Pikeys and a stolen substation neutral cable = BANG!!!

[Kirchoff]

Hi,
I heard about this on the news. Apparently all the local residents' appliances blew up due to a voltage spike.

I couldn't figure out at first how this happened? Then I started wondering whether, because the secondary winding of the transformer is a 'star' winding and the centre the 'earth/neutral tap it must have caused an inbalance in one of the legs leading to the voltage surge?

I'm not sure if I'm right as I've not worked for many years (ill health).

Any other ideas?

 

[JD Hogg]

Yeh I live quite near a grid substation, does anyone know why the transformers hum?

 

[Justin pearson]

they dont know the words

 

[JD Hogg]

Lol ...

 

[Justin pearson]

no i dont,but i know they are expensive very b***dy expensive and we were planning for work that was going to happen ten years later.

Im not sure if that was because the maintenance teams were very slow, or it just took that much planning. But i suppose if you are switching of a power station in a big city i can imagine it takes some planning.

Think of all the tea and sandwiches they would have to make for the workers.

 

[Engineer54]

Yeh I live quite near a grid substation, does anyone know why the transformers hum?

IN very simple general terms, ionisation of humid air is the hissing noise and the the humming is the the iron core lamination's vibrating under load... All in most circumstances very normal!! However, differences in these humming noises can to a trained ear indicate underlying problems with a TX. Also different TX constructions, different humming noises....

 

[Knobhead]

One to add to the experience pile. I got a call to a plant, all the office equipment had failed. On arrival I tested the supply to the office and all seemed OK. Called the lads from office services to come and sort their junk out. (It was junk, it's a wonder you didn't need to shovel coal in to the computers). All power supplies smoked! I'm puzzled now, how could 4 go wrong together. Got talking to the plant manager and he tells me that the lighting had "gone weird" for a while. Went back and checked the Ph -N voltages again and all OK.
It wasn't until lunchtime when I got talking to some of the other electricians that things became clear. Two of them had installed a 150mm 3½C cable in to the plant main switch board that morning and had disconnected the neutral - earth cables to make it easier to get their cable in, when finished they reconnected them. To quote one of them "I thought there was a bit of a spark!" What galled me was I'd fitted an engraved warning label to the earth bar when I installed it!

 

[Knobhead]

and on a foggy wet evening they crackle, and spit i wouldnt want to walk round them with a step ladder on my sholder. I forget exactly what they told us on our training, in nottingham where they have fake power station training centre is, as it was so long ago now but getting within 1.2m of some of the bits will arc across and give one hell of a packet.

Which one did you go to? The first one I went to was Ashover Hall but this was closed down and a new one opened at Radcliff power station. Ashover Hall was a beautiful place, when you went out to do the practical switching part of the course it was like going on a nature ramble. When I want to Radcliff the HSE had ruined the course it was so watered down. The “students” were no longer allowed to touch the U/G link boxes, I had a great time at Ashover paralleling transformers with solid copper links, lots of pretty blue sparks.

 

[mechelec]

I have spent a lot of time at ratcliffe on various courses, they have a pretty good 11KV network which they power up so that you can sectionalise, isolate and test on etc. If I remember correctly it is run at less than 11KV with very fine protection.
Im not sure what will happen to the training centre as western power have taken over from eon. The last time i was there there were rumours of some staff being finished. They had recieved a price to move the training network and werent sure if they would move it or leave it at ratcliffe. I would imagine the training centre would struggle to keep going if WPD stopped sending their guys there.
Ashover hall had finished when i started but I have heard stories of how good it was. my mate used to take his bike and go cylcling in the evenngs when he was sent. Did you stop in the digs Tony?

Engineer54 I wish there was some sort of monitoring on the LV side. The times I go to customers off supply and they say "you took your time I waited and waited for someone to come out". I ask how long was you off supply before you rang. usually it went off hours before and they think we know. We totally rely on customers calling us. If your power is off and you dont call no one will ever know, be it one house or an industrial estate (LV). Another point is if we have an OC LV mains fault it can make it easier to find if all affected customers call in.
On the HV if a pigeon farts on the line control know. it has been more geese flying into the line this week funnily enough i have had 3, 2 were on saturday. bloody tasty too on sunday.

 

[Justin pearson]

Ashover Hall it was indeed a nice place

 

[Knobhead]

Digs! The place was like a 5 star hotel. We’d go off to the local pub in the evening, on arriving back at the hall a buffet would have been laid out by the housekeeper. 4 meals a day!

As to the 11KV it was what the label says. The system was feed by four .433/11KV high reactance transformers, it could kill you quite well. You know the voltage was right as when you isolate an OCB you get a hiss as the contacts part. I was very twitchy doing live phasing out for the first time at Ashover, and to be honest over the years I was no less twitchy doing it.

Below is the set up at Radcliffe.

View attachment 7211

 

[aptsys]

As we know electricity will always follow the path of least resistance and any electronics circuits with filter circuits will be fried.

Electricity will follow ALL paths of resistance.

 

[mechelec]

Electricity will follow ALL paths of resistance.

I was just trying to explain basically if the neutral is lost as a return path in a piece of connected equipment then electricity will follow the path of least resistance in the equipment back to source to make a circuit!!!!!!

 

[aptsys]

I was just trying to explain basically if the neutral is lost as a return path in a piece of connected equipment then electricity will follow the path of least resistance in the equipment back to source to make a circuit!!!!!!

...AND all the other return paths. Not just the path of least resistance.

 

[Knobhead]

If you want to know what’s happening to the phase-neutral voltages you have draw out a vector diagram of the currents and then transpose voltages on top of it.

Basically to comes down to:
The phase with the lowest load will have the highest Ph-N voltage. That’s the phase that first blows things up. That drops the loading on that phase even more causing more imbalance and an even higher voltage rise.

End result, one estate with irate customers wanting compensation and a couple of house fires!

I will try to do a vector for you, but it’s not easy so give me time.

 

[Dunc]

I love these posts about the MV/HV side of things. Most of my work is domestic with the odd commercial/light industrial job so most of this stuff goes right over my head but interesting reading none the less.