Discuss suspected faulty 6.6kva transformer in the Electrical Forum area at ElectriciansForums.net

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I think I have a faulty 66kva transformer but have little to no experience with them (delta star I believe)

Call out due to reported problems from some of the tenants (offices that are rented within part of the building)
some RCD protected ring final circuits disconnecting. Investigated at the local sub-main distribution board and discovered some strange goings-on... The voltage of all three phases with reference to each other and with reference to neutral are as you would expect. When L1, L2 & L3 were tested with reference to earth I have readings of 130, 290 & 380 volts (approximate values) the voltage from earth to neutral is 130 volts.

I toddle off to the factory side of the installation and when on site I am greeted by "electricians" scratching their heads and removing RCD socket outlets and replacing for standard.

I make my way over to the substation with a privately owned 6.6kva transformer. The 1st part of the installation is a bus bar chamber. I gingerly remove the lid and see that one of the 3 phases is shorting with the neutral... Booyah; I've found the problem. I isolate the supply, separate the phase conductor from the neutral conductor and re-energise. I test the installation, pat my self on the back and go home.

Today I get called out due to the same symptoms. I go to the original distribution board within the offices and discover that the same problem exists. Back over to the substation I go and the same readings are observed. I gingerly check the neutral conductor is mechanically sound and i'm satisfied that it is. The earthing arrangement inspires confidence to, 2 earth rods within enclosures, a good connection to the casing of the transformer and a sound connection to the isolation switches and busbar chamber

maybe one for Engineer54? ...if you're not to busy bashing partP and the NICEIC :)

Thanks Rob, I'm not sure of the size of the transformer

Edited for simplicity:

the secondary side of my delta star transformer appears to generating 3 phase perfectly and a good, mechanically sound, centre tapped neutral. the voltages I observe with reference to earth (across each phase and to neutral) would suggest that the earth is poor. But how can it be with the following arrangement; 2 x earth rods to side of transformer, earth connection on casing of transformer, earth bar running the length of the wall (about 12 inches off ground and showing signs of discolouring (not sure what this is for)) and 10 earth cables leaving the room to various extraneous conductive parts ie; gas, water, structural steel etc..

And why would the installation remain energised while L3 has been sat in contact with the neutral conductor?
 
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you may have a bit of a wait as the forum has been offline a lot over the last day or two. some of the guys may not log on for a day or so.
 
6.6kva is tiny fella. I'm sure that's a mistake.

Have you done any electrical testing yet? Sounds to me like your phases are still floating, probably a high resistance N->E joint.
 
The Beama RCD handbook covers a lot of information about RCD tripping, but I think RoB2 has it down pat really. From your readings there is a N-E potential, though it is surprising the L-N readings were OK with a line to neutral short.
 
Agreed, there is a red herring there. Either you made a mistake logging the voltage readings or that wasn't a real short, otherwise it would have gone bang or taken out a fuse. The problem does seem to be that the neutral isn't earthed at the star point. Or perhaps I should say, *a* problem, as there might be more to it than meets the eye. Find out how it is meant to be earthed...
 
No mistake with my collecting voltages.
It certainly was a short, but why it didn't go bank or poof has me very puzzled
neutral seems fine.. the problem is with the earth, but i don't understand why. please see my edit at the bottom of my original post. still need your thoughts!
 
It has to obey the laws of physics - you cannot have mains voltage across a dead short. So either the L-N voltage at that busbar was zero, or it was not a short. When you are measuring L-E and getting funny readings, presumably you are using the CPC instead of poking your probe in the mud. Thus the earth rods etc are not part of that test, what you are finding is that the neutral point is not connected to the MET. Have you checked this connection?
 
Can't offer much help but agree with the above, how can you have a correct L-N voltage when L is shorted direct to neutral??? Daz
 
6.6kva is tiny fella. I'm sure that's a mistake.

Have you done any electrical testing yet? Sounds to me like your phases are still floating, probably a high resistance N->E joint.
Yes, or maybe some damage sustained with the phase to neutral problem?
Certainly 130V neutral to earth if the neutral is supposed to be connected to earth is neither acceptable nor safe .
 
I'm not sure what to make of it yet, i have to consider floating neutrals? poor earth? but as I say; all visible connections appear electrically and mechanically sound

not sure how to upload pictures so here is a link

Dan
 
So I'm seeing a picture of a short that's blown itself open, and a picture of an earth bar. Now, let's see a picture of the star point connection...
 
Now, let's see a picture of the star point connection...

I need you to help me understand...
on the secondary side of the delta star transformer i have 3 phases and a neutral conductor that enter an isolator and then busbar chamber (shown in one of the 4 photos) I also have an earth conductor form the transformer (not shown in photo) to the earth bar that you can see. hopefully you can also make out 2 earth rods in the photo. you refer to the star point. I could only point out the "star point" on a diagram of said transformer.

Dan
 
In a TN system, the neutral is earthed at the source of energy, in this case the transformer secondary, If it were not, there would not be any points within the system where the voltage to earth was defined, nor would current be able to return via earth or the CPCs to the transformer winding in the event of a fault. Many forms of protection would not function. In your system, you are seeing a significant N-E voltage that is not due to drop along the N conductor or a poor connection in it as you are getting correct readings L-N, Therefore we must assume at this stage that the N-E connection is broken between the secy terminals and the earthing system and MET for the installation.
 
In a TN system, the neutral is earthed at the source of energy, in this case the transformer secondary, If it were not, there would not be any points within the system where the voltage to earth was defined, nor would current be able to return via earth or the CPCs to the transformer winding in the event of a fault. Many forms of protection would not function. In your system, you are seeing a significant N-E voltage that is not due to drop along the N conductor or a poor connection in it as you are getting correct readings L-N, Therefore we must assume at this stage that the N-E connection is broken between the secy terminals and the earthing system and MET for the installation.
I concur. That 130V neutral to earth is a pretty obvious clue that something is broken.
 
Am I correct in assuming that the casing of the transformer is connected to the star point and hence providing the main earth to the MET. If so the use of earth rods also connected to the MET is simply additional and the done thing?

I am going to isolate the supply this evening and separate the earth conductors from the MET. I can then test the installation with reference to the casing of the transformer and the two earth rods.

For reference... when I observed the readings FROM L1-E = 395 VOLTS, L2-E 310 VOLTS & L3-E = 180 VOLTS. This was done from the MET, The earth point within the busbar chamber, the casing of the transformer and from one of the earth rods (all still connected in situ) including the Neutral to earth tests (130 volts observed)
 
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No the star point isn’t connected to the tank. There will be a separate connection between neutral and the MET usually in the incoming switch terminal chamber. It is made removable so testing can be carried out.

I’m concerned about you working on this as you don’t seem to have the experience required.
Who is the site AP?
 

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