Electrical distribution Neutrals

[Bstolle]

Elecrical distrabution question if I may. I am not an EE and only an avid farmer with a nessesity for electrical knowlage. Here is my question. I am using two unlike transformers with the same values.( 5oKVA single phase 120/240-600V). Starting at the utility pole where I have 200amp 120/240V and a neutral of course. I am intending to abandone the neutral at this point and step up the two hots to 600V. On the other side I intent to step back down to 120/240 and use the centre tap of the transformer to re-establish the a neutral. Asuming that this makes sense here are my question.
Should I ground the centre tap of the transformer? Or should I only ground the nuetral at the ditribution panel?
OR... should I carry the neutral through the entire systeme from the Utility company?
I am concered that if I do this wrong I may create stress on the utity transformer or some sort of induction voltages in the system. Please some clearity on this would be a great help.

 

[davenn]

hi Bstolle
welcome to PF

considering your self confession of a lack of knowledge with this sort of thing

I would suggest it would be very unwise for your to do this yourself. You could be endangering yourself and others
Please get a registered electrical company to do the work for you. At least it can certified as safeRegards
Dave

 

[Bstolle]

Thanks for the reply. I have somewhat undersold my knowlage. I have worked quite a lot in the field in the past. I have a good understanding of what I am doing and will not do anything I don't understand. Having said that I am no further ahead with my question. I have consulted two electricains that I know but niether had exsperiance with this kind of systeme and were no help.

 

[psparky]

I believe the nuetrals of your Y secondary transformers should be grounded regardless of the nuetral being active or not. They are grounded way back at the power plant and are grounded at the transformer close to your house.

I assume you are pumping up to 600 volts to avoid voltage drop over a long run.

Just my opinion...see if others agree.

Also, I would recommend (its the law) getting an electrical permit with your building inspector inspecting your rough in and finish. Which leads to another point...speak with your local building inspector first, see what he would do.

 

[Bstolle]

Thanks for your response,
Yes I am stepping up to avoid voltage drop. For clarification I will abandon the neutral at the utility source. I will run the a continuous ground from the Utility,( ground plate on main disconnect switch) through the step up-transformer and all the way to but not including the step-down transformer.
I am doing this to avoid any weird interaction between the two ground locations. I understand this to be the correct approach. This is according to my interpretation of the electrical code.
I will then re-establish ground at the step down transformer and also ground the neutral at this point. The distribution panel is 30feet away from the transformer. I will also ground the neutral in the panel.
Thoughts.

 

[psparky]

Thanks for your response,
Yes I am stepping up to avoid voltage drop. For clarification I will abandon the neutral at the utility source. I will run the a continuous ground from the Utility,( ground plate on main disconnect switch) through the step up-transformer and all the way to but not including the step-down transformer.
I am doing this to avoid any weird interaction between the two ground locations. I understand this to be the correct approach. This is according to my interpretation of the electrical code.
I will then re-establish ground at the step down transformer and also ground the neutral at this point. The distribution panel is 30feet away from the transformer. I will also ground the neutral in the panel.
Thoughts.

BTW...are you in USA or the Congo for example?
Electrical building codes do vary, especially from 3rd world countries to 1st world countries.

What you are saying sounds reasonable, but you can't just get comments/opinions from a physics message board then wire a relatively high power system.

You need facts.

Grounding is a tough subject. Generally, you can ask 10 different people and get 10 different answers.

 

[Windadct]

120/240 are single phase - best bet IN EVERY CASE of this is to make a diagram - can you do this, even lspice will work.

I have read three times - and I think what you want to do will work, and even be legal - but a diagram and a licensed electrician are your best friends.

so you want to Step up 240 to 600V then connect to a remote step down 600 to 240 - and re-establish the neutral there? For your first ? the location of the grounding point depends largely (totally) On local code.

Based on my understanding of your explanation - you do not need to carry the neutral.

Your concern about stress is largely addressed by a diagram.

 

[jim hardy]

"IEEE Green Book " is the basis for electrical code grounding practice.. and a lot easier to understand.

Your remotely located stepdown transformer becomes what is called a "Separately Derived System"
whose neutral must be earthed and bonded very close to the transformer, similar to the way electric company earthed their neutral at the pole.

