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!

well a 180 degree phase shift is pushing instead of pulling, i.e. a load-source reversal. Other phase shifts can be seen as reactive power - or a mix of reactive and either generation or dissipation..
You can use an inverter that supports bi directional power flow(*) by driving ts generating voltage out of phase with the mains to emulate the action of a capacitor or an inductor, or more commonly a generator with some extra capacitance or inductance in parallel. Such 'reactive power' can be used to tame some power factor issues. It is not as cheap as fixed capacitors, but it is programmable.

Mike.

(* )so power goes from the dc bus, battery or whatever out onto the mains for some of the cycle period, but then for the rest  of the cycle current flows back from the mains to recharge the DC bus. - a pure capacitor, or inductor averaged over the full cycle, adds nothing to the power, just time shifts it...

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cable and therefore a voltage difference (higher at the inverter end) is inevitable - presuming Ohm's Law hasn't been repealed.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).

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

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