Ungrounded System to Grounded

[PQ]

Hi Guys,

Our receiving power is 69kV from our Utilty w/c passes thru our 10MVA 69kV/13.8kV Delta-wye Power Transformer. Our distribution voltage is 13.8kV. and Back-up Power is a 5 Units 2000kVA, 480V, 3P, Diesel Generating Sets which connected to our 13.8kV sytem thru 5 units Step-up Transformer 2000kVA 480V/13.8kV Wye Delta, connected individually to each generator. My Question is, How can we make our system a grounded system during Back-up power since our step-up transformer is delta on the High Side? . Some of our 13.8kV distribution is using 3- 7960/230V 1P Transformer. Is it possible to install a zigzag transformer at the step-up transformer PCC (point of common Couplin)

 

[DanDel]

The short answer is, yes it is possible. The full answer requires significantly more time and money in design.

 

[busbar]

Usual ANSI practices are covered in ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in …Distribution

With phase-to-neutral loads on the 13.8kV bus, you are consigned to a solidly-grounded system.

[TOC (only) of C62.92.4 at

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.4-1991_desc.html

]

 

[gordonl]

I haven't seen that before, but your going to need a pretty beefy zig zag because it needs to supply neutral and ground fault current.

 

[peebee]

No answers here, but lots of questions:

Is your utility transformer grounded? I assume so, otherwise you probably would not be asking this, but it can't hurt to check.

This is a 3-wire system only? No neutral? Please verify.

It sounds like you never parallel the generators and utility, please verify.

Are the utility and generators both considered separately derived systems?

What are you using for transfer switching and/or generator disconnects? 3-pole or 4-pole?

Is solid grounding a requirement, or is impedance grounding OK?

How about B-phase grounding? I've never heard of this on MV systems, but I can't find anything in NEC that would prohibit it -- I could be wrong. May not be a good idea either -- I'd be interested in any comments on that. . . .

What's the motivation for grounding? Can you just leave it ungrounded?

 

[alehman]

It seems like you will need a grounding transformer for each generator or some method to make sure a single grounding transformer is on-line whenever operating on any of the generators. You probably don't want it on-line when connected to the grounded 13.8 transformer.

Maybe it would be easier to get rid of the L-N loads. I wouldn't recommend operating the system ungrounded anyway though.

 

[PQ]

peebee,

Yes! the 10MVA transformer is grounded at secondary, It is a 13.8kV system 4wire wye, but during the operation of our genset which is a 480V 3P wye generators we connect to the system passing through a Step-up transformer 480v/13800v wye-delta. Which means that during emergency power our system is ungrounded

 

[digitrex]

Get a new 13.8kV feeder panel and install a zig-zag transformer which will provide a neutral earthing at 13.8kV level.

 

[alehman]

The simplest solution may be to replace the generator transformers.