What caused this burn out?

[Nick]

Maintenance on the LV network is usually on the basis: "if it blows, fix it". There's very little preventive maintenance.  I suspect it's cheaper that way.  I don't know about the 11kV network but I suspect there's relatively little preventive work and again it may well be cheaper to fix it when it blows rather than look for trouble in advance.

When I was working in the West End of London next to Covent Garden, we had two 11KV underground subs blow near us in four years - a real problem for me as CTO of a hedge fund - it made all our DR planning worth while.

There are videos on YouTube of this happening - it's quite spectacular - the pavement seem to be on fire. The noise & sound is pretty neat too...

I also had an 11KV feeder explode in the building when working for NYSE/Euroext (LIFFE as was) at Cannon Street back in about 2002 - the explosion destroyed the junction box (about the size of a human coffin and made of heavy steel plate), bent and twisted the plant room steel companion ways and blew a set of heavy wooden doors off their hinges.

We all though a bomb had gone off. The real problem was that the building infrastructure team were doing testing with the Fire Brigade elsewhere in the site at the time and they'd disabled the fire alarms, so when the explosion happened with all the wood flying around and fire & stuff, the alarms didn't go off.

However, no-one needed an alarm to exit that building sharpish. Not good.

[Refugee]

The one in my photo would have been fine at the time the plans went in but with all those stupid delays getting the plans approved the solar panels were NLA.
They must have just planted new ones that produced more power for the same area.
A cable that only needed to power a little farm when new was a bit small for 7.6 MW peaking when the drop cable is in full sun light.
It resulted in some roast copper and ashes marinaded in vinyl

[ppppenguin]

This company has a pretty no nonsense policy on the sort of grid conenction it needs to make a commercial PV scheme worthwhile: http://www.haymakerenergy.co.uk/document...ements.pdf

[Refugee]

You are right as of when they applied for planning and the panels they had in production were in stock.
The power output got a bit over 5MW by the time they got the go ahead due to the old panels being NLA.
With a bigger drop cable they can just about get away with it due to the loading on the far end of the run of the original cable splitting the output of the solar panels.
They are covering up the south facing sides of most of the heaps of mining waste with solar panels around this area.
They just use the existing cables that ran the mining gear to feed the power in.

[Ed_Dinning]

Watch out for local voltage rise on your network and damaged appliances.

Ed

[Refugee]

You are right.
The burn out happened on a cable that feeds a village south of us.
On our cable there is a 5MW array feeding into what once fed a coal pit head so the regulation should remain reasonably good.
It once fed a pretty heavy intermittent load from the winding gear plus all the fans and water pumps.

[reset]

Although repairs have been carried out the burn pattern can still clearly be seen on the wood however very little if any to vegetation close by.
The heat clearly came from the drop wire running to ground level as the wood is only burned on one side and only to the top of the drop wire.
The drop wire is on the south side of the pole.

It's hard to reconstruct an event after repairs are made. I've seen this type damage from wet oak limb contacting two lines, kudsu vine growing up the pole and contacting the high line.
Was the racoon guard there before the event? (The angle iron with the barbed wire is what I call a racoon guard.) Sadly racoons are large enough to bridge the gap between the line and the ground wire and they do like to climb poles.
The fuse will protect a wire overload but it takes much less power to burn a power system out than it takes to blow the fuse. I was involved in a rebuild that burned about 24" of 3/8" X 6" buss bar. The system was 3 phase Delta (no direct ground, it finds ground threw faults). Two of the fuses blew which would have normally killed power but the third line was finding a return path threw the equipment ground system and a power feed in a neighboring distribution center.
We put out the fire and it would restart. We had to pull the third fuse under load.

Anyway, it could be a small animal bridged the gap or a wet tree limb found its way into that spot? As you say it appears to have started there and went down.

[Refugee]

Nothing touched the wires. It was a 100Amp drop cable and they plugged 7.6MW of solar panels into it. 200A at 11KV.
It sat there for a few months like a ticking time bomb and went off on a cloudless sunny day while the wind was blowing in the wrong direction for the smoke to cast a shadow on the solar panels.
All the heat was flat against the pole so that there was no damage to the vegetation or vermin grille. There is so burn marks on the pole above the top of the drop cable.

[reset]

So you are saying there are no transformers in the system????

[reset]

So you are saying there are no transformers in the system????

Let's see 7.6 Mw at 11 Kv.
P = E * I
I = P / E
I = 6.6 Mw / 11 Kv = 690 amp
690 amp in a single phase system. This being a 3 phase system it would be about 400 Amp per phase.

P = E X I = 11kv X 400 = 4.4 Mw per phase
For three phase multiply by 1.732
4.4 Mw X 1.732 = 7.62 Mw

So the 11Kv feed would be at about 400 Amps when fully loaded.

If your home is fed 400 volts you would draw 1 amp from the 11 Kv line for evey 27.5 Amps you use. So your 200 Amp service would only draw a little over 7 Amps at full load.

The transformer increases the current as it reduces the voltage. A wire the size of your finger can easily carry a Mw at high voltage but would be under rated to power an electric stove at low voltage.

An amp is an amp at any voltage level but it represents a lot more power at 11 Kv than 220V.