MAJOR problem with supply...

[dlt27]

Sorry title was a bit dramatic, but wanted to get as much knowledge as possible.
I was working at a biomass factory today after they had a fire. Extension leads were apparently the cause.
When doing a test at the db I noticed that L1 phase was at 272v to earth. L2 phase was 222v to earth and L3 phase was 240v to earth. Any phase to neutral was 240v. I then carried out a high loop test on all phases and got between 76 and 80 Ohms.(TNC's/PME).
Due to these readings I went to 11kv transformer that is in the building and done the same test a lv side of panel and was still getting same results. I then took a feed out to earth rods/ earth mat of transformer, but was still getting the same figures. I have got past test results that said ze was 0.2 Ohms a couple of weeks ago.
I also noticed when any circuit is turned off you get 30v from earth to neutral and earth to live. Also rcd's are failing to trip on push buttons.
Just wondered if anyone had an explanation for these findings and if so could you please explain in simpletons terms.
Much appreciated in advance
(I'm guessing a problem with neutral at star point of transformer).

 

[Guest55]

and i bet that avatar pic really is you as well , your holyness !

 

[Guest111]

Put simply if you lose the connection to the star point then you lose the neutral,so on a 3 phase system imagine you had a socket fed off red phase and neutral and another from yellow phase to neutral if you lost the main neutral then the neutral bar in the dis board effectively links the sockets on red and yellow phases,so instead of each socket getting 230v they end up getting 415v and bang goes anything thats connected to them.which is why when working on a 3 phase system you should NEVER remove a neutral link with the supply energized.

 

[dlt27]

I understand that but any phase to neutral is 240v So neutral isn't broken, but possibly failing.

 

[Knobhead]

If the OP wants to PM me I’ll go through the testing required. I’m not going in to it in open forum or until I’m satisfied he will be safe.

 

[GEOFF S]

Thos sound like a fault on one of the phases to earth, the fault being a high impedance earth fault. This in effect drops a voltage accross Rb of the Tx. Test to true earth with a large screwdriver.

 

[Engineer54]

I'll not confuse the issue and comment on the probable cause of the differing Ph to E voltages you are experiencing, i'll leave that to Tony. But i can guarantee you that this factory doesn't have a TNC-S/PME earthing system!! What you have seen, probably at the LV main switchboard, is the TX's N-E link. More often than not these days this is where the N-E point is made rather than in the TX's LV take off box.

 

[Knobhead]

The OP has a list of tests to carry out and will phone me when he has the results.

When we get to the bottom of it then I’ll leave it to him to pass on the outcome.

 

[Silly Sausage]

and i bet that avatar pic really is you as well , your holyness !

You're getting the idea!

Seeing as this a forum for discussing electrical stuff, I just thought I'd chip in my 2p worth.
However, you wouldn't get me within a 100 yards of that throbbing transformer and fiddling with N-E links.
I'll leave that to you big hairy arsed electricians.

 

[dlt27]

Tony's info was invaluable. Haven't got to the bottom of it yet, but HV guys are saying they think the same as you guys. A failed or failing neutral. However they haven't carried out any tests out yet. Aalso apparently there was a disturbance on the network (possibly a transient voltage) last week.
The HV guy (transformer installer)said he thinks the transformer might of been damaged and when I asked what protection the transformer had against overvoltage etc he said there was protection, but it was never set up correctly because the client wouldn't have a protection survey done. (Don't know if you guys could educate me what protection an 11kv transformer would have and what a protection survey is).
The outcome at the moment is that he said the client could have an expensive bill coming and also that he can't do nothing until western power isolate the supply to the transformer.
One final thing just wondered is it standard practice for the supply cables to the transformer to run on tray across the mains room with nothing enclosed around them. Didn't like the thought of 11kv just sitting there waiting to get damaged.

 

[Engineer54]

All secondary protection as well as some primary protection to MV installations is by use of protection relays using current and potential transformers. The overvoltage protection relay number is 59, but can be a dual under (27)/over (59) voltage relay..

Very roughly a protection relay survey is basically giving setting values to all protection relays, ACB's, MCCB's MCB's, etc A time/current graph is usually produced showing/proving protection setting curves discriminate between each device from MV switchboard side through to the LV switchboard side and often if the survey is a comprehensive one, all the way back to final circuit DBs....

You don't however need a protection relay survey to set an appropriate over, or under voltage relay setting which will gave both a tripping voltage and a time element.... If this TX is owned by the DNO, it would make sense for them to set any protection relays that have been afforded. I'm damn sure that when this TX first went into service, a full set of testing would have been conducted as well as an Engineer or commissioning technician setting protection levels to ACB's/MCCB's (adjustable) on both the MV side and LV side of things...

 

[Knobhead]

A photo of this setup would help.

As for the 11KV, nothing out of the ordinary for it to be run alongside other cables. It doesn’t spontaneously combust if you sneeze near it.

Usual protection will be O/C & E/F on the MV side, O/C & REF on the LV switchgear to protect the transformer secondary. Unless the supply is O/H no over voltage is usually provided, if it is it’s the suppliers responsibility at the intake point.

 

[Engineer54]

I've rarely seen an overvoltage protection relay being provided, and when they have been included, it's normally because of a dual under/over voltage relay being provided. Under voltage relays on the other hand ''Are'' commonly used, especially in an automated main switchboard system...

 

[dlt27]

Just to let everyone know. Got to the bottom of it today. Transformer guys came in today and discovered the neutral to earth link was missing in the transformer. Apparently it has been like it since it was installed in May. What made me chuckle was that the guy said it happens quite often. Thanks for all the advice.

 

[Engineer54]

Just to let everyone know. Got to the bottom of it today. Transformer guys came in today and discovered the neutral to earth link was missing in the transformer. Apparently it has been like it since it was installed in May. What made me chuckle was that the guy said it happens quite often. Thanks for all the advice.

I was guessing that there would have been a loose connection on the N-E link!! Now i would have thought a floating neutral would have shown differing PH-N voltages as the loads on each phase increased/decreased...

It's more usual these days that the TX LV N-E link being made at the main switchboard, it makes any earth fault protection relay monitoring easier....