounded 2

[nightfox1925]

In the case of a Delta (primary) - corner delta grounded (secondary), is one of the phase physically connected to its core or body? If we purchase this type of transformer, is one phase actually pre-connected to its body? I appreciate if someone also provide me a link for this?

Thanks

 

[dpc]

The delta will not be grounded to transformer tank, in all probability. You would need to ground the delta external to the transformer.

I would just buy a standard delta-delta transformer.

Actually, what I'd probably do is try to find a different solution. Corner-grounded delta systems are rare and not something most electrician or linemen are used to dealing with. Best to avoid them if at all possible.

 

[nightfox1925]

Thanks DPC. If the primary is Delta as well, are we going to get voltage-to-ground reading (on R-S, S-T, R-T) of Vprimary/1.732 even if the transformer is not grounded? If we do get this reading, what does it imply?

 

[davidbeach]

On an ungrounded delta voltage from phase to ground can be most anything, but is almost always less than phase-phase voltage. With reasonable balance in the load and circuits connected you should get close to phase-phase voltage divided by the square root of 3 measuring between any phase and ground, but the balance can be so delicate that the voltmeter pulls the voltage down and they all read less than the expected value.

I'm not sure what you mean about "voltage-to-ground reading (on R-S, S-T, R-T)". Either you would be measuring R-G, S-G, T-G or you would be measuring R-S, S-T, T-R, but there is no R-S to G voltage that would have any meaning.

 

[nightfox1925]

Oops sorry for the error DPC, yes it was supposed to be R-G, S-G and T-G. According to field personnel, the transformer was opened and both primary and secondary have 9 terminals; for 480V, 460V and 440V. Could the other 9 terminals at the secondary signifies that there is also tap selection on the secondary?

 

[davidbeach]

What does the transformer name plate indicate?

 

[busbar]

One other disadvantage of a fixed-winding-to-ground transformer winding is the inability to check the winding’s insulation resistance.

 

[sberbece]

As dpc said in his answer above, corner grounded delta systems are very rarely, and will only give you problems in extending the distribution system and protecting it.

Your best solution will be to buy a delta-delta transformer, which have the main advantage of insulating the zero sequence networks = less 1 phase short circuit current. Unfortunately Dd transformers are more expensive than Yd transformers. This is because all HV insulation is rated for the HV level, in comparison with Yd transformer, where its HV insulation is gradually reduced through its neutral (earthing point).

In addition, you can monitor the insulation of the phases to earth in the distribution system, by installing an earthing transformer, usually called zig-zag transformer.