XFormer Grounding Question

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DPDT

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We are purchasing a 300kva 3 phase 2400 volt delta primary to
480/277y secondary.The only loads supplied will be 3 phase 480
volt motors and one 480/277 primary to 120/240 secondary 15kva
xfmr.We do not want to ground xo of the secondary 480/277y of
the 2400 volt xfmr as there is no need for a neutral conductor
in our circuits.We also have ground detection lights on our 480
system which needs an ungrounded system to work properly.Is it
legal not to not ground the wye secondary system.(250.20
B2).Will it cause problems to "float" xo?
 
If you only have line-line loads on the 480V system, and you meet the ''conditions of maintenance and supervision" requirements, and you have the ground fault detection hardware, then it is legal to leave the 480V system ungrounded.

There are no more problems leaving X0 floating than there are problems leaving the delta floating on an ungrounded delta system.

There _are_ known problems with ungrounded systems. Remember that it is really more correct to call such a system 'capacitively grounded'. Certain types of ground fault can 'pump' the secondary line to ground capactance up to well above the line-line voltage. You can end up with a nice steady 480V line-line, but get a couple thousand voltage line-ground. This excessive voltage can cause one fault to cascade into many faults.

I would strongly suggest you consider a 'high resistance grounded' system. This offers all of the benefits of an ungrounded system, with the additional feature of controlling the line-ground voltage. Because you have X0 available, adding resistance grounding would be almost trivial.

-Jon
 
Floating winding

Floating winding

I agree. There is nothing that requires grounding of XO if it is not used. However, without a ground reference, any surge or spike on the primary can travel through to the secondary (transformer winding capacitance) and cause damage. I have seen lightning hit an overhead POCO line and go right through the transformer to appear as a huge spike on the secondary that burns out the secondary equipment. The transformer is not damaged because it is robust enough.

Keep the ground detection working and have qualified staff to find the problem.

Some VFD will not work if fed from corner grounded system, which is what you have if one leg goes to ground on an ungrounded service.
 
RB1 said:
How is the voltage "pumped up" on the secondary side?
Click to expand...

The entire ungrounded system (transformer secondary, wiring, loads, etc.) has no 'galvanic' path to ground, but is surrounded by grounded, bonded metal. In effect the entire power system is a capacitor. A certain amount of current will flow through this capacitance, even though there is no galvanic path. By 'galvanic path', I mean a path by which current flows through actual movement of charges, eg. electrons.

If you have a nice solid 'bolted fault' at some point in this system, then 'capacitive charging current' will flow through the fault. On the non-faulted phases, you are getting capacitive current flow to ground, and this current flow is balanced by the galvanic current flow through the fault.

My understanding is that certain types of intermittent faults, or faults through inductive loads, can cause the system capacitance to get charged up with a voltage that exceeds the line-line voltage. I am not good enough with the details to explain this, and have to refer you to other sources:
http://www.mikeholt.com/safety.php?...tage - Dangers of Ungrounded Systems (2-21-2K)

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=2866

-Jon
 
Transformer grounding

Transformer grounding

I am going to swap out an old 45 KVA transformer for a new one. The only problem I saw with out taking the cover off was there didn't appear to be a grounding electrode conductor run back to the grounding electrode. If the primary feeding the transformer has an equipment ground and it is all bonded does it still need an GEC run back to the GE or can I use the equipment ground. I am trying to ask a qusetion, this is a very difficult sight to navigate.
 
250.30(a)(3)

250.30(a)(3)

2008 NEC 250.30(A)(3) is pretty clear. X0 needs to be grounded, not just bonded. X0 can be connected to ground bus by bonding and ground bus connected through grounding electrode conductor to grounding electrode.
 
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