Read up on "separately derived systems" . Draw yourself pictures.

Also, it seems to me your 600 volt stepup transformer is also a "separately derived system". You'll want it grounded too, with provisions for clearing a ground fault in the 600 volt wiring. I think you'll need two new grounds, one for your 600 and one for yoyr remote 240/120; and probably a green or bare ground conductor run along with your two 600V wires..

old jim

https://www.progress-energy.com/assets/www/docs/business/Grounding.pdf
http://ecmweb.com/nec/grounding-and-bonding-separately-derived-ac-systems
http://www.mikeholt.com/technical.p...tems&type=u&title=Separately Derived Systems [Transformers,%20Generators,%20etc.]%20%284-10-2K%29

 

[Bstolle]

Thanks Old Jim,

I appreciate the sharing of your knowledge. I will get a diagram together in the nest few days which should clarify thing a bit.
Cheers

 

[Bstolle]

Good day gentleman,

Please see the attached drawing of my system. From reading the links provided by Old Jim I feel that I am on the right track with this layout. any insights would greatly appreciated.

Cheers

 

[psparky]

In Ground A, don't you want to ground the nuetral of T1? I see you grounded the case there, Ok i would guess as long as the center tap of T1 is grounded to the earthed ground?

On ground A, Is the ground that actually goes into the Earth also attached to both panels? The lines cross but not sure there is a node there. Or perhaps the center tap is already grounded to the panel.

Please clarify.

 

[jim hardy]

Okay, great start...
Sparky's right, T1 600 volt secondary ought to be grounded somehow even if through a fault-current limiting scheme..

And of course there'll be fuses in both the 600 volt lines sized to protect the transformer windings and your 1000 ft cable, whichever is of smaller capacity.

Now - grounding the centertap of T1 might run afoul of a NEC phrase about creating parallel paths for fault current , depending on where physically is the tie between "ground A" and T1's enclosure.
You want fault current on your 600 volt side to not mix with fault current on the 240 volts side - the objective being to keep exposed metal at low potential wrt earth.

Equipment bonding jumpers

You must run an equipment bonding jumper to the secondary system disconnecting means, regardless of where the system bonding jumper is installed. Where the equipment bonding jumper is of the wire type, size it per Table 250.66, based on the area of the secondary conductor in the raceway or cable [250.102(C)].
Grounding electrode conductor (GEC)

You must install a GEC to connect the neutral terminal of an SDS to a grounding electrode of a type identified in 250.30(A)(7) [250.30(A)(3)]. You don't have to do this, however, if your SDS is for a Class 1, Class 2, or Class 3 circuit, and is rated 1 kVA or less [250.30(A)(3) Ex No. 1]. Size the GEC per 250.66, based on the area of the secondary conductor.

To prevent objectionable neutral current [250.6] from flowing onto metal parts, the GEC must originate at the same point on the SDS where the system bonding jumper is connected [250.30(A)(1)].

http://ecmweb.com/code-basics/grounding-and-bonding-separately-derived-systems

If i read the above correctly, you'd run a green wire from T1's enclosure down to Ground A and another one from same bolt on T1's enclosure to T1's 600 volt side neutral. Even though that neutral really doesn't go anyplace.

Perhaps a genuine construction oriented EE will confirm or correct ?

old jim

 

[tfr000]

Why don't you just ask the power company if it can drop 480V to wherever it is you want it? That's what comes into my workplace's building, and there are some large-ish transformers to convert it down.

 

[jim hardy]

By the way
your 1000 foot 600 volt run - is that in conduit, direct buried cable, or strung above ground on poles?

old jim

 

[sophiecentaur]

Guys. We normally would not be encouraging anyone to get involved with anything as potentially dangerous as this. I would say that, if someone needs to be asking these questions on a forum such as PF then they should not trust themselves to do the job themselves. It is one matter to discuss general practice in established and accredited supply organisations but this is the equivalent of telling someone how to build fireworks and atom bombs.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Thread will stay closed, and will be deleted in a couple of days.

Helpers -- please report this kind of thread early so we can avoid helping OPs to break the law and do dangerous things.

OP -- check your PMs.