Last night, actually early hours of this morning, we were getting multiple short power cuts but also one of our RCDs recurrently tripping and a UPS switching to battery every few seconds for less than a second even between observed power lossss. I didn't, owing to the hour and the need to set up some sort of voltage divider, get to look at the supply on an oscilloscope, but I think we can assume all was not well. The actual voltage measured by the UPS remained at the usual 244-247.
This morning the voltage has dropped to 230-233 and the frequency to
49.8.
My question is how they do this, when Gridwatch confirms the national frequency is a usual 50.1Hz? It is a rural, mid-Wales overhead supply.
So at around 0400-0500 they let the frequency drop.
there is no 'usual' frequency. Only an average value of 50.0000000Hz.
When I was off grid and being run from a massive container sized genny whilst they fixed an 11KV problem outside my house, I got a lot more trips.
Instability on the mains will trip RCDS that are 'on the limit'
If you were actually measuring incoming voltage and not UPS output voltage a high voltage indicates that there is less load on your local area - that is fully consistent with 'wee small hours'
a low frequency merely means they pulled more capacity off the grid than they should have done - in all probability a wind farm or two tripped out and dropped off.
A cursory glance at gridwatch suggests they stopped importing so much from France and Holland at that point and CCGT was practically all off. That would have left very little in the way of short term frequency control on the grid.
That, or the UPS being wrong, were the only things I could think of. I can't see how even an overloaded distribution line could actually cause a progressive phase shift like that. BTW, I meant 50.01Hz for the national frequency, not 50.1 as I wrote.
How accurate is the frequency meter being used? I wouldn't trust the UPS. My APC unit displays to two decimal places but measurement increments are 0.25 Hz, logs only show 49.75 or 50.00 Hz...
I'd expect an actual power cut of at least several minutes to half an hour to do all the SIDE stuff required to connect in a temporary generator and again to disconnect it. Check your DNO's web site for power cut information. I doubt they'd bung in a genset unless the fault is going to take > 12 hours to fix and affects a fair number of customers.
Reduced voltage could simply be the result of changes in the local distribution to isolate the fault and restore as many customers as possible as quickly as possible. Done by manually opening/closeing switches in the local distribution. With a "bumpy" supply all you're likely to notice is the bumps stopping when they disconnect the faulty section.
I suppose it is possible but it would have to be a large and distant fault. The frequency has generally been 50.0 since mid morning but the voltage has fallen to 221.5v. It doesn't normally fall more than a volt or two fo about 242v at peak time. There are reports of incident tape across the entrance to our nearest substation where the 11kV is probably distributed (because they tend to report work there when it fails) so perhaps it is flooded.
When BBC TV Centre was built it was connected to the West London 11kV ring and there was also a 'back-up' feed direct from the main board at Battersea. Sadly that main board, which was part of the West London ring, blew up on the scheduled opening night for BBC2.
Sounds like the grid is not all one grid then, or that would cause some issues. This problem came up twice in my lifetime, during the very cold winter of 1963, and during the miners strike with rolling power cuts, but the stress even then drove the cycles down. Brian
That's sort of to be expected. If you are being fed via another route that route is supplying it's normal load plus what ever has been added by the arrangements to keep most of the affected are fed.
Not normally run as a ring though, at least not around here. Our line comes from the primary sub-station to the village but the air switch that would connect it to the village is normally open. The line the otherside of the valley from the primary to village has a normally closed air switch at the village. There is also another line that goes over the tops to a village in the next valley via a normally open air switch that could conect to a third line from the primary to that village.
There are other air switches in the lines every few miles so faults can be isolated and by opening/closing the other air switches every where can be fed apart from that isolated section.
It's quite concievable for just one of those lines being required to feed both villages and all the valleys. (Two faults close to the primary). That could easily treble the length of line and more than double the load.
Aye, the distribution is pretty resiliant, as above. I also have a sneaky feeling that some of the 11 kV lines from one primary can be fed from a neighbour primary again by opening/closing air switches.
When my cable got its short, they isolated it at the nearest overground poles, and that meant just me and my neighbour were 'off grid'
You don't need 'air switches' when you can unlink at the pole.
So: My short tripped a ring, power cut happened, men up poles, everyone else got power back, took me a day for a container genny to be parked in the field next to me..
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