How does exported PV power 'get past' the local supply Transformer?

[Dave Edwards]

I was looking at how my electricity meter works with the PV panels I have, and specifically how the meter stops, and a little red light comes on solid when I'm exporting.
I think I understand all of this, so I'm not looking for information about that, but when it is happening, when that red light is on and my consumption is less than the PV is producing so I'm exporting to the grid, how does that exported power get past the transformer on the telegraph pole near my property? The transformer must be capable of working both ways?
We have a single phase supply, and I'm assuming the supply to the transformer is a pretty high voltage.
When the PV was originally installed, would they have changed the transformer to be able to cope with exporting?
Would this transformer have to be upgraded when exporting more than 4kW from a single phase domestic connection?

 

[Deleted member 26818]

Your electricity doesn’t go past the transformer (though if it did, it would get stepped up to whatever the supply voltage is), it just gets used by your neighbours instead of electricity from the grid.

 

[davesparks]

Is yours the only property fed from this pole mount transformer?
If there are other properties fed from the same transformer then they will effectively be using the exported power.
If yours is the only property then yes it will go out through the transformer, though I wouldn’t be surprised if it was entirely wasted through the transformer and cable losses rather than be of any use.

 

[Dave Edwards]

Thanks spinlondon and davesparks.
We are out in the sticks, the transformer has two outputs, one to my neighbour and one to us.
I totally get what you're saying that the power will find it's 'easiest' route of being used, but that just shows I didn't ask the question properly. What I'm asking is if the transformer is capable of transforming in both directions, but it looks like you're both saying it would do that if 'forced' to.
I was thinking that when they were first designed and installed those transformers wouldn't have been 'expecting' any current flow in the opposite direction, so I just wondered how this is achieved, but you obviously seem sure they're designed to do just that!
Thanks.

 

[littlespark]

A simple transformer works both ways. Step up, and step down, UNLESS some form of limitation was designed into it. I doubt that would be the case with pole mounted LV transformers.

Your invertor reads the supply voltage, and produces its voltage slightly higher to 'push back' the grid supply. That way you use your 'free' electricity rather than the grid.

When you are exporting, your suppliers meter might say something like REC and the flashing red light will stay on. That's how mine works anyway.

 

[davesparks]

I was thinking that when they were first designed and installed those transformers wouldn't have been 'expecting' any current flow in the opposite direction, so I just wondered how this is achieved, but you obviously seem sure they're designed to do just that!
Thanks.

A transformer is (very) basically two coils of wire wrapped round an iron core, pass alternating current through one coil and the resulting magnetic field will induce an alternating current in the other coil.
The ratio of turns of wire in one coil to the other dictates the change in voltage.
so if coil A has 10 turns and coil B has 100 turns then applying 240V AC to coil A will result in 2400V AC being induced in coil B, or applying 240V AC to coil B will result in 24V AC being induced in coil A.

 

[loplug]

the transformer will work in both directions when there is no load on 'your' side of the transformer the voltage will step up and return to the grid

 

[Lucien Nunes]

Transformers have no sense of 'direction' and power can flow from any winding to any other. You can even have real watts flowing one way and wattless VARs flowing the other way. Once you've fed it the necessary magnetising current to create the magnetic flux in the core, which will typically come from the grid side, current will flow whichever way helps maintain the voltage ratio across it in proportion with the turns ratio.

Real life transformers are complex beasts to analyse in detail, but fundamentally simple in principle.

 

[Dave Edwards]

Thanks guys. Good clear explanations.
Much Appreciated.

 

[static zap]

( without getting all complex-)
(...pun Intended...)
Returning power to the grid happens more often than you would think ,
Bad power factor loads return power to the grid - but a little out of step,
(and often get it wasted) ...

 

[Lucien Nunes]

Pun appreciated - but I think this could be misunderstood.
Static is referring to the effect of loads with non-unity power factor consuming power for part of each half-cycle (1/100 of a second) and returning a fraction of it for the other part. When considered for any period longer than that, however, the power flow is unidirectional and by definition towards a load.

Here's an odd one - when you ride a passenger lift by yourself, going down draws power from the grid, but going up returns it.