Loss of one electricity phase causes low voltage (95 V) for everyone

We have an interesting power supply problem at the moment, which Northern Powergrid are trying to fix. Power to our village is present (for everyone, AFAIK) but the voltage has dropped to 95 V (barely enough to turn on the LCD display on my power-monitoring plug!). NP say it is due to a circuit breaker having tripped just one of the three phases in the supply to our village.

Is it plausible that this could cause low voltage for everyone, as opposed to full voltage for those houses that are on good phases and total power loss for people on the phase that has tripped?

Assuming that your local neutral and earth are still at roughly earth potential, perhaps before your local transformer the imbalance between phases somewhere in the higher-voltage supply has raised a neutral and your supply transformer is working between that and one of the active phases?

nib

I think so, though I cant quite visualise exactly how right now (coffee infusion hasn't quite levelled out)

A three phase transformer takes all phases and magnetically couples them not only to the output phases as you might expect, but also to each other.

"A 3-phase transformer or 3φ transformer can be constructed either by connecting together three single-phase transformers, thereby forming a so-called three phase transformer bank, *or by using one pre-assembled and balanced three phase transformer*which consists of three pairs of single phase windings mounted onto *one single laminated core.*

The advantages of building a single three phase transformer is that for the same kVA rating it will be smaller, cheaper and lighter than three individual single phase transformers connected together. This is because the copper and iron core are used more effectively."

Now the analysis of what happens to such a beast when you lose a single input phase is beyond me at this stage in the morning, but it certainly includes the possibility of weird s*it on *all* the output phases.

It's not *quite* like that. Upstream of the final three phase transformer there IS no neutral. As such.

That final transformer - probably a grey or green box somewhere local, will be a *single* iron core with six windings fed from three 11kV (sometimes 33kV) lines.

The neutral is generated by the connection of three of the output phases to a common point. Which is then earthed.

Whether or not you can regard that as 'raising a neutral ' under fault conditions when only two phases exist is moot. But it certainly isn't happening *upstream* of the transformer.

And as pointed out above, three phase transformers are *not* three single phase transformers in the same box.

Their operation under fault conditions is not something I was ever taught or had reason to study, so I can't say more than it isn't nearly as simple as one would hope for it to be.

"Also to each other" - that's the crucial thing!

We definitely got "weird s*it". My desktop PC switched off as if there had been a power cut but my LED desk lamp was still on, as were the router and the various mesh network nodes around the house. My wife, in her home office in the bedroom, was not aware of any problem. I checked the "fuse box" for a tripped ring main - fine! The fridge and freezer were "on" (display of temperature but no motors... but then the thermostat may not have switched them on. Then I noticed that a mains power-monitoring plug was displaying a very faint "95.6 V". The miracle is that so many electronic devices were getting their full power - switched-mode power supplies are wonderful beasts! I know a lot are rated for input voltages from about 80V up to 260 V, and either 50 or 60 Hz, so they can be used anywhere in the world - just with a greater duty cycle if the mains voltage is lower.

Reminds me of the switched-mode PSU project that we all did at university

I would probably have known how to calculate the effect of losing one phase of a 3-phase transformer - but it was 45 years ago and the "use it or lose it" principle means I've forgotten.

power came back a little while ago so all computers, router, mesh nodes etc - and washing machine! - are now working.

Round here it tends to be falling branches. The 11kV is too widely spaced for a squirrel to stretch...

Well back in the day it was a reasonable way cost-benefit to string up overhead 11kV lines. With pole mounted transformers, when electricity was a luxury. not a necessity.

Then the trees grew. And the demands grew. And the dependency grew.

No one lays out overhead 11KV any more if they can help it.

It's all undergrounded at quite considerable cost.

But you save an enormous amount in maintenance. Tree pollarding here is now an annual preventative measure.

As are outages due to trees that are not on the annual schedule.

Have had similar here (though I don't know whether it was a lost phase, or lost neutral) anything "electronic" just carried on, the fridge was distinctly unhappy.

Indeed. Your 230v supply is the difference between 2 phases of a 380v

Andy Burns snipped-for-privacy@andyburns.uk wrote in news:ml6o4nFkfevU1 @mid.individual.net:

Watch out for fridges & freezers on brown-outs, it can lead to a compressor stall & burnout. Common in India where power supply irregularities are more common is a little monitor box plugged in before fridges to cut the power to them if it is badly out of spec. Possible that our modern appliances will monitor the supply and do similar, I don't know.

Many of today's switched mode power supplies are habppy from 48vDC to

Yup.

Nice project!

Precisely. Same here. And my CH boiler just crapped out....

Good work!

If you imagine a 3-phase transformer with the three output sides forming the star-connected final distribution to premises, with each domestic property working from one phase to neutral...

and the input sides connected to a delta-connected incoming mains, and just one of the incoming mains goes open circuit (say L3)...

then one of the output phases works as normal, giving 230V (the one with the transformer input connected betweeen L1 and L2). The other two output phases both see the outputs from transformers with inputs connected in series between L1 and L2 (via the open-circuit L3), so you would expect them to see half volts.

So I would expect one phase to keep working at 230/240V and the other two to see 115/120V.

nib

Too many words! Like this:

nib

Except those iron cores are not separate

On a phase failure here, I switched off at the incomer until it was repaired.

I didn't even need to do repeated measurements with a meter, to determine the all clear. Just watched a local traffic light, until the lighting on it was "normal again".

On the electrical system, the dining table serves up:

2) A rise in voltage. Incandescent bulbs pop. Bad for motors (heat from excess voltage). Bad for the audio power amp in your multimedia rack. Switch off at incomer and wait for service.

Paul

If the squirrel story was true, it was a phase-to-earth, I guess.

That's no comfort to a user in an all-overhead area !

We used to get a lot of brief one-second power cuts - just long enough to force all computer equipment to reboot :-(

The best excuse I had from Northern Powergrid was calves in a farmer's field using a wooden HV pole as a scratching post. Which was plausible if it made the wires swing so the touched each other or an overhanging tree... except they specified a field which did not have calves in it at that time. The truth was that (as so often happened until we kicked up a stink) NP were not pruning back branches *before* they got close enough to cause shorting.

After the brownout problem had been fixed, our power came back perfectly, but then later went off for three and a half hours while the engineers made further corrections. Been fine ever since.

Intriguingly, the woman who took my call and read the engineer's report said it was a fault in "a 20 kV line". Is there such a thing? I know of

I was sitting at my desk when my PC went off. Bugger: power cut, I thought. Hang on, my LED desk lamp is still on. Has the ring main tripped? No, the router and mesh node next door are still on (on same ring main). No circuit breakers have tripped. Then I noticed my power-monitoring passthrough plug on my desk (*) had a barely visible number. What did it say? 96 V? WTF!! I went upstairs to warn my wife and she was completely unaware, so her wifi connection via another mesh node and the one connected to the router was quite happy with 96 V instead of

We switched off everything with SMPSUs to avoid them overheating. We'd have turned off the fridge and freezer but those are on Kasa smart plugs

- mainly for the power-monitoring capabilities rather than the remote switch on/off capabilities, and it means pulling the fridge/freezer out to reach the switches (and there was too little mains to control the switches remotely). So those had to stay on - or else empty the fridge/freezer to make them light enough to move...

If I'd had a tungsten light bulb to hand, it would have been amusing to see how dim and orange it was. I forgot about the dining room which is the only room which still has tungsten bulbs, in the form of 40 W candle bulbs in a candelabra. Or is it candelabrum if there's just one?

(*) I'd been using it to monitor how much power a 5 V Raspberry Pi PSU used to see if I may have been overloading its 5 V output (if mains power is less than 10 W then output power can't be any higher than that so 5V at 2A is not being exceeded).

Ah yes, equipment being able to monitor mains voltage. My first indication that something was up was a strange error code on the washing machine that I was about to switch on. I'd just got to my PC to look up the code in the PDF instruction manual when the PC switched itself off. I looked up the code later on: "mains voltage too low". There are also other codes for "mains voltage too high" and "mains frequency out of spec".

Fridge and freezer are a bugger for us: the switches to turn them off are behind the appliances and can only be reached by emptying the fridge/freezer to make them light enough to pull out to reach the switches. They're on Kasa smart switches - but the mains voltage was too low for those to be operated remotely.