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Voltage variations of grid supplies.
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adam2
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PostPosted: Mon Sep 12, 2011 12:40 pm    Post subject: Voltage variations of grid supplies. Reply with quote

It would appear that the actual voltage supplied varies a lot, and that these variations are increasing.

The nominal declared voltage in the UK used to be 240 volts, plus or minus 6%
Some years ago it was altered to 230 volts, plus 10% and minus 6%

The current limits are therefore from 217 volts up to 253 volts.
In most urban areas the actual voltage supplied tends to be around 240/245 volts and does not normally alter much.

In rural and suburban areas, the voltage varies a lot more and tends to be close to the upper limit at low load, and close to the lower limit at peak load.
Allowing for voltage drop in the consumers wiring, the voltage at the point of use could be as low as 200 volts.

Modern appliances should tolerate such a range, but performance of heaters and motors can be marginal near the bottom end of the range.

Short term during breakdowns, the voltage can drop well below normal.
These breakdowns appear to be becoming more frequent.
In some circumstances (such as theft or failure of the neutral) the single phase voltage can reach as much as 440 volts.
As well reported elswhere, theft and the consequent overvoltages, is a growing problem.

Brief but extreme overvoltage is often caused by lightning or storm damage.
This appears to be happening more frequently, perhaps due to climate change.

When planning a new electrical installation, it would seem advisable to allow for worsening power qaulity in years to come.
UPS units and voltage regulators can be useful, but cost money and add losses.
Surge divertors help to an extent if of good qaulity.
Brief failures can kill fridge motors, devices to prevent this are available.

Or go off grid ?
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Potemkin Villager



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PostPosted: Wed Sep 14, 2011 10:28 pm    Post subject: Reply with quote

Had an experience of this earlier today about 0830. We are connected to a pole mounted transformer on a rural 20kV line about 8 miles from our nearest 110kV sub station so the grid is not that weak.

I was working on the computer when I noticed some relays in a power supply started chattering and then a CFL started flickering however the computer remained fine. I put a meter across the mains and it was reading 120V and then collapsed to zero. The period at half voltage was about 3 minutes. I then unplugged our fridge and freezer and supply came back on at 240V about 1/2 an hour later when I reconnected the fridge and freezer.

I have left the meter connected today and at various stages have read between 230 and 250 volts.

It is easy to be fooled as to what is happening with computers and CFLs as they are quite voltage tolerant unlike our old washing machine and spin dryer motors! An old fashioned high energy bulb is a much better and very sensitive indicator of any strange things going on.

What are these protective devices you are referring to adam2? I had in mind making up a simple undervoltage detector and using a relay to
put enough of a L-E fault to trip the consumer unit RCCD but this would not be too clever if we were away for a few days and the fridge and freezer stayed off untill we got back!
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adam2
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PostPosted: Wed Sep 14, 2011 11:36 pm    Post subject: Reply with quote

Most loads are fairly tolerant of voltage reductions, as you found.
Refrigeration appliances are however vulnerable (fridges, freezers, de-humidifiers, ice makers, air conditioners etc)
Sollatek make a range of protective devices
http://www.sollatek.com/pdf/Manuals/Guard%20Instruction%20Booklet%20Rev%201-0%20Mar%2004.pdf
Which give a good degree of protection.
The fridgegaurd is of course for fridges etc.

This prtects against undervoltage, and also has a start delay.
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An Inspector Calls
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PostPosted: Sat Nov 05, 2011 4:47 pm    Post subject: Reply with quote

I'm on a rural supply with my very own pole-mounted transformer.

Some time ago I noticed very low supply voltages to the house. We had storage heaters then, and when they came on our voltage collapsed to below 200 VAC. On grotty audio power amplifiers, the ripple hum was quite loud!

So I complained to the supplier, gave them some numbers, and within a fortnight we had a brand new transformer.

Supply voltages have never been a problem since. But walking round the fields, I see many of our old type pole transformers still in service!

