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Old 25th Feb 2024, 9:51 am   #1
Tractorfan
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Smile Inrush currents en mass?

Hi,
I hope I've posted this in the right place. If not, my apologies.
This year, so far, we've had a couple of major power cuts ranging from 8 to 22 hours. We managed with candles and the gas hob.
Now, this set me wondering how the system copes with the enormous inrush current of a couple of hundred fridges, freezers and other appliances starting up on re-connection?
Given that a motor can draw four to five times its running current on start up, that first second or two when the breaker goes in must be interesting. Is there likely to be a brief voltage spike? I unplugged the telly and digibox just in case.
Cheers, Pete.
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Old 25th Feb 2024, 10:22 am   #2
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Default Re: Inrush currents en mass?

I don’t know the answer but I think you are wise to unplug sensitive and expensive devices.
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Old 25th Feb 2024, 10:40 am   #3
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Default Re: Inrush currents en mass?

There will be switching going on and this will create transients, The network is a lot of unmatched transmission lines connected with stubs all over the place. These can act as pulse shaping networks to some of the frequency components and you can wind up with kilovolt transients.

This was taught by the power guy on my undergrad course it looked messy and scared a lot of students off. Only later did I twig its relevence to RF reansmission lines and things became clearer. Some of us worked out the relevence for ourselves, many didn't.

Unplugging stuff is wise.

Even stuff turned off. Those modern EMC filtering X and Y capacitors are usually on the power input side of the switch and can get flashed over using up their life expectancy. I tend to keep stuff not being used unplugged or switched off at the socket to avoid using up capacitor life expectancy.

David
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Old 25th Feb 2024, 10:56 am   #4
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Default Re: Inrush currents en mass?

If you have half a dozen soft switched PCs on the same circuit and there is a power cut the breaker will normally go when it comes back up - then you have to go round unplugging every thing before switching back on.
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Old 25th Feb 2024, 12:01 pm   #5
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Default Re: Inrush currents en mass?

Motors were traditionally expected to be a major cause of inrush disturbances but the huge number of switched-mode power supplies which are now in use will contrubute greatly. These include electronic ballasts in most forms of modern lighting.

Presumably, in view of these inrush currents causing voltage disturbances, the lastest amendment to the Wiring Regulations requires transient overvoltage protection for any new or updated installation:

"where the consequences caused by overvoltage could result in:
(i) serious injuryto, or loss of, human life
(ii) deleted, May 2023
(iii) significant financial or data loss.
For all other cases, protection against transient overvoltage shall be provided unless the owner of the installation declares it is not required due to any loss or damage being tolerable and they accept the risk of damage to equipment and any consequential loss." (Regulation 443.4.1 of BS7671, Amendment 2).

Most people might be justifiably concerned about protecting expensive consumer electronics but forum users might be just as concerned about "heritage" equipment.

For anyone sufficiently interested, one good explanation can be found here. (Warning - you will suffer advertisement before the presentation).

https://www.google.com/search?q=efix..._Rc14c2g,st:20
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Old 25th Feb 2024, 12:09 pm   #6
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Default Re: Inrush currents en mass?

Quote:
Originally Posted by Radio Wrangler View Post
There will be switching going on and this will create transients, The network is a lot of unmatched transmission lines connected with stubs all over the place. These can act as pulse shaping networks to some of the frequency components and you can wind up with kilovolt transients.

This was taught by the power guy on my undergrad course it looked messy and scared a lot of students off. Only later did I twig its relevence to RF reansmission lines and things became clearer. Some of us worked out the relevence for ourselves, many didn't.

Unplugging stuff is wise.

Even stuff turned off. Those modern EMC filtering X and Y capacitors are usually on the power input side of the switch and can get flashed over using up their life expectancy. I tend to keep stuff not being used unplugged or switched off at the socket to avoid using up capacitor life expectancy.

David
I watched a Youtube video yesterday about early submarine telegraph lines, in the context of Lord Kelvin's and Messes Nyquist and Shannon's theories. Very enlightening, and I'm sure relevant here.
Separately, I became aware of the Ferranti Effect.
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Old 25th Feb 2024, 12:19 pm   #7
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Default Re: Inrush currents en mass?

William Thomson as he was at the time was certainly a smart cookie and used science, understood what was going on. He was a jump or two ahead of the stuffed-shirt assumptions.

I always thought about those transatlantic cables, if their design was uniform along their length, they would have a simple characteristic impedance, a loss per unit length and a propagation velocity. Rather than loading with inductive coils etc to change Zo, wouldn't it have been easier to engineer the source and load impedances to match what the cable dictated?

The section of the course I did was looking at the power grid structure and considering lightning strikes as the chief source of transients.

David
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Old 25th Feb 2024, 3:14 pm   #8
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Default Re: Inrush currents en mass?

