Delta/delta transformer

[Samb0]

Hi guys, I'm new to the forums and hoping to get some advice on a power transformer.

So basically this is a portable machine powered by a 1000v supply having core balance earth leakage on the protection relay and backup earth leakage on the NER. My issue is that part of the machine is powered by a 1000v/415v delta/delta 20kva transformer. The 415 side of circuit has some small 415v dol motors and some control circuitry. My issue is that since the secondary 415v side has no earth reference the equipment supplier is saying to jump the 1000v earth accross and use that for a secondary earth. When testing voltages from the 415 side to primary earth we are getting l1 218v l2 236v l3 264v, these readings are with losd and no load.

So, is the primary earth a suitable reference to detect ground faults on the secondary side.

Thanks in advance,

Sam

 

[davidbeach]

May in part depend on the local codes. You haven't said where in the world you are.

 

[Samb0]

Oh I'm from Australia.

 

[waross]

My issue is that since the secondary 415v side has no earth reference the equipment supplier is saying to jump the 1000v earth accross and use that for a secondary earth.

Jump it across to where?
You my be able to use the same ground point or grid, but it must be connected to the delta secondary in some way.
The delta may be grounded:
With a corner ground.
With a phase center tap (if the transformer supplies four wire delta).
With a wye/delta bank or transformer.
With a zig-zag transformer.
The equipment supplier may not be your best source of technical advice

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

Howdy Sam,
Since you have a delta-delta transformer, there is no means to provide a "ground reference" on the secondary side. When the equipment supplier says "... to jump the 1000v earth across and use that for a secondary earth" he knows not what he is talking about. No worries though mate, this is likely legal, even in Australia.

The primary GF protection will not provide any protection for a ground on the secondary side. This applies to any (delta-delta or delta-wye) double-winding transformer.

Your line-to-ground voltages are not exactly symmetrical, and might be a cause for concern. However this might not be a major issue, as long as the line-to-line voltages are symmetrical (ie better than [say] 2%).

Regards,
GG


"Wish I didn't know now what I didn't know then." -- Bob Seger

 

[Samb0]

Thanks for the reply guys. There is no grounding transformer or tap on the delta secondary and I have the same opinion as you groovyguy.

Thanks again guys