Complain! As I understand it, supply voltage requires statutory compliance - for safety reasons.
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adam2
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PostPosted: Sat Nov 05, 2011 7:27 pm    Post subject: Reply with quote

The low voltage experienced was most likely due to the transformer being overloaded, the new one is almost certainly of greater capacity.

The older transformers still in use should be fine provided that the load is modest.

The improved voltage being due to the transformer being larger, not because it is newer.

If the voltage is outside the legally required tolerance, then complaining to the network operator will normally produce an improvement.
BE CAREFULL THOUGH !! before complaining, as it is not widely known that additional loads of more than 3KW or motors of more than 1HP should be notified to the supplier before connection.
The customer may be required to pay for network improvements to meet the extra load.
I have heard of this being enforced retrospectively as a result of complaints of low voltage.
If the voltage is only low when the customer is useing in excess of that permitted, then I advise keeping silent.

There is a rumour that the lower limit of voltage may be reduced to 207 volts, this being 230 minus 10%.
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PostPosted: Sun Nov 06, 2011 12:23 am    Post subject: Reply with quote

Yes, of course, it has nothing to do with newness, simply transformer rating.

It's perhaps a give-away that at the same time they changed the incoming line fuse holder, discarded the 40 A fuse and replaced it with 100 A!

I wouldn't recommend silence. It should be an accepted norm that one can connect an electric cooker to a domestic supply, and there'll be circumstances when that will cause problems on old transformers. If it goes uncorrected there's a risk that the poor regulation of the supply may fail to clear fault loads.
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kenneal - lagger
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PostPosted: Sun Nov 06, 2011 5:54 am    Post subject: Reply with quote

Have you offered to pay for a new transformer for your shiny new heat pump, inspector? That must be well into the 3 to 8kW range and is probably what caused your problems. Or are other consumers subsidising your installation?
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adam2
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PostPosted: Sun Nov 06, 2011 6:09 am    Post subject: Reply with quote

Silence MIGHT be the best option IF the consumer has connected loads in excess of that permitted, and IF the voltage is low only at times of high load.
Night storage heating is often a substantial long hour load, and at a time when extra load is welcomed. The network operator will often replace a transformer for free in such cases.

They are not so keen on electric showers and cookers as these tend to use a lot of current, but produce little revenue due to the short hours of use.
Whilst it may seem reasonable that an electric cooker should be able to be supplied, this is not the case.
A cooker is almost cetainly in excess of 3KW and therefore should be notified, electric showers likewise.
Many rural supplies are only 40 amps, which is unlikely to be judged sufficient for an electric cooker or shower.
In practice the equipment, including the 40 amp service fuse, will often survive short term overloads to say 60 amps. It would however be unwise to complain that the voltage is low when the 40 amp service is loaded to 60 amps !

Many old fashioned rural dwellings with a 40 amp supply were fine with coal or wood being used for most cooking, space heating, and water heating.
That would allow normal use of lighting, entertainment, refrigeration and other low loading appliances, together with moderate use of electric kettle, washing machine, portable heater, power tools and the like.
Such properties are sometimes "improved" by the addition of large electric cookers, multiple electric showers, and a tumble dryer.
A 100 amp service is marginal for that lot !
It may come as a shock to find that the customer has to pay many thousands of pounds for a supply upgrade.
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lancasterlad



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PostPosted: Sun Nov 06, 2011 10:12 am    Post subject: Reply with quote

I had the opposite problem - the voltage was above the specified limits. GS Light bulbs would last no time at all. I complained and the local supplier fitted a monitoring unit for a week or so, agreed there was a problem and made the adjustments at the local transformer.
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kenneal - lagger
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PostPosted: Mon Nov 07, 2011 3:31 am    Post subject: Reply with quote

An Inspector Calls wrote:
.......

The load of the heat pump is 3 kW. Why on earth would you think that it might be 8 kW?

Do you know anything about heat pumps?


