The contributor from Isle of Man that I would never challenge on technical issues pointed out to me that current can be injected into the grid by shifting the phase rather than raising the voltage, and I was confusing myself considering DC theory rather than AC.

I believe that with DC the voltage has to be raised, as in battery charging, but with AC the voltage doesn’t have to be raised as the phase can be shifted.

Please don’t ask me to explain it!

My thinking is that that theory might be correct at the point of injection - but if you want the generated current to flow along a cable (e.g. from the inverter to the CU) there will be current flow and resistance in the cable and therefore a voltage difference (higher at the inverter end) is inevitable - presuming Ohm's Law hasn't been repealed.

   - Andy.

it is a higher voltage, but only at some points in the cycle, rather than by an equal ratio for the whole cycle -

Example

When  in phase 220V AC is always twice the voltage of 110V AC and if they were connected by a resistor, the current would be set by that resistor and a 110V voltage difference, and energy would flow out of the 220V supply pulling it down, but into the 110V one, pulling it up..

But now, if you shift the phases for example again, by say 90 degrees, then that is no longer true, as  for some regions in the 50Hz cycle the voltage from the 110V is more than that from the 220...

- at the  moments the 220V is at or near its zero crossings, the 110V is at or near peak, so the current in the resistor is still flowing but the 110V source is pumping into the 220...
Mike.

mapj1 said:

Such 'reactive power' can be used to tame some power factor issues. It is not as cheap as fixed capacitors, but it is programmable.

...and, side note, is sometimes stipulated by DNOs (e.g. where they specify generation at the point of supply shall be at a specific power factor other than unity).

Sparkingchip said:

Both the inverter and main board are downstream of the suppliers meter, so the electric the main board draws from the inverter is not recorded on the suppliers meter.

Unless it's a SMETS2 smart meter.  They are set up to record both import and export.  With the ending of the FIT scheme for new installs, that's the way a householder can get paid for any generation that they export.

Not quite.

Meters with import and export registers (and they don't have to be smart meters) will only see a reduced consumption. If it's reduced below zero, obviously that'll be recorded as an export.

That is not to say you can't have both improt and export in the same billing period of course... neither register runs backwards.

Only then does the meter  "reverse".

Old mechanical meters were fitted with a pawl system so they didn't record exported power (hence FIT's 50% deemed export).

Nowadays smart meters record imported and exported separately, so even people on the original FIT scheme get paid by actual exports (but then have the option to switch to an smart export tariff, which usually pays better).

   - Andy.

I made the mistake a couple, or maybe more years ago, of using the DC and water analogy for explaining the operation of a Solar PV Inverter and I was corrected on this. Oddly(?) I just searched for my error and it did not appear to be there.  For that matter, nor was much else!

Have the older posts been removed?

Clive

Perhaps "archived" would be more accurate than, "removed".

I think the phrase you are really needing is 'lost'. There is a general problem of searching or old posts or threads that makes it hard to find things by topic, and this is compounded by the fact that all the pre IET_EngX stuff has been taken off-line.

Mike

it is a higher voltage, but only at some points in the cycle, rather than by an equal ratio for the whole cycle -

Testing my thinking here... if the inverter is producing some reasonable approximation to a sine wave and is slightly out of phase with the grid voltage, then won't we get a small amount of "wattless current" - i.e. some power then momentarily, for a small fraction of the cycle, flows the wrong way (from grid to inverter in our case) like we do with normal inductive or capacitive loads? I can get my head around this happening with normal capacitive or inductive loads as they can store a little power in the magnetic field or between the plates of the capacitor - but would an inverter have the ability to cope with receiving that power? (can it shove it back into the d.c. bus?) ... or is the output somewhat different from an ideal sine wave? Or have I got my head on upside down again?

   - Andy.