Back in the late 60s, I spent some time at Eggborough power station. There was an interesting clock on the wall of the control room. It clearly had two inputs - one was from the station mains frequency which was driving its second hand (can't remember if there were actually any others) forward, whilst it was driven backwards by the station instrumentation feed, from a pendulum clock which produced pulses ever second (and minute, quarter hour etc for various printing recorders).
So the hand moved around the dial, twitching back every second, generally hovered around 12 o'clock, but did move around a little over time.
A long time ago, one of my set lab experiments was to run up fairly large three-phase generator, synchronise it with the mains, get it generating power and do various test measurements. Fortunately, my professional career was more concerned with valves and transistors, and I never needed to do it again!
Mains driven clocks are allowed to drift off by so many seconds a day. Before privatisation it was kept very close to zero, but needed to catch up (or even anticipate future slow running) at night. Since then the full range of drift has been more likely to occur. If memory serves (probably not) the legal requirement was about 30 seconds.
The clock will have been to show the difference. We had seperate clocks at work which showed the difference.
There was an excellent series (of 3 programmes) called the Secret Life of the National Grid shown before Christmas. You may be able to find it with the BBC iPlayer.
Routine load shedding, it's been happening for decades. 5% then 10% load shed, acheived by a voltage reduction, I'll let you work out the percentage reduction in voltage to get the reduction in load.
After that it's rota disconnection as happened in the 1970s.
Yep. Think of the grid as a huge generators to which lots of little engines are geared. As demand goes up the voltage and frequency start to fall..the whole contraption is 'climbing a hill'
Throttles are opened on all the engines online, and new ones are geared to it to help a bit. Meanwhile the word goes out for anyone who can do without the grid to get off it, and if anyone has any backup power units spare, could they please start them..
If the whole shebang cant make it over the top, then passengers are thrown out till it can.
You will get far more familiar with this as more silly wind power is added to the grid.
I'm not overly keen on this frequency based automatic load shedding (as in switching off) either. Sounds like a recipe for grid instabilty if you ask me. Frequency drops, lots of stuff drops off line, Whe Hey! too much capacity frequency goes up, stuff comes back online, frequency drops...
And how much stuff is about that can be switched off for an indeterminate period without warning? Not a lot, storage heaters maybe but they aren't on when there is peak demand and you could only switch 'em off for a limited period otherwise people would be cold(er) the next evening.
Some form of dynamic load directly controlled by the grid would work but I'm skeptical about an open loop system.
No, that's not a problem really..as there are time delays and hysteresis built in.
But the whole thing is pretty silly really as the amount of kit that can be load dumped in this way is probably vanishingly small.
Exactly.
Not a lot, storage heaters
Actually it isn't open loop, the feedback control is in the frequency.
I am very mixed about this, On the one hand its a small effect, and wont save the planet, on the other hand its actually bloody simple and cheap to build into - say a fridge freezer - the intelligence to do it and to move the largest part of that load to off peak times.
Probably a far fare better way to modulate demand, would be for people to work 24x7, preferably from home, and spread the 'human activity' part of the load throughout the night as well as the day..
Frankly this is just another bit of 'this will help a teeny bit, but such a teeny bit its barely worth spending money on' like switching off the telly at night, that's been blown up to 'This will save the planet' by the eco-fantasists and the 'green' big business interests.
I suspect that in the average household, not a lot but I have a friend = who works for a water company and they regularly get paid for NOT = pumping at certain times.
works for a water company and they regularly get paid for NOT pumping at certain times.
He must work at Dinorwig :-)
Whilst there are a few industrial processes that are not critical when they use power, by and large electricity is currently (sic!) used BECAUSE it is always there, always on tap, when you need it.
"The train at platform five will be leaving when a squall hits the outer Hebrides, and we can suck enough power off the grid to get it going, or when everyone has finished watching Corrie, whichever is the soonest"
If the frequency drops then the first response is the governors on all the generators on the system. They will always actively control the energy input such that the frequency is maintained at the target of
50Hz.
If the frequency still drops then load that is on a disconnection agreement is removed in a controlled manner. This is usually commercial heating / cooling / non critical processes.
Then there is voltage reduction in a number of stages to further reduce load that isn't on a disconnection agreement (most domestic load for instance)
If the frequency drops further then load gets shed in defined blocks it could be an entire city, or an area , or a particularly large industrial user - that load then stays off, there is no automatic reconnection.
If by this stage the frequency is still dropping then the generators automatically disconnect to protect themselves.
The grid is then dead.
Then sometime later certain contracted for 'black start' sites commence generation.
Starting with a diesel generator, to power the fuel pumps etc to run up a gas turbine (kerosine fueled) which then runs the auxiliaries to run the main generation at that particular site (usually coal but could be gas or oil) You might get some hydro running too.
Once part of the grid is energised it is interconnected with other generation and load, controlling generation and load such that the frequency and voltage is within limits. Gradually everything is reconnected.
What happens at each stage is defined in advance, as are the frequency targets (beyond the published +/- 0.5Hz) and the voltage reductions. Consumers on a disconnection agreement know that they could be disconnected at little or no notice.
Two stages, as I said earlier, 5% and then 10%. Carried out very easily these days with the automated control systems in use. It's worth noting, however, that the load reduction is usually carried out by the Distribution companies through their control rooms.
Unlikely to be a whole city, unless it's an emergency load shed. Normally the rota disconnections are planned on an 11kv feeder by feeder basis, again carried out these days by a simple command from a central control room. I disagree with your comment that there's no automatic reconnection, rota shedding is usually for 3 hour periods.
If we're talking emergency load shedding, that usually implies something catastrophic has happened, eith on the generation or transmission systems. I seem to recollect it happened some years ago during summer, when part of the grid was out for maintenance and there was another failure on either a
275 or 400kv cable somewhere around north London meaning effectively that the country was electricaslly split in two. Lasted for several hours if memory serves correctly.
Let's also be fair though, for the conditions you mention we are talking an absolute catastrophe. That we don't get therm is why control engineers suffer high blood pressure.....
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