Yes, I do. I used to carry out energy surveys for a company selling heat pumps in the mid seventies, so I have had some experience of them. They sold a range of German made air source heat pumps ranging from 2kW, for domestic hot water supply only, to 16 kW output, for larger houses, inputs from 500W to 4kW.

I owned a 2kW model which we placed in our loft to make use of the solar gain from the dark coloured roof tiles. When we moved to our current farm we took it with us and placed it in our generator shed to make use of the engine heat and provide hot water for our milking parlour. Unfortunately the generator caught fire and the heat pump went up with it.

The reason I bought it was that it was an end of range model offered to staff at a very good price and because of the small size I could set it up in a situation where it was in a warmed environment to ensure it worked at a high COP most of the time. Most installations cannot guarantee that situation.
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DominicJ



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PostPosted: Mon Nov 07, 2011 2:08 pm    Post subject: Reply with quote

The supply side stuff is quite interesting, how would I check?
Quietly...
Being that I have an electric oven/hob, that was at some point gas?
Although I now run the shower off the combi boiler, but very much doubt the electric one was installed by a competant person, never mind the DNO notified?
Generaly speaking, I tend to avoid using big loads at the same time (my oven/hob can pull at least 50amps) but did one have my washer/dryer/dishwasher/two hobs and one oven going, and just though, this could be silly....
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adam2
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PostPosted: Mon Nov 07, 2011 2:41 pm    Post subject: Reply with quote

DominicJ wrote:
The supply side stuff is quite interesting, how would I check?
Quietly...
Being that I have an electric oven/hob, that was at some point gas?
Although I now run the shower off the combi boiler, but very much doubt the electric one was installed by a competant person, never mind the DNO notified?
Generaly speaking, I tend to avoid using big loads at the same time (my oven/hob can pull at least 50amps) but did one have my washer/dryer/dishwasher/two hobs and one oven going, and just though, this could be silly....


Your electricity bill should show the maximum KVA available from your supply.
Enquiry of your supplier might also give this information.
The holder for the service fuse will have a rating in amps marked on it, but this is is the maximum, it might contain a smaller fuse.

Most domestic supplies are 60, 80 or 100 amps, though a few are only 40 amps, and a few are 125 amps. All single phase.
3 phase domestic supplies exist but are not common.

Additional loads of more than 3KW are meant to be notified, though I suspect that most are not.
Unless the supply is known to be ample, I would avoid useing an electric shower at the same time as a large cooker.
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Potemkin Villager



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PostPosted: Tue Nov 08, 2011 8:37 pm    Post subject: Reply with quote

kenneal - lagger wrote:


Unfortunately the generator caught fire and the heat pump went up with it.



Shocked Shocked Rolling Eyes

The more I hear of kenneal's powerswitch adventures the
more I am convinced he should tell all in a book!
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adam2
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PostPosted: Tue Nov 08, 2011 9:16 pm    Post subject: Reply with quote

Roger Adair wrote:
kenneal - lagger wrote:


Unfortunately the generator caught fire and the heat pump went up with it.



Shocked Shocked Rolling Eyes

The more I hear of kenneal's powerswitch adventures the
more I am convinced he should tell all in a book!


Generators DO catch fire, not very often, but often enough to make placing them away from living spaces or valuables a good idea.

BTW, I had just gone to sleep next to a 500KVA generator (having convinced myself that it made a gentle purring sound) when the wretched thing blew up.
Covered in hot oil Sad not able to sleep for rest of the night, pyjamas and blankets thrown away.
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kenneal - lagger
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PostPosted: Wed Nov 09, 2011 2:25 am    Post subject: Reply with quote

My generator that caught fire was right next to a 1500 ltr diesel tank. While my wife was calling the fire brigade and we waited for them to arrive I was sheltering behind a nearby building directing a garden hose onto the tank to try and stop the diesel boiling out from it and catching fire as well! Luckily the fire brigade arrived in time.

One day a book might come of our experiences. I'm just practising here first.
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