Question. Sub station switches.

[Derlyn]

Question for the toppies who are familiar with and who regularly do work in sub stations.

Are the switches in substations designed to be able to be switched on and off under load numerous times daily as is presently the case with loadshedding ?

[GCE]

The older MV switches - No , especially oil MCB
Every time it opens under load there is an arch which the oil suppresses - The oil heats up and stirs itself , as it settles the carbon floats to the surface - Next switch , same story until eventually the layer of carbon or the amount of carbon still trying to settle out of the oil becomes to much and it flashes across to earth.

The new switches are either gas surrounding the contacts or a vacuum - Both have pressure switches to drop the switch out in the case of a drop in pressure or vacuum - They are capable of switching more frequently under load which is why so many main sub stations have been upgrade so that it can be done remotely over a cloud based system.

We have used 2 x vacuum MCB on a 11KV feed with a genset feed stepped up to 11Kv as a change over switch and had no issues

The problem that is happening is that the remote switches being vacuum are being switched but when load comes back the oil switches further down the line are coming out on overload as the diversity factor disappears and all geysers etc come on at the same time.
This is where a lot of the damage is coming from.

The older switches also tend to drawn moisture in and on switch on flash across . I had a clients CT on the MV side crack and flash across last week - Took almost 6 hours to clean up and get them on again .

At the end , no circuit breaker , LV or MV , gas or oil , is designed to take constant switching - All have a mechanical lifetime

No electrical equipment likes to be cooled down as it allows the ingress of moisture

[Isetech]

I was watching the council electrician switching on a sub station the other day and I had to laugh. The oke tied a piece of rope to the switch stood outside behind the wall and pulled the switch

[Isetech]

What I have been trying to figure out is why there are bigger spikes when the power switches off that when it switches on again. Maybe a ripple effect down the line as the power switches off.

[Justloadit]

What I have been trying to figure out is why there are bigger spikes when the power switches off that when it switches on again. Maybe a ripple effect down the line as the power switches off.

It is the inductance in the line (many transformers after the feeder) which causes the flash over and spikes.

[GCE]

I was watching the council electrician switching on a sub station the other day and I had to laugh. The oke tied a piece of rope to the switch stood outside behind the wall and pulled the switch

Some are designed to fit the cord , which allows you to stand outside - others have a remote switch facility and now they have fitted delay timers that allows you to exit before the switch closes
All for safety - When MV switchgear goes bang you don't always get the chance to walk away

[Isetech]

I saw the results of another 11kv mini sub that went pop last night, it was replaced by midnight and went bang a couple of minutes after it was replaced, today I got to see the damage. I think the ladies and gents doing the replacement are a little nervous this time, they have been sitting waiting for a replacement since 9 am this morning, still nothing.

The part I dont understand the electricity department indicated today that some suburbs are exempt from load shedding due to sub stations being damaged beyond repair.

My question is will this suburb/road be exempt from load shedding, considering every time the power is restored the breaker trips, and now it has blown up twice in 1 day?

[Isetech]

Something else I noticed, the old lead cables hanging in mid air still from the floods. the road was washed away in numerous places exposing the lead cables, It cant be good for old lead cables to hanging 8-10 meters without support.