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Voltage variations of grid supplies.
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Potemkin Villager



Joined: 14 Mar 2006
Posts: 776
Location: Narnia

PostPosted: Wed Nov 09, 2011 9:22 am    Post subject: Reply with quote

adam2 wrote:


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.


It sounds like you were trying to save on your overnight subsistance!

I have never made a habit of sleeping with gennys but once spent
a night in an estate car in the Yorkshire Dales trying to sort an intermittent fault on a wind turbine in a howling gale. Gentle and purring
are not words I would use to describe the experience.

Amazing the daft things we do when young and keen. Rolling Eyes
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mobbsey



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PostPosted: Mon Nov 14, 2011 7:31 pm    Post subject: Re: Voltage variations of grid supplies. Reply with quote

adam2 wrote:
It would appear that the actual voltage supplied varies a lot, and that these variations are increasing.

Changes in voltage will be absorbed in older equipment by changes in reactance -- trading off voltage for current. Shouldn't be a problem but it can push the efficiency of your equipment off by changing the power factor, and that costs money (particularly a problem with industrial motors and such things because it affects heat dissipation).

In new equipment most switched mode supplies won't blink an eye at swings in the input voltage -- they just modify the duty cycle to accommodate the change. In linear supplies (ones with heavy transformers) the output change is a small fraction of the input change -- e.g. at 12 volts the change in output voltage is about one twentieth of the input.

The bigger problem, and this seems to affect rural areas more, are power spikes from switching the inductive load of the grid itself -- if you look at these spikes on an oscilloscope they look a little like the seismic traces of earthquakes. Good old linear power supplies and big heavy inductive loads don't worry about those, but the switched mode supplies of an awful lot of modern consumer electronics can accumulate damage if this happens on a regular basis -- until they fail. What affects how well the PSU mops up spikes is the cost of the components. Often you find that the better the supply, the more insults it'll take.

Personally, I think the more you can avoid using electricity the better. It's not just that it costs money; you become dependent upon the machinery involved, which obviously presents problems when it goes wrong. E.g., recently my trusty power drill sheared its gearbox after 11 or 12 years of use; I've really enjoyed using a brace and bit/hand drills since then, but it does make you appreciate the role of electricity in our everyday life far more!

Off-grid is a good way of doing this because it forces you to rationalise your use of power in order to afford the equipment involved. However, generators are just a techno-fix -- if you really want to master energy use then you need to go non-fossil fuel off-grid. That doesn't preclude the use of combustion engines (either internal or external), but the lack of convenience makes you focus on the demand-side of the problem rather than the simplistic supply-side of the equation.
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kenneal - lagger
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PostPosted: Mon Nov 14, 2011 10:12 pm    Post subject: Re: Voltage variations of grid supplies. Reply with quote

mobbsey wrote:
Off-grid is a good way of doing this because it forces you to rationalise your use of power in order to afford the equipment involved. However, generators are just a techno-fix -- if you really want to master energy use then you need to go non-fossil fuel off-grid. That doesn't preclude the use of combustion engines (either internal or external), but the lack of convenience makes you focus on the demand-side of the problem rather than the simplistic supply-side of the equation.


Couldn't agree more. When you have to tot up the power being used already before you decide whether to switch another appliance on you have an awareness of the value and limitations of that power.
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Potemkin Villager



Joined: 14 Mar 2006
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Location: Narnia

PostPosted: Wed Nov 16, 2011 9:04 pm    Post subject: Re: Voltage variations of grid supplies. Reply with quote

kenneal - lagger wrote:
mobbsey wrote:
Off-grid is a good way of doing this because it forces you to rationalise your use of power in order to afford the equipment involved. However, generators are just a techno-fix -- if you really want to master energy use then you need to go non-fossil fuel off-grid. That doesn't preclude the use of combustion engines (either internal or external), but the lack of convenience makes you focus on the demand-side of the problem rather than the simplistic supply-side of the equation.


Couldn't agree more. When you have to tot up the power being used already before you decide whether to switch another appliance on you have an awareness of the value and limitations of that power.


But "lagger" ( :>) ) how can all this be squared with ZCB's plans to install mega massive amounts of wind generation capacity all of which is to be conventionally grid connected?

If the voltage variation of grid supplies remains at minus 100% for an extended period then all these wind turbines will be left sitting around with zero capacity factor no matter how much wind is blowing.

Shocked Shocked
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kenneal - lagger
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PostPosted: Thu Nov 17, 2011 6:27 am    Post subject: Reply with quote

I wasn't suggesting that every body should be off grid just that people should experience it for a while to make them think about what they are using.

CAT do this with school groups who visit and stay in the Eco-cabins they have there. These have a limited power supply from PV and a small pond supplying water to a hydro turbine. If people leave lights on or use too much from the power sockets the lights go out and the power goes off. The kids are left with emergency lighting only, not very bright at all, and have to wait until someone comes along to switch them into the main grid. Works a treat apparently.
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RenewableCandy



Joined: 12 Sep 2007
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PostPosted: Thu Nov 17, 2011 7:25 pm    Post subject: Reply with quote

Apparently there is a new standard (G5/4) for mains quality (lack of harmonics, spikes, etc) and upsets can be caused by neighbours using large/sudden/unusual loads.
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Potemkin Villager



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PostPosted: Thu Nov 17, 2011 7:58 pm    Post subject: Reply with quote

kenneal - lagger wrote:
CAT do this with school groups who visit and stay in the Eco-cabins they have there. These have a limited power supply from PV and a small pond supplying water to a hydro turbine. If people leave lights on or use too much from the power sockets the lights go out and the power goes off. The kids are left with emergency lighting only, not very bright at all, and have to wait until someone comes along to switch them into the main grid. Works a treat apparently.


