Zigzag Transformer Sizing

Status
Not open for further replies.
E

ernric43

Member
Location
United States
I have several 13.2KV to 480V Delta Delta ungrounded transformers. My insurance company is recommending that I add zigzag transformers with a grounding resistor to eliminate this condition. How do I go about sizing the zigzag transformers? I currently have 750KVA, 1000KVA, and 2000KVA delta delta transformers.

Thanks,
Rich
 
What are the insurance guys concerned with; that may make a difference on how to make them happy.

If you just want to be able to sense ground fault, install something like a BENDER IRDH275.
[h=1][/h]
 
Zigzag

Zigzag

The insurance companies wants the transformers grounded in some fashion. They feel that the casualty risk is too high with ungrounded transformers.
 
ernric43 said:
I have several 13.2KV to 480V Delta Delta ungrounded transformers. My insurance company is recommending that I add zigzag transformers with a grounding resistor to eliminate this condition. How do I go about sizing the zigzag transformers? I currently have 750KVA, 1000KVA, and 2000KVA delta delta transformers.

Thanks,
Rich
Click to expand...
You can get an approximate idea of the size required by looking at the maximum current that will flow in the grounding resistor when a line to ground fault occurs. Will this be high impedance or low impedance grounding?

With high impedance grounding the transformer would be more like a control transformer in size than a power transformer.
 
ZIGZAG TRANSFORMER

ZIGZAG TRANSFORMER

What is a zigzag transformer . what are they used for . how are they wired ?
 
Ernric...

If you want a copy of my "Zig-Zag Xfmr Sizing Using Gi.Fi.E.S. (Pronounced JIFFY’s)", contact me off-forum!
If you find it worthwhile you can post it!

Regards, Phil Corso
 
insurance

insurance

kingpb said:
What are the insurance guys concerned with; that may make a difference on how to make them happy.

If you just want to be able to sense ground fault, install something like a BENDER IRDH275.
Click to expand...


They don't just want to sense the ground fault, they want the transformers grounded in some way.
 
A zig-zag transformer derives a neutral from 3 'hot' phases. This neutral can then be grounded.

A stand-alone set of wye coils doesn't do a good job of deriving the neutral, because the current flow through a single wye coil is limited by its inductance; it is like the primary of a transformer with no secondary load.

You _can_ use a wye-delta transformer to derive a neutral. The wye is connected to the supply and the wye neutral is your derived neutral. But if any current flows to the derived neutral it will induce current to circulate on the delta, avoiding the current limiting of the stand-alone wye.

As far as how to size a grounding transformer, the question is: how much current is expected to flow, and for what duration.

If you want to solidly ground your system the way an ordinary wye secondary is grounded, then you need to size the zig-zag transformer to carry the expected fault current for sufficient time to trip OCPD.

If you instead use a high resistance ground system, which gives continuity of service benefits combined with grounding benefits, then you need to size the zig-zag transformer to carry the expected _continuous_ fault current through the grounding resistance.

-Jon
 
ernric43 said:
They don't just want to sense the ground fault, they want the transformers grounded in some way.
Click to expand...

Perhaps they have acquired some engineering data that has led them to believe this is necessary and somehow makes a facility less dangerous. Would they be willing to share that knowledge?
 
kingpb said:
Perhaps they have acquired some engineering data that has led them to believe this is necessary and somehow makes a facility less dangerous. Would they be willing to share that knowledge?
Click to expand...

I just installed a grounding transformer because the AC drive manufacturer insisted that the supply be grounded.


The customer 'lost' the original transformer and the replacement one they bought was delta-delta (why pay for engineering, sales guys know how to pick equipment). The quickest transformer we were able to locate was a used wye-delta.
 
GoldDigger said:
If the individual winding ends were accessible on the delta-delta, you probably could have rewired it as a zig-zag.
Click to expand...

Snark alert.
Except for single phase 'pole top' units, when was the last time you saw accessible winding ends? Out of all the transformers you have dealt with in your career how many of them been reconnectable, maybe 1%?
 
Status
Not open for further replies.
Top