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Grid Voltage Reductions?

 
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jev



Joined: 24 Nov 2005
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PostPosted: Sun Oct 09, 2005 8:49 pm    Post subject: Grid Voltage Reductions? Reply with quote

Topic split from UK Energy Reports

With regard to John Hemmings post on this topic.

Quote:
We need to preemptively knock up parliamentary questions on these issues (including what are the effects of reducing mains voltage, what voltage reductions would be expected etc etc)

I understand what your saying about reduction in mains voltage etc but I believe that:- Power Consumed in watts = The Voltage x The Current in amps. Therefore to a degree if the mains voltage was dropped slightly I would imagine most devices would have a range of voltage tolerance where if voltage dropped the current would naturally need to increase slightly to allow it to aquire the power it requires to operate. Most household devices have internal transformers and/or transformer/rectifiers in to reduce or increase the voltage to what is required by the device and/or convert it from AC to DC etc. At the end of the day the power consumed (the energy part of it) would need to be roughly the same to operate a piece of equipment effectively no matter what voltage it is supplied at. There's no way we could go around significantly altering our mains voltage and still expect our current equipment to still work, plus the supply network probably wouldn't cope with the increased current at lower voltage - also more power loss occurs in transmission at lower voltage, this is why the national grid is at about 400,000 volts. Interestingly solar panels and I believe wind turbines generate their power in DC (which needs to be rectified to AC for consumption). As most of our electrical items work (at least on the primary side of the transformer) from AC this could lead to problems even if you do have a solar array (wired to the grid) - if and when the grid fails. Would the domestic inverter, which I expect transforms the low voltage DC from the panels to 230V AC for use, be able to cope in a stand alone situation and if it could the consumer would have to be very aware of overloading it/possible damage to it?
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mikepepler
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PostPosted: Mon Oct 10, 2005 7:31 am    Post subject: Reply with quote

jev wrote:
Power Consumed in watts = The Voltage x The Current in amps.

Correct, but remember that current = voltage / resistance, so as you drop the voltage, the current drops too. So you could say:
    power = (voltage x voltage) / resistance
so reducing the voltage does reduce the power consumption. This also means the grid won't have any problem (that I'm aware of) with a reduced voltage.

jev wrote:
Most household devices have internal transformers and/or transformer/rectifiers in to reduce or increase the voltage to what is required by the device and/or convert it from AC to DC etc. At the end of the day the power consumed (the energy part of it) would need to be roughly the same to operate a piece of equipment effectively no matter what voltage it is supplied at. There's no way we could go around significantly altering our mains voltage and still expect our current equipment to still work

This is true, but a lot of electronic devices can operate over a range of voltages. For example, you might find a PC's CPU can operate from 1.25-1.4V, so if you start off in the middle of that there's a fair percentage range you can move within (though if you move much the clock rate may need to change). When you consider the power delivered is proportional to the square of the voltage, you don't need to drop the voltage much to get a big saving, so I think most electronics will remain OK. Things like vacuum cleaners, fridges, kettles, etc. are less fussy up to a point, and they're also the energy guzzlers!

jev wrote:
Interestingly solar panels and I believe wind turbines generate their power in DC (which needs to be rectified to AC for consumption). As most of our electrical items work (at least on the primary side of the transformer) from AC this could lead to problems even if you do have a solar array (wired to the grid) - if and when the grid fails. Would the domestic inverter, which I expect transforms the low voltage DC from the panels to 230V AC for use, be able to cope in a stand alone situation and if it could the consumer would have to be very aware of overloading it/possible damage to it?

