Out of my depth

[nunthe wiser]

I was asked to install a small consumer unit at the end of step down transformer. The consumer unit is fed from a panel board put in place for a future wind turbine project . The consumer unit feeds a socket and a light. This is all in a metal container that is earth roded with bare tape and rods. Now I checked my voltages at my income r of panel board and everything was okay apart from one phase was 20 volts higher than the others with respect of earth and neutral. I connected up my board to test my loop impedances and when a load goes on the board the neutral reaches 3 phase voltage when measured to other phases and 240 to earth. I figured the neutral must be gone but when measuring with no load on system everything seems fine . From the transformer plates they are delta delta and the neutral is through crimped. I'm told the reason for the step up - down transformer is because upsizing the cable would cost the same but would impossible to work with. I have no knowledge of design of this level of work, should the neutral be larger and,as it is it's to high a resistance for a return. It's got me stumped and it was supposed to be a simple job. Long winded I know.

Thanks for all help given

 

[Sintra]

I would check the continuity of the neutral between your consumer unit and the source.

 

[nunthe wiser]

It's hundreds of metres of 4 core 70 mm I insulation tested it and no faults I can't find a value m ohm per metre for 70 mm but I will interconnect my neutral with each phase and make sure they are the same. I only have a seaward multi tester is this suitable for such a large cable run, Ill have to get an end to end resistance when I return. Supposedly cable was not damaged when laid .

Thanks for your reply

 

[Pat H]

Your multimeter most likely won't be accurate at the fractions of ohms you need to measure.

If the turbine is a future project what's the current supply? 3 phase down this hundreds of meters of cable?
Not sure I see what's feeding the step down transformer and what you are getting out of it?

 

[nunthe wiser]

There's a 415 to 1000 v transformer at fed from panel board at supply authority meter, then the 4 core 70 upto the 1000v 415 step down transformer which feeds a panel board in the shed. I took a small rcd consumer unit fed from a 32 a Mccb of this panel board. And when I went to live test the neutral potential rose so I isolated, my supply is to give power to engineers so they can have power when doing turbine install which is going to be fed from panel board in shed. There has to be a problem with the cable if there's no neutral return. I'll get an end to end resistance of the neutral. I don't believe it's a fault with what I've done but, I can't just walk away and leave it.

L1 n 240
L2 n 262
L3 n 237 same to earth

When energising my board which is on l2 l2 n 0
L2 to earth 260
L2 to l1 440
L2 to l3 440
L3 to l2 415
N to earth 260
All voltages were measured with my multimeter, I did use my multi tester loop, insulation continuity etc.

 

[Lucien Nunes]

From the transformer plates they are delta delta and the neutral is through crimped

Well that could be the problem! A neutral is only a neutral because it's connected to the star point of the source. If the transformer is double-wound (i.e. has separate primary and secondary windings) then the source for your panelboard is the transformer secondary, not the supply down the armoured cable. You can't just borrow the neutral from there, i.e. the primary side because there's no connection between the two circuits. The neutral core might be connected to the rest of the planet, but as far as your delta secondary is concerned, it's floating.

Your voltages while unloaded merely show that the tiny meter current can flow through the stray capacitance of the transformer, which by virtue of being fairly symmetrical floats around where you would expect the neutral volts to be.

We need full specs of the transformer to understand what is going on here. E.g., was there ever supposed to be a neutral at the panelboard? Was the intention to power any 230V loads from a 400-230 stepdown?

 

[westward10]

Totally agree you cannot derive the neutral from the delta secondary.

 

[nunthe wiser]

It will be Monday before I can get some pics of the plates. The neutral was provided at the panel board so I connected to it. I believe the design is not fit for what is required. Just to clarify the transformers are at metering 415 3 phase in 1000 v 3 phase out then cable run, step down transformer 1000v in 415 3 phase out. Maybe the system was never designed to supply single phase neutral. If it was then someone is at fault at the design stage. I'll get pics uploaded Monday night. I don't deal with three phase transformers at all anything I learned in college is forgotten. But I'm interested in the why.

I'll tell the client to consult designers and engineers to see what was in spec. Maybe the system won't be suitable for turbine, any controls would probably need 3ph +n.

 

[nunthe wiser]

Thanks for all you're help.

 

[Pat H]

Not that I have much to do with 3 phase but isn't one method to have a center tap on one winding as Neutral?
Or do you need a further transformer to drop 440V to 230V?

 

[westward10]

Not that I have much to do with 3 phase but isn't one method to have a center tap on one winding as Neutral?
Or do you need a further transformer to drop 440V to 230V?

Yes the neutral is taken from the star point.

 

[westward10]

I am guessing because of the long run it is stepped up to 1000v to enable better transmission then it is stepped back down. Where it is stepped back down you need a delta/star transformer.

 

[David Prosser]

Not that I have much to do with 3 phase but isn't one method to have a center tap on one winding as Neutral?
Or do you need a further transformer to drop 440V to 230V?

I think that would give a two phase (and neutral) supply.

 

[nunthe wiser]

Its just a small board for a socket and light and it just showed up my ignorance, time to do some research on the subject.

 

[Des 56]

Not that I have much to do with 3 phase but isn't one method to have a center tap on one winding as Neutral?
Or do you need a further transformer to drop 440V to 230V?

A four-wire delta (4WD) electrical service is a three-phase delta service with a center-tap on one of the transformer windings to create a neutral for single-phase loads. Motors loads are commonly connected to phases A, B, and C, while single-phase loads are connected to either phase A or C and to neutral. Phase B, the “high” leg, is not used for single phase loads.

Interesting solution
However,that pesky neutral would still not become available