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Posted By: RODALCO LV meltdown in Titirangi - 12/31/07 10:15 AM
Not being on standby this week i was called to a complete poweroutage of our local POCO substation in Titirangi at around 15.00 hrs.
Luckily i was not far away and working in an other substation scoping some jobs for the new year.

Could I please investigate what was going on ?
At the same time reports came in that an LV line was melting away about 5 km's away in a side road.

Arriving at the sub, T1 transformer (33kV / 11kV, 10,000 kVA) was out and tripped on master trip.
T2 was not in the sub and was removed for major overhaul in the workshop.

Checking the area around the TX, nothing obvious from damage, no Buch holtz alarm or oil leaks or blown insulators.
Spoke to control room and decided to reliven T1 with T1 incomer out to give a quick soaktest before applying a load.

OCB closed TX humming nicely for about 20 seconds, then whissshh!! , TRIP. (Sounded like a sky rocket).
Some black blobs were visible atop the TX but no smoke.

OK we have a problem here, went through the process off testing, isolating, earthing the TX supply , outgoing and VT.
After i got my access permit i could have a closer look and found the 11 kV junction box vented itself and spilled thick insulating oil through one of the gaskets.
Posted By: RODALCO Re: LV meltdown in Titirangi - 12/31/07 10:29 AM
Ok ,First priority was to get the power back on and had to do the 11kV switching to close open point switches and get the power back on in the affected area. The faultman was doing the western part while i did the northern part in liasion with the control room. Power restoration was achieved at around 1800 hrs.

We decided to work out a plan and get materials organised and attack the TX job tomorrow.

In the street where the OH wires melted, an upgrade project was being done but fuses were removed and replaced with knifelinks. The poor old 400 Volt supply from a neighbouring substation was backfeeding the LV supply on the outage which of course caused the wiring insulation to melt , some conductors came down, and local customers saw glowing wires sparking at the poles. About 7 spans were directly affected here.

The reason why to use fuses is obvious here, in this case the wires acted as fuses and succombed.
Luckily no one was injured here and line crews are temporary resagging the stretched lines here and redo terminations for now. These lines will need to be replaced in the new year.

Some photo's will follow in the next couple of days

Regards for now, 32 minutes till the New Year.
Cheers, Raymond
Posted By: Trumpy Re: LV meltdown in Titirangi - 12/31/07 10:01 PM
Gidday Ray!,
Sounds like a curly one there mate.
I find it strange that the Buchholz relay never tripped, I was under the impression that that was what they were designed to do in a fault like this.

This sort of thing is what I used to fear all the time with HV work, as in resetting and re-energising transformers, you just never know what is about to go wrong, you can test until the cows come home, but the real proof is when the supply goes back on.
I've been lucky sometimes and un-lucky other times, being unlucky is really hard on the nerves.

I hate the use of knife-links as a temporary Band-aid during upgrades, it's either fuses or nothing with me.

I can't wait to see the pics, when they arrive here, should be good.

BTW mate, hope you had a relatively nice, quiet (Faults and Fires-wise)Xmas/New Years break. wink
Posted By: RODALCO Re: LV meltdown in Titirangi - 01/01/08 12:06 AM
We had a look in the TX termination box this morning.
The lead armoured 500mm² cable blown out and shorted to earth.
The REF relay at the 11 kV panel had the flag up which tripped the master trip.
The junction box was full with penotrol oil which was all carbonised up. The hard part is to get new oil that over the next couple of days.
The jointers are sourcing parts and preparing the replacement cable.
The TX meggered ok and the windings are luckily not damaged.
The other subs carried the extra 10 MW load ok overnight via the tie switches although there were a few dips around midnight.

I will post you some pics to add to this thread.

Happy New Year anyway, Raymond
Posted By: Trumpy Re: LV meltdown in Titirangi - 01/01/08 08:26 PM
Here are the pics from Ray:

T 1 transformer at Atkinson Rd substation, Titirangi. At the top is the terminal box, the 11 kV cable goes to the back of the TX to the stands LHS, the 33 kV is on the middle near the header tank:

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Close-up of terminal box and marks from oil vented through gasket:

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Terminal box open, left hand cable damaged, this being the Blue phase:

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Terminal box from a different angle:

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Cable gland removed and cable disconnected from box.
The black pipe was used for bleeding the reservoir.
The red lead was from the 5kV megger:

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Close-up of damage to the cable:

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11kV stands, the fault current melted the heatshrink adhesive from the rain-shields on two phases:

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More to come later......
Posted By: Trumpy Re: LV meltdown in Titirangi - 01/04/08 08:59 AM
The rest of the pics:

Melted span across driveway:

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Cable end and overheated insulation:

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Hollywood Avenue:

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Cables after re-sag, the whole lot needs to be replaced in the next couple of weeks, cables are thermally stretched and soft-drawn after the surge:

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(Thanks for all of the photo's Ray}
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