How does a main board know when to draw power from the grid or an Solar PV inverter?

Hi,

It might be bit silly but how does a main board/busbar etc know when to draw power from the grid or an inverter? Lets say its sunny and your PV system is generating plenty, how does the main board decide to supply the loads via the inverter and not the incoming fuse cutouts? Similarly, how does the excess current flow back to the nearