Outside Electrics

[DetlefSchmitz]

Yes, I understand that, but I thought that they sensed (and thought that eric was talking about them sensing) the grid supply voltage, rather than just their own output (which presumably will also be 'capped'). Is that not the case?

As you presumably understand, it was eric's comment about voltage drop affecting what situations would cause the inverter to shut down that I didn't really understand.

If you are suggesting that they have a second cable running to the supply point to measure the voltage without voltage drop, in addition to the cable that they have to feed power to that point; then no they don't.

 

[JohnW2]

If you are suggesting that they have a second cable running to the supply point to measure the voltage without voltage drop, in addition to the cable that they have to feed power to that point; then no they don't.

I wasn't clear enough. No, I wasn't suggesting a second 'sensing cable' (experiencing virtually no voltage drop) but, rather, that the inverter would sense the voltage at the point is was connected (in the case being considered, fairly remote from the main installation).

The (5.7V) VD that eric seemed to be considering was that due to 4 kW worth of solar-generated electricity (after the inverter) travelling in the 32m cable run back to the house. That would mean that the voltage of the generated electricity would be 5.7V less than at the inverter. However, since (in the absence of a second 'sensing' cable such as you mention), all the inverter knows about is the voltage at its end of the cable, I don't really see how that VD comes into the equation, and hence can't really understand ...

... [the inverter] needs to monitor the voltage and auto shut down if not within 207 to 253 volts .... So 32 meters of cable at 4 mm² at 17.5 amp looking at 5.7 volt drop, which would mean if the voltage reaches 247.3 volts the system will lock out ...

What am I missing ?

Kind Regards, John

 

[DetlefSchmitz]

(pressed wrong button)

 

[DetlefSchmitz]

I wasn't clear enough. No, I wasn't suggesting a second 'sensing cable' (experiencing virtually no voltage drop) but, rather, that the inverter would sense the voltage at the point is was connected (in the case being considered, fairly remote from the main installation).

The (5.7V) VD that eric seemed to be considering was that due to 4 kW worth of solar-generated electricity (after the inverter) travelling in the 32m cable run back to the house. That would mean that the voltage of the generated electricity would be 5.7V less than at the inverter. However, since (in the absence of a second 'sensing' cable such as you mention), all the inverter knows about is the voltage at its end of the cable, I don't really see how that VD comes into the equation, and hence can't really understand ...
What am I missing ?

In your example, if the network is (validly) at 253V then the inverter would see 258.7V and therefore be unnecessarily obliged to turn off.

 

[JohnW2]

In your example, if the network is (validly) at 253V then the inverter would see 258.7V and therefore be unnecessarily obliged to turn off.

It was eric's example, not mine.

Maybe I'm getting there! Given that the supply voltage was only 253V, the only way in which the inverter could 'see' 258.7V is if it were generating that voltage in order to get 17.5A (~4 kW) to flow through the cable (with 253V at the other end) which, as I said, is something I would assume it would 'refuse to do'. So, yes, I think I now understand what is being said (and agree with the 'bottom line' - but not so much because it "sees 258.7V" (since I suspect that it never would) but, rather, since it refuses to generate an output >253V, hence cannot send current down that cable if there is 253V (or more) at the other end.

Is that about right?

Kind Regards, John

 

[DetlefSchmitz]

It was eric's example, not mine.

Maybe I'm getting there! Given that the supply voltage was only 253V, the only way in which the inverter could 'see' 258.7V is if it were generating that voltage in order to get 17.5A (~4 kW) to flow through the cable (with 253V at the other end) which, as I said, is something I would assume it would 'refuse to do'. So, yes, I think I now understand what is being said (and agree with the 'bottom line' - but not so much because it "sees 258.7V" (since I suspect that it never would) but, rather, since it refuses to generate an output >253V, hence cannot send current down that cable if there is 253V (or more) at the other end.

Is that about right?

Well yes. Of course it never sees 258.7V because it shuts off before that happens. That is just to use the number you quoted to explain that it would see more than 253V if it didn't shut off. I have 15m of cable from the inverter, and made it thicker than recommended because we have a constant 250V supply. If my inverter stopped working I would have to start paying for electricity!

 

[JohnW2]

Well yes. Of course it never sees 258.7V because it shuts off before that happens. That is just to use the number you quoted to explain that it would see more than 253V if it didn't shut off.