Now this is an interesting one. The inverters used with small scale solar and wind systems tend to sense the waveform of the AC on the grid and generate a signal to match it. While it would certainly be possible to design an inverter to generate over a range of voltages, I don't know if this has been done. If not, I would expect that if the grid voltage moves outside of a pre-determined range, then the inverter would simply disconnect, which would be a shame!
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johnhemming



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PostPosted: Mon Oct 10, 2005 9:40 am    Post subject: Reply with quote

Lightbulbs and kettles tend to take what they are given. The domestic load is quite substantial.
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revdode



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PostPosted: Mon Oct 10, 2005 10:15 am    Post subject: Reply with quote

The problem with reducing voltage is it actually increases losses, the gird uses HV to reduce current so reducing power losses due to resistance. If you reduce grid HV and try to delivery the same amount of power you increase current and increase grid losses. Alternatively if you change substations you increase local delivery losses although I don't think the increase in losses dropping from 230V to 200V would be that great. It isn't always possible to change substations to achieve this, our local delivers 240V-250V and its tapped down as far as it goes.
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mikepepler
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PostPosted: Mon Oct 10, 2005 1:52 pm    Post subject: Reply with quote

revdode wrote:
If you reduce grid HV and try to delivery the same amount of power you increase current and increase grid losses.

That would be true - but we're not trying to deliver the same amount of power here - we're trying to deliver less - that's the whole point. We reduce the voltage the customer sees, and the current drops too.
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revdode



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PostPosted: Mon Oct 10, 2005 4:21 pm    Post subject: Reply with quote

mikepepler wrote:
revdode wrote:
If you reduce grid HV and try to delivery the same amount of power you increase current and increase grid losses.

That would be true - but we're not trying to deliver the same amount of power here - we're trying to deliver less - that's the whole point. We reduce the voltage the customer sees, and the current drops too.


This may be true of simple light bulbs but many appliances including fluorescent fittings will drawing more current to compensate. I'm pretty sure that dropping the voltage will actually result in some of these loads failing. This problem (like most energy problems) can't be sorted at the supply side alone.
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mikepepler
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PostPosted: Tue Oct 11, 2005 8:35 am    Post subject: Reply with quote

revdode wrote:
mikepepler wrote:
revdode wrote:
If you reduce grid HV and try to delivery the same amount of power you increase current and increase grid losses.

That would be true - but we're not trying to deliver the same amount of power here - we're trying to deliver less - that's the whole point. We reduce the voltage the customer sees, and the current drops too.


This may be true of simple light bulbs but many appliances including fluorescent fittings will drawing more current to compensate. I'm pretty sure that dropping the voltage will actually result in some of these loads failing. This problem (like most energy problems) can't be sorted at the supply side alone.

It's true for light bulbs, kettles, heaters, cookers, most (maybe all?) motors. Not sure what will happen with fluorescent light bulbs, but I don't think they'd draw extra power - they'd just stop working after a point. Basically, all the energy-guzzling devices we have in our homes will reduce their consumption with falling voltage.
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revdode



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PostPosted: Tue Oct 11, 2005 10:20 am    Post subject: Reply with quote

mikepepler wrote:

It's true for light bulbs, kettles, heaters, cookers, most (maybe all?) motors. Not sure what will happen with fluorescent light bulbs, but I don't think they'd draw extra power - they'd just stop working after a point. Basically, all the energy-guzzling devices we have in our homes will reduce their consumption with falling voltage.


I agree with the simple loads up to but not including motors although my experience is industrial rather than residential. A lot of motors are constant power loads, if you reduce voltage the motor will draw more current up to a point. In older motors this can be the point at which it melts. Fluorescent lights with electronic balasts are also constant power devices, I think a lot of switched mode power supplies also fall into this category.
Reducing supply voltage on heating elements (kettles, heaters etc.) will increase heating times reducing (although not elimating) the savings made. Reducing supply voltage to older fluorecent fittings and incandescent fitting will reduce power drawn and also reduce light levels, again in an industrial or office environment this may not be acceptable.