To limit inrush currents on restoration of the supply after a large scale breakdown it is common practice to restore the supply in sections.

If only a small area was affected, then they just hope for the best and hope that fuses in substations withstand the brief but significant overload.

The problem is not confined to motors and switched mode power supplies! consider also things like thermostatically controlled space and water heaters.
1,000 3kw immersion heaters in a district do not add that much load under normal conditions. Only 5% operate at any time. Following a power cut the stored water will have cooled and almost every heater will operate at the same time, imposing an extra load of nearly 3 MW.
Similar arguments apply to other loads.

On some oversea grid systems, the power company can remotely disable water heaters, mainly to limit peak loading. After a power cut this is used to limit the load for a bit until conditions stabilise.
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Old 25th Feb 2024, 3:29 pm   #9
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Default Re: Inrush currents en mass?

When I was working, I had in my home office a rack containing a few 'blade' servers, some Cisco routers, a firewall and some fibre optic switches, all powered by a 1KVA rated UPS. Mains glitches would regularly trip the 16A breaker on the radial feeding the rack.

(The UPS was a direct on line type so the SMPS part of it was running all the time).

Big data centres have some serious power management systems that will bring up things in a controlled way if faced with doing a cold start. Personally I don't do anything specific here any more to handle power failures, I accept the risk of possible equipment damage, in a way I like subjecting kit to occasional power challenges in a semi controlled way, if only to flush out any problems or failing components.
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Old 25th Feb 2024, 3:32 pm   #10
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Default Re: Inrush currents en mass?

The in-rush current is 'compensated' by a suppressed voltage waveform. A superposition of in-rush currents from many load points all starting at the same time will effectively suppress each in-rush current level/waveform, as the upstream voltage is itself suppressed.

One presentation of what many loads of the same type can do is the flat-topping seen on a voltage waveform due to many vintage switchmode and capacitor-input filtered power supplies. Another presentation is when a local short circuit occurs, and the voltage waveform throughout a distribution network will exhibit a depressed portion up until the short is removed.

There is typically no damage from suppressed voltage per se, unless inductance or load switching happens as a result of sagged voltage, and that is more likely to be a nuisance than damage.
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Old 25th Feb 2024, 8:54 pm   #11
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Default Re: Inrush currents en mass?

I remember that, during the three-day week energy crisis era of the 1970's, when they got all the TV transmitters to shut down at 10.30PM , there was a power surge shortly after as thousands of people all over the country simultaneously stood up, turned on their room lights and switched on their electric kettles. To alleviate the situation, TV shutdown times were staggered.

I read in the papers last week that a particular model of high power smart EV charger is being withdrawn from sale as it is not sufficiently secure from hackers. The authorities are concerned that a malicious attack could cause them all to be turned on simultaneously, tripping the grid.
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Old 26th Feb 2024, 5:59 am   #12
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Default Re: Inrush currents en mass?

SMPS have a nasty characteristic that they try to look like constant power loads. If the supply voltage is low, they compensate by taking more current by changing their switching periods.

So current goes UP as voltage goes DOWN. Um, that's negative resistance!

The avionics industry is worried about this. As more and more systems, instriments, radios radar use 'smart' switching power regulators, a lower and lower negative resistance gets imposed as the load on a battery and generator. The stability of the battery voltage control of the generator comes into doubt.

I suspect that something similar is looming for the electricity distribution network. The thermostatic load issue is undestood, but the SMPS business is on a cycle by cycle basis.

The distribution system has not only many loads being brought back on line, but also many different generators with different regulation schemes scattered all over the place. Think of the numbers of small solar inverters. They are arranged to shut off feeding the grid if the grid voltage goes too low, but the business of the grid coming up will bring them on-line in various ways.

It's all rather scary once you start thinking about it. The new requirement for surge suppression in new installations starts to look sensible.

David
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Old 26th Feb 2024, 7:27 am   #13
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Default Re: Inrush currents en mass?

It sounds like a prime case for the Internet of Things. Tie all the parts together, for effective monitoring and control. Could that be done in a secure and affordable manner? Can we afford not to do it?

Would it in fact be more secure and affordable to use a radio signal with national coverage?
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Old 26th Feb 2024, 8:42 am   #14
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Default Re: Inrush currents en mass?

There already is an invisible sort of coordination system. some generators inputting to the grid use the frequency to control how hard they drive. If the frequency is low, they increase their output, backing off if the frequency is too high. All while limiting their max output power to protect themselves from overload.