Fair enough - but, as I said, they were eric's numbers, not mine

I think I now understand, but I also think that the way things were worded was rather confusing (at least, to me!). As you seem to agree agree, the inverter does not really shut off 'because it sees' a voltage of 258.7V (or any other voltage above 253), since, as I think we've agreed, it never will actually 'see' such a voltage (except maybe very transiently).

Maybe it's just me, but I find it clearer ('less confusing') to think of the situation as being that the inverter shuts off if, given the voltage at the other end of the cable and the VD along that cable, it would have to raise its output voltage to above 253V (although it doesn't ever actually do that, except perhaps very transiently) to get current to flow in the desired direction.

Do you think that's a reasonable way of thinking about it?

I have 15m of cable from the inverter, and made it thicker than recommended because we have a constant 250V supply. If my inverter stopped working I would have to start paying for electricity!

Fair enough, and that seems consistent with what I say above - i.e. that your inverter will shut off if it would have to (but doesn't) raise its output voltage to >253V to get current to flow in the right direction (given supply voltage and VD).

Kind Regards, John

 

[DetlefSchmitz]

I'm really not sure why you are having such a problem with Ohm's Law. I don't have anything else to add I'm afraid.

 

[JohnW2]

I'm really not sure why you are having such a problem with Ohm's Law. I don't have anything else to add I'm afraid.

I suppose my problem was that Ohm's Law doesn't really apply to a voltage which doesn't exist

Kind Regards, John

 

[ericmark]

I used a program which compensates when the cable is not at maximum rating, so maybe figures not spot on. I am unsure of the rules on shut down, as far as I am aware the shut down is to stop the supply being made live when some one is working on the system, so I would assume it needs a manual reset? The same with EV charging, using voltage to detect loss of PEN needs a manual reset.

This is it seems a major problem, as often the home user is unaware the system has tripped out until it is too late. i.e. come to use the car.

My house roof faces East/West and when I looked into solar panels it seems this can work out better as the output is for a longer time, but in my case it would not work as sun raises above the hill latter and sinks behind the hill earlier as we live in a valley, yes I know direct sun light not required, but coupled with three story house and problems with roof access not really a good idea.

The balance is assuming by the age of 90 either dead or in a old peoples home, it has to pay for its self in 20 years, really less on balance, want to be able to use the money it saves. Resale valve of house does not matter, as likely dead before it is sold, so want it really to pay for its self in 10 years.

The same with electric car, I do on average 10 miles a week, the electric bike is however a good idea as hobby is photography so I can park a bike easier than a car, and the electric bike allows me to get to places I want to take photos.

I have considered visitors, and want to build a garage, and having a 32 amp outlet and a charging lead seems a good idea for when son or daughter visits once they have an electric car, would not buy the £280 lead until they get an EV, but having a 32 amp commando socket ready makes sense.

The same applies to solar panels, at the moment there is a large tree which casts a shadow on the garage roof, but neighbour who owns the tree has solar panels and I think he would like to see the tree gone. When building the new garage it may be a good idea to make it so solar panels can be used?

There is no reason why the voltage sensor should not be in the house, even when solar panels are remote, but it would depend on the normal voltage. If the normal voltage is 230 +/- 10 volt then the volt drop does not really present a problem, but I have seen in last house when suddenly the voltage changed, it was always on the high side, 240 - 250 volt then went to 225 - 235 volt, which was enough to stop my old fluorescent lamp working.

There is nothing to stop the DNO moving the tapping on the supply transformer, and as the load is increased with EV charging and heat pumps the voltage variation is likely to increase, they are allowed 207 to 253 volt and likely they will not replace supply cables while they can maintain supply within those limits.

Many years ago we saw a special consumer unit with an auto transformer built in, the idea was to drop the voltage when high so fluorescent lamps and the like did not use more power than required, the switch mode power supply removed the need for these devices, however it does introduce the need for surge protection devices instead. However if the reason for the auto disconnect was simply to stop over voltage then there are ways around the problem, but the reason is more to do with faults like loss of PEN, so you want the voltage monitor as close to the DNO supply as you can get it, I am sure one can design an inverter which has a remote voltage monitor, but can you design it and also get the type testing required to be able it use it?

When the RCD first came out we were told no electronics allowed as they may fail, but today they do have electronics, things move on, so having EV charge points or solar panels with remote voltage sensors may happen in the future.

And this is a problem when trying to think ahead, years ago before the take over I lived in Hong Kong, and I would write home daily using fax, the faster the fax worked the less it costs, so at home I had a fax machine which auto disconnected the phones to speed up the fax, and had double phone outlet sockets one supplied from fax machine, and one direct, so if the fax machine was unplugged I could move plug and still use phone, it worked well.