I'm not trying to poor cold water on this idea, I know some American suppliers have had some success with similar practices, I'm just pointing out it isn't as simple as it seems at first. From memory I think the power saving is around half of the theoretical simple saving assuming simple loads. This is all being dragged from the deepest recesses of my brain, I haven't studied electrical transmission for over twenty years so it's a little fuzzy.
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mikepepler
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PostPosted: Tue Oct 11, 2005 10:56 am    Post subject: Reply with quote

Cheers revdode, that's useful info, I have no experience on industrial motors. I guess brushless induction motors behave differently to brushed motors - I was thinking of things like vacuum cleaners running more slowly. You could well be right about switch-mode PSUs as well - these are used in the more modern low-energy lights aren't they?

I see your point about kettles boiling more slowly and the total energy being the same, but I think the important point here is reducing peak demand, so maybe this is enough to help, just by spreading the load out over a few more minutes.

So, half the theoretical saving? Sounds like it's worth doing in an emergency...
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Bandidoz
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PostPosted: Wed Oct 12, 2005 1:19 am    Post subject: Reply with quote

NGT already have "reduce voltage" as part of their "shed load" policy. This buys them a little time to warn their "interruptible" users that supply is going to be cut off. IIRC the voltage reduction can only be in the range of 10% or so, beyond which the frequency starts to drop.

Bear in mind that a 10% voltage reduction would result in a 19% power reduction of passive loads. (0.9*0.9 = 0.81)
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RogerCO



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PostPosted: Wed Oct 12, 2005 6:25 pm    Post subject: Reply with quote

I thought that the generating companies (including domestic microgenerators) were legally obliged to keep the supply within fairly tight limits on both frequency and voltage.

I'm sure I got this link from a post hereabouts (the meter appears to be not working today).

The legal limits are referenced here
Voltage +-6% (nominal 230v ?), frequency +-1% (nominal 50Hz)

Reducing load or frequency is a possible way to respond to excess demand, but as someone mentioned above most modern electronic equipment uses switched-mode power supplies so the power drain is constant over a very wide range of volatge & frequency (having the big advantage that you can produce a single model to work off any voltage or frequency used worldwide).

Incidentally in case you wanted to know what the Govt plans to do in response to a fuel crisis read this pdf
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Bandidoz
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PostPosted: Thu Oct 13, 2005 12:43 am    Post subject: Reply with quote

The tolerance on frequency is a lot tighter than that on voltage. Voltage reductions are the first response to keep the system "on-frequency". The normal operating target is specified in here:

http://www.ofgem.gov.uk/temp/ofgem/cache/cmsattach/7189_9904b.pdf

Quote:
Target Frequency: That Frequency determined by NGC, in its reasonable opinion, as the desired operating Frequency of the Total System. This will normally be 50.00Hz plus or minus 0.05Hz, except in exceptional circumstances as determined by NGC, in its reasonable opinion when this may be 49.90 or 50.10Hz. An example of exceptional circumstances may be difficulties caused in operating the System during disputes affecting fuel supplies.


Picotech (who do PC-based digital oscilloscopes) ran a monitor on mains supply in this experiment:

http://www.picotech.com/experiments/mains_voltage/results.html

Quote:
Incidentally in case you wanted to know what the Govt plans to do in response to a fuel crisis read this pdf


Interesting document - particularly about "rota disconnections". So now we know where to hang out Razz

Quote:
'V' LIST CONSUMERS: VITAL SERVICES
- Coal Mines.
- Major airports and associated control facilities.
- Railway operations.
- Gas reception terminals; gas storage installations including gas boosting and
compression equipment; gas compressor stations and principal development and
control sites which provide facilities for the control of gas supply systems and
emergency procedures.
- Licensed Electricity generators, including nuclear stations so as to ensure the safety
requirements of their nuclear licences continue to be met; electricity transmitters and
distributors.
- Essential water and sewerage installations.
- Hospitals as agreed in consultation with health authorities and NHS trusts.
- Ports and Docks.
- Postal, telecommunications and broadcasting services
- Oil refineries and vital oil pipeline pumping stations.

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