11kv pole loop fault, over voltage ?

[PoultryMike]

Hi first post, I'm a pretty well experienced electrician of 15 years now working solely in Agricultural electrical
We have a lot of voltage sensitive equipment on our farms which run auto ventilation and alarms ect

Today we had a huge bang from the 11kv pole transformer on one of our sites, the generator kicked in but out of 18 alarm dialers I think all 18 plus some pcbs on our ventilation panels have been fried.

When sp energy had finished it looks to me as the fault was worn insulators on the 11kv loop on top of the pole and not a transformer problem, this short on the pole happened around 1pm yet all the affected alarm pcbs faulted whilst on generator power 415v at around 2.15pm
My question is... could the loss of the 11kv loop bring in a flash of over voltage or is it possible the generator over generated and caused over voltage? On some pcbs 50ma glass fuses have survived yet the panels are dead

Thanks very much
Mike

 

[James]

If the installation was isolated from the grid at 1pm
and running on a generator set after that, how could a grid fault cause damage, 2 hours after the site was disconnected from the grid?

 

[PoultryMike]

If the installation was isolated from the grid at 1pm
and running on a generator set after that, how could a grid fault cause damage, 2 hours after the site was disconnected from the grid?

You know what James I don't bother with forums because of answers like thst, totally unhelpful, read it ..... my question was when the incident happened could that loss at the loop cause the problem

 

[Lucien Nunes]

James has a point. It does seem unlikely that damage inflicted from the DNO supply before the transfer switch transferred, didn't show up on any of the units until later, when they all went down.

It would be interesting to know exactly what has failed on the PCBs. For example, burst electrolytics, VDRs cracked open, PCB tracks blown open, switching transistors shorted, are all potential clues (pun intended) to the level and duration of the overvoltage. It's usually possible to distinguish between say a loss of neutral or AVR failure that puts 300-400V onto 230V 1P loads, and damage from some short-sharp transient event like a flashover from the HV.

I don't bother with forums because of answers like thst, totally unhelpful,

Luckily, we help grumpy people as well as happy ones Say hello, hang out for a bit, see what people come up with. There are some good brains on this forum.

 

[James]

You know what James I don't bother with forums because of answers like thst, totally unhelpful, read it ..... my question was when the incident happened could that loss at the loop cause the problem

It was not a comment to make you look stupid or be unhelpful, it was a genuine question intended to provoke thought from yourself and others as to how could a fault on the grid cause a failure in devices 1 hour after being disconnected from the grid.

some devices may have an issue with a large voltage between earth (grid) and earth (mass of ground) that is the only thing I can think of that is still connected assuming correct disconnection from grid whilst running on site generator.

 

[James]

You know what James I don't bother with forums because of answers like thst, totally unhelpful, read it ..... my question was when the incident happened could that loss at the loop cause the problem

Would it be more helpful if I said
maybe but it would require a lot of unusual and unexpected failures to happen simultaneously?

 

[timhoward]

Looking from a completely different angle…. How were the alarm dialers communicating with the outside world, pots lines, internet, 4G ??
Was a local cell tower affected too?
Just wondering if they weren’t able to report until later or they had already reported and communications were delayed for some reason.

 

[UNG]

Today we had a huge bang from the 11kv pole transformer on one of our sites, the generator kicked in but out of 18 alarm dialers I think all 18 plus some pcbs on our ventilation panels have been fried.

Are the diallers supplied at mains voltage or from a 12 / 24v PSU

When sp energy had finished it looks to me as the fault was worn insulators on the 11kv loop on top of the pole and not a transformer problem, this short on the pole happened around 1pm yet all the affected alarm pcbs faulted whilst on generator power 415v at around 2.15pm

Was the generator voltage you quote actually measured or have you just assumed that was the value, what was the phase to neutral / earth voltage on the generator

My question is... could the loss of the 11kv loop bring in a flash of over voltage or is it possible the generator over generated and caused over voltage? On some pcbs 50ma glass fuses have survived yet the panels are dead

I suppose anything is possible when working with electricity where momentary spikes can cause some major damage, is it possible a voltage spike on the earthing system has caused your problem
You say the glass fuses on some PCB's have survived this suggests other haven't is this relative to where the initial fault occured,
I would certainly be checking that the generator is functioning correctly and is providing the correct phase to phase and phase to neutral voltages on load as generator maintenance is often neglected and when extra load is added across a site very little thought is given to whether the generator has the capacity to supply it

Forensic analysis of a fault is very difficult from afar and alot of important information can be lost in the rush to get things back online in the minutes / hours following the fault and documenting the timeline of events can be important in substantiating a claim

 

[marconi]

I would investigate first how you know the time of the fault on the transformer and the alarm diallers. Is the alarm dialler clock set to the right time? Does the generator auto start have a clock?

Could you tell us more about the supply change-over switch eg make model and whether there is any staggering of power up following a black out as loads move over to DG. And what happens when DNO supply is restored - are all loads en masse returned to 11kV/400V. The distinction is 'short break' or 'no break' in power supply and derivatives of these.

James is a goodun so please do not shun him.

Edit: The rhyming was unintended but I am quite chuffed with it.

 

[marconi]

What Causes High Voltage In Diesel Generators? What Can We Do To Prevent Generator From Being Over Voltage?-Tide Power Technology Co., Ltd., - https://www.tidepower.uk/news/428.html

You said the damage happened some time after the 11kV event. I wonder if the DG has poor voltage regulation with changing rpm or load leading to overvoltage at its output. The link is to a nice short piece on possible causes of over voltage and things to check on the DG.

I also wonder about what happens to the installation neutral as the changeover from DNO to DG happens. How is the DG neutral connected? Is it connected to earth somehow? What is the earthing system when on DNO supply?

Is the DG lightly, moderately or heavily loaded up? Which large equipments might cause transients when they start and stop?

Would your client consider putting the low power sensitive electronics on a no-break supply?

These are my musings on what you reported which I hope are helpful.