This is certainly a very admirable and useful demonstrator which perhaps needs implemenation on a much wider scale to get the message across to a significant percentage of the adult population as well. I wonder how many folk here remember the three day week and selective power cuts?
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RenewableCandy



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PostPosted: Thu Nov 17, 2011 10:55 pm    Post subject: Reply with quote

Ah those were the days!

Guests were due and RenewableBro and I thought how nice it would be for them if there were a roaring fire in the hearth...so we lit one. And it didn't take, and didn't take, and we thought funny it worked last time.

Enter guests and parents to find RenewableBro and myself practically kippered and almost unconscious in a thick soup of smoke...we'd only NOT realised the chimney had been blocked with olde papers to keep the heat in!!
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kenneal - lagger
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PostPosted: Fri Nov 18, 2011 3:14 am    Post subject: Reply with quote

So Renewablemum & Renewabledad were greenies as well then?
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Potemkin Villager



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PostPosted: Fri Nov 18, 2011 12:32 pm    Post subject: Reply with quote

RenewableCandy wrote:


Enter guests and parents to find RenewableBro and myself practically kippered and almost unconscious in a thick soup of smoke...we'd only NOT realised the chimney had been blocked with olde papers to keep the heat in!!



I managed the same effect with an infrequently used fireplace here
because crows had thoughtfully built a nest in the chimney pot to keep the heat in.

As Candy discovered blocking a flue and hence restricting the draught has rather detrimental effects on combustion efficiency, user comfort and safety. Luckily Candy and renewbro did not turn blue and lived to tell the tale.
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RenewableCandy



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PostPosted: Fri Nov 18, 2011 8:08 pm    Post subject: Reply with quote

kenneal - lagger wrote:
So Renewablemum & Renewabledad were greenies as well then?
Well yes and no, Minister. This was the 1970s and we were learnng to live with power cuts and the like. We were also in the middle of nowhere and hence orf the Gas grid.

Interestingly, they were among the first people, as far as I know, to get Clearview stoves (but that was about 20 years on from this little tableau).
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kenneal - lagger
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PostPosted: Fri Nov 18, 2011 8:37 pm    Post subject: Reply with quote

Excellent machines, Clearview stoves. We got one about 14 years ago and used it continuously for about 7 years. It had a couple of years off, was reconditioned - new liners, door seals and a spot of paint - and has been used ever since then by my daughter and son-in -law in their log cabin.
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adam2
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PostPosted: Tue Apr 30, 2013 10:24 am    Post subject: Reply with quote

To return to voltage variations, last week I was requested to look at the wiring in a cottage "because the lights go very dim if we use the shower"

On inspection I found the cottage had no grid supply of its own but was supplied from a neigbouring farm via a check meter and buried cable.

Voltage at farm intake 220 volts
Voltage at barn from which the cottage was supplied 210 volts
Voltage at cottage about 200 volts until the shower was turned on when it dropped to about 170 volts.

The farmer stated that the voltage has recently been reduced and that it was formerly about 250.
Starting with 250 volts gives a lot more leeway than starting with only 220 volts.

Supply cable was only 4mm, and on a 45 amp fuse "because the 30 amp one kept blowing"
I have advised against use of an electric shower.
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Tarrel



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PostPosted: Wed May 01, 2013 10:59 am    Post subject: Reply with quote

Actually, current would be my biggest concern with voltage fluctuation. Would I be correct in thinking that, say a 3kW load was running at 250V, the current flow would be 12 Amps, but if the same load was running at 200V, the current flow would be 15 Amps? If so, cabling that was under-specified, or "on the limit" could be at risk of overheating or fire if the voltage dropped.

Correct me if I'm wrong.
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adam2
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PostPosted: Wed May 01, 2013 11:40 am    Post subject: Reply with quote

Tarrel wrote:
Actually, current would be my biggest concern with voltage fluctuation. Would I be correct in thinking that, say a 3kW load was running at 250V, the current flow would be 12 Amps, but if the same load was running at 200V, the current flow would be 15 Amps? If so, cabling that was under-specified, or "on the limit" could be at risk of overheating or fire if the voltage dropped.

Correct me if I'm wrong.


It depends on the type of load.
A simple resistance heater will use LESS current on a lower supply voltage. There is no question of overloading wiring.
If the heater uses 12 amps at 250 volts, then at 200 volts it will only use 9.6 amps.
The heat output would be much reduced, to about 2KW.
The problem would arise if the reduced heat output was noticed and compensated for by the use of additional heaters.
Consider an area heated by a pair of 3KW heaters, at 250 volts they will each use 12 amps, or 24 amps in total.
Now suppose that the supply voltage drops to 200 volts and each heater uses 9.6 amps, or 19.2 amps in total, no problem.
But what if someone notices that the space is not sufficiently warmed and uses a third heater ? Each of the three heaters now uses 9.6 amps or 28.8 amps in total. That might be a problem if the original wiring was only just suitable for 24 amps.
In theory the worst that could happen would be a blown fuse, but in practice a problem might occur.

Other loads such as switched mode power supplies use constant power, and at a lower supply voltage will use more amps.
3KW worth of IT equipment and electronic lamp ballasts will indeed use 12 amps at 250 volts, and 15 amps at 200 volts.
A properly designed installation should be designed accordingly, but in practice it might be a problem in cases that are already marginal.

When assesing the current demand of an installation, I presume an actual voltage of 250 volts for heaters as this is close to the maximum that can reasonably be expected.
In the case of switched mode power supplies then I presume an actual voltage of 200 volts as this is about the lowest that can reasonably be expected.
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