Why Grounding Transformers Used w/Delta Sec. Transformer?

[arnab ghosh]

Just curious about why grounding transformers are used with a delta secondary . What would happen if I connect a phase to ground load to the secondary of a star delta transformer without adding a grounding transformer?

 

[Hesch]

why grounding transformers are used with a delta secondary

A Δ/Y-coupled transformer cannot transfer a neutral current (no neutral on the Δ-side). In a power-line no power is transferred due to the neutral current as 0V*1000A = 0W, so the neutral current only results in ohmic losses (P=I²*R.Therefore we want to eliminate the neutral current so that the phasecurrents: Ia + Ib + Ic = 0 (KCL). Of course these phase-currents also result in ohmic losses, but they result in a power-transfer as well.

So Δ/Y-transformers are inserted to minimize losses. The grounding on the Y-side is used to keep the voltage potentials within some "known space".

 

[arnab ghosh]

A Δ/Y-coupled transformer cannot transfer a neutral current (no neutral on the Δ-side). In a power-line no power is transferred due to the neutral current as 0V*1000A = 0W, so the neutral current only results in ohmic losses (P=I²*R.Therefore we want to eliminate the neutral current so that the phasecurrents: Ia + Ib + Ic = 0 (KCL). Of course these phase-currents also result in ohmic losses, but they result in a power-transfer as well.

So Δ/Y-transformers are inserted to minimize losses. The grounding on the Y-side is used to keep the voltage potentials within some "known space".

Yes that is okay . That is the use of inserting Y-D transformers . But I wanted to know why Grounding Transformers(see the figure) are used with the secondary of a Y-D transformer when we need to connect single phase-ground loads. Is it because it completes the circuit and avoids voltage surges?

 

[Windadct]

There can be a couple reasons - but the one that comes to mind is that a Grounded circuit may be required for safety and code purposes (created by the grounding transformer), but the Delta secondary is needed due to phase shift. -- Each Y-D or D-Y transformer creates a phase shift, in doubly ( or more) fed systems it may be necessary to match the phase shift - in another source.

 

[Windadct]

Oh -- In your diagram I do not feel this is for the connection of neutral loads. -If it is for connection of loads I do not like the naming of this a grounding transformer - we connect loads to neutrals, and not to ground in AC Systems. This if it is for a neutral load, I suspect for a relatively small percentage of the main transformers capacity. In an industrial setting the bulk of the load could be a large motor or system, yet the controller may need the neutral point connection.
There is also the issue of filtering the - transformer in this case would pass the 3rd ( and 3 multiple) harmonics. ( are there solid state loads ??)