Think of the three-phase grid as if it was driving a lot of synchronous motors as loads, and had a lot of synchronous generators. Treat it as one huge rotating machine with some parts linked by shafts, some linked with electrical equivalents. The speed is whatever solves the equation of power versus revs of generators and motors. If there isn't enough generation, the whole thing slows until the available power is sufficient for the reduced load. Something, somewhere has to try to control the speed. We have really good frequency standards so it's easy to monitor the speed/frequency with good accuracy. So we can have various generators all watching the frequency and turning their wick up or down to try to keep 50Hz. Reality intrudes and the frequency is allowed to wander a little to allow smaller generators to contribute. These are often renewables and so preferable to shovelling more fossil fuel on the fires. Another complication is that for synchronous clocks, the frequency is compensated to bring the number of cycles in a full day to the right value so sync clocks neither gain nor lose time.

Add a lot of SMPS, smart inverter motors etc into this mix and the voltage to power relationship slope is reduced and could in the limit go negative.... hence the risk of instability.

You can't do meaningful step and impulse load variation testing on these sorts of systems without loud bangs and tons of mangled metal. So instead, low magnitude random noise modulation of the load or excitation is employed. Cross-Correlators can be used to get the step and impulse response of the system computationally from comparing the system response to the noise, with the noise itself. You can also FFT this into frequency response plots as well.

Attempts at doing real transient tests on nuclear power stations have not ended well.

So electronic instrumentation allied to computational techniques are the best show in town. Attemps to verify directly can leave a glowing hole in the ground.

David
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Old 26th Feb 2024, 10:59 am   #15
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Default Re: Inrush currents en mass?

All this makes me think what would happen if the National Grid ever crashed - or even separated in two. It's never happened yet although I understand the procedures exist, but obviously can't be tried!

I guess restarting would be getting one power station up and running, at the right frequency, bringing another station up, syncing it in frequency phase and voltage and switching in, then another (bearing in mind that some power stations suck a fair bit of power till they're established), connecting some users, then bit by bit adding more generation and more load.

Syncing two separate sections of Grid, if it ever split, sounds nigh-on impossible. One would have to be progressively shrunk as its generators and users were isolated and reswitched into the other.
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Old 26th Feb 2024, 11:35 am   #16
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Default Re: Inrush currents en mass?

Theoretically we're in sync with mainland Europe, but there are multiple links and loop lengths big enough for worrisome phase shifts, so I think the links are all DC interconnectors. At least that would allow us to come up out of sync with the continent and then lock up as we get to an almost right frequency and wait for the phase to drift in-sync. I guess this means using grid freq to control generation has to be continent-wide.

Glad it's not my problem.

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Old 26th Feb 2024, 11:56 am   #17
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Default Re: Inrush currents en mass?

If the grid is down then there are no volts, so just bring up the alternators from zero volts and the current will rise proportionally. You only need the voltage rise to be in 20ms chunks, not seconds.

I remember being told about the problems when British Leyland shut down each afternoon with the voltage and frequency swings, seemed a far worse problem than turning on.

Bit like running a generator if you have one, whilst 2.5kVA will start and run a 9" angle grinder, smaller then tweak the engine speed control to drop the revs.
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Old 26th Feb 2024, 12:05 pm   #18
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Default Re: Inrush currents en mass?

I've been (very) peripherally involved with these problems when doing design work for electric vehicle chargers. It turns out that these heavy battery charging loads are actually quite useful things to have on the grid, because almost all of them can be controlled remotely, if necessary, to add or remove load. For those managing the balance of generation and load, it turns out to be quite handy to have some big chunks of load which can be quickly instructed to switch off or slow down for a while. Nobody's going to mind their car taking a few minutes longer to charge if it prevents a power cut due to something big tripping out.

There is an enormous amount of effort going in to keeping the grid stable as we move from lots of big coal-fired generators powering electric fires to distributed sources of generation running switched-mode power supplies. Much bigger brains than mine are working on it. Part of the solution turns out to be rather low-tech in concept: a flywheel, known in the trade as a "synchronous compensator". Some of these are purpose built, and some are actually re-purposed fossil fuel power stations. Even without burning any fuel, a big spinning mass attached to a synchronous motor helps to keep the grid stable in the event of sudden changes in load or generation. There are already quite a few of these in operation on the UK grid.

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Old 26th Feb 2024, 12:25 pm   #19
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Default Re: Inrush currents en mass?

Some 40 years back or so, I went on a trip to Eggborough power station. Whilst in the control room it was noticed that there was a tv set up on a shelf. When the tour guide was asked about the tv, he explained that it was not there for pleasure but so that the controllers could check when the advert breaks were coming up during sports matches and soaps. The method was to ramp up electricity generation to coincide with the advert breaks, as that was when customers went to put the kettle on!

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Old 26th Feb 2024, 12:34 pm   #20
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That makes sense - when the advert cue came on (top RH corner of the screen), the boiler coal feed could be increased, more steam pressure raised so that when the valves were opened, the power could be increased. And not too far ahead either, so the pressure release safety valves never lifted.

OK that's simplistic, but a few moments glimpse into the future can have massive benefits!
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