However when broad band came in I found these extra lines slowed up broad band, and people had moved to email anyway, so stopped using fax machine, and all the work fitting the phone sockets became redundant, specially as moved to cordless phones, and this is the problem, what is good today maybe redundant in 10 years time, and fitting cables ready is often pointless, tubes under drives etc allowing cables to be pulled in latter yes good idea, but guessing what will happen in 10 years time, simply does not work.

 

[JohnW2]

... I am unsure of the rules on shut down, as far as I am aware the shut down is to stop the supply being made live when some one is working on the system ....

It would, for that reason, make sense for the inverter to shut down if DNO-supplied voltage fell below some level (particularly if completely 'lost'), but I don't see how that can be the explanation for shutting down due to 'excessively high' DNO-supplied voltage - which is what you have been talking about.

Is not a more credible 'explanation' that if the (DNO) supply voltage is already at the 'maximum permitted' level (i.e. 253 V), then an inverter which successfully delivered energy into the network would inevitably raise the voltage in the local part of the network to at least a little above that 'permitted maximum'?

Kind Regards, John

 

[ericmark]

but I don't see how that can be the explanation for shutting down due to 'excessively high' DNO-supplied voltage

If the PEN is lost, then the supply can be anywhere between 0 and 400 volt for a single phase from a three phase supply, 0 and 460 volt for a split phase. To be zero other phases have no load, and for 400 volt phase your using has no load, neither is likely to happen, so the 207 to 253 volt was considered as showing the PEN is OK, however not worked it out, but seems likely you could get 70 volt PEN to true earth, and the normal max is 50 volt, seem to remember latest amendment had reference to this, so not sure if 50 or 70 now, but 253-207 = 40 volt so it would seem likely using the two limits will disconnect with a loss of PEN.

However the voltage will continue to vary until fault is corrected, so one would assume to correct volts will go to zero at some point, but not sure of the rules on reconnection, I think it has to be manual done?

Loss of PEN is rare, however we must ensure when it does happen a street full of solar panels can't keep each other running, so three methods of auto lock out, over voltage, under voltage or not at 50 Hz.

This has caused problems, as where a fault caused a high load for short time, wind farms, solar farms and the likely can auto disconnect, leaving a load which the rotating generators can't satisfy, we rely on the rotating mass to ensure a few second overload does not bring down the system, solar panels and wind farms use inverters so no rotating mass.

 

[JohnW2]

If the PEN is lost, then the supply can be anywhere between 0 and 400 volt for a single phase from a three phase supply, 0 and 460 volt for a split phase. To be zero other phases have no load, and for 400 volt phase your using has no load, neither is likely to happen, so the 207 to 253 volt was considered as showing the PEN is OK, however not worked it out, but seems likely you could get 70 volt PEN to true earth, and the normal max is 50 volt, seem to remember latest amendment had reference to this, so not sure if 50 or 70 now, but 253-207 = 40 volt so it would seem likely using the two limits will disconnect with a loss of PEN.

Are you suggesting that the requirement for an inverter to shut down in the event of 'over-voltage' (>253 V, or whatever) is only relevant in the case of a TN-C-S installation (given that the explanation you have provided obviously only relates to TN-C-S)?

Kind Regards, John

 

[RandomGrinch]

I suppose my problem was that Ohm's Law doesn't really apply to a voltage which doesn't exist

Kind Regards, John

Now we're talking superconductors!

 

[ericmark]

Are you suggesting that the requirement for an inverter to shut down in the event of 'over-voltage' (>253 V, or whatever) is only relevant in the case of a TN-C-S installation (given that the explanation you have provided obviously only relates to TN-C-S)?

No as there are other faults which could happen, with a street full of solar panels if the DNO want to work on the supply or cut it for other reasons, when they cut the power the solar panels could maintain the supply, however likely it would go under/over voltage or stray from 50 Hz so the 207 - 253 volt limit would switch off the inverters one by one and ensure the supply did become dead.

I have a similar thing with my battery charger, under 7.5 volt it assumes 6 volt battery, and over 15 volt it assumes battery not connected. Under 2 volt it assumes also battery not connected, so when I remove battery leads the croc clips are dead, and only become live when I press the start button. Pain in the neck when battery fully discharged, have to connect a good one in parallel for it to charge.