I were thinking like, about power cuts

[Bill Wright]

Our power was off for three hours today. It was quite a wide area. Today was very cold here. What I was wondering was, what happens when they put it back on? I mean, every thermostatically controlled device would be 'on', every battery-backed device would need recharging. The load must be enormous. Hypothetically, if you could measure the resistance the power was feeding into, what would it be? What tiny fraction of an ohm? Presumably the power network has some reactive and inductive elements, so what happens there when there's this sudden rush of power? How do they physically switch the power back on? Maybe the switch is immersed in something. Doesn't the sudden demand cause problems for the grid?

Bill

[Tim Watts]

On Monday 08 April 2013 20:29 Bill Wright wrote in uk.d-i-y:

I suspect they would isolate areas, restore the HV and then switch the areas in one by one.

[The Natural Philosopher]

well transformer leakage inductance takes care of a lot of it.

It is. Mineral oil

not much. ebeverything slows down a little.

[Sam Plusnet]

Vacuum circuit breakers are used at the lower end of the distribution system (11kV).

[Dave Liquorice]

The 11 kV switches around here are air but then they are more manual routing switches than overload/circuit protection and wouldn't normally have to switch under load.

It's switching off that is more of a problem than switching on. Off you have to kill the arc quickly, by quenching it in oil, blowing it out with compressed gas, or trying to open the contacts fast enough that it can't sustain itself. On, just bang the contacts together quickly so that any arc is short in length and duration and the contacts are well together and thus low resistance to carry the line charging current and any connected equipment in rush without excessive heating.

[The Other Mike]

And increasingly SF6 at higher voltages.

[zaax]

Imagine the power that is needed when an arc furnace is switched on by a steel making factory

[harry]

Some dangerous equipment has "No volt release" devices.

However electric heaters (fires, hotplates etc) are a real fire danger. Always shut them off if there is a power cut.

[Nightjar]

I hope they have more sophisticated detection systems these days, but that was sometimes the easiest way to find where the fault was. Keep switching areas back on until one causes the system to trip again, then switch on all the other areas and send the engineers to the one with the fault.

Colin Bignell

[The Other Mike]

Not a problem in the UK. Ask in an Indian newsgroup.

[Peter Johnson]

That's one of the reasons they built the pumped storage power stations at Ffestiniog and Dinorwig in Wales, and others in Scotland. They deal with the surges when people put the kattle on after Coronation Street etc.

[whisky-dave]

We used to phone up the local hospital and teh nearest substatiojn before f iring up our 6.6MV generator which was poweeredd by something like 8ft dia meter flywhhel a 50KV and many hundred amps. Apparently teh electrical fiel ed generator used to be detedcted by the hospitals equipment aboput 1-2 mil es up the road.

[Brian Gaff]

This is why they normally switch areas back on in stages of course, they are not that stupid!

Brian

[Brian Gaff]

More to the point, what happens to the solar cells generating power into the grid when the main power fails?

Brian

[John Williamson]

They turn themselves off for the safety of people working on the grid to rectify the fault.

[Dave Liquorice]

That's how they sometimes do it around here but it's not remote switching it's men driving around the manual switches opening/closing as required. I think they do have some kit that can measure ground currents to help zero in a bit quicker but that still requires them to go around putting it in place and energising the line(s). Once the fault is found that imediate section is isolated, other sections resupplied via other routes and work on the fault commenced. Once fixed the switches are set to "normal" and the alternative supply arrangements removed without having to interupt the supply again.

They seemed quite pleased the other year when we called the DNO to report one of the isulators on our pole arcing/smoking as they repeatedly tried to energise the line after it had tripped. They'd almost found it but saved 'em a bit of time.

[Dave Liquorice]

The invertor shuts down and isolates them from the grid.

[Tim Watts]

On Tuesday 09 April 2013 13:23 Brian Gaff wrote in uk.d-i-y:

They shut down - it is a condition of the inverter design that they sync to only exisiting power and shut down when grid power does off.

[Tim Watts]

On Tuesday 09 April 2013 13:35 John Williamson wrote in uk.d-i-y:

And to stop harry from powering the entire road and bankrupting his neighbours in FIT subsidies ;->

[harry]

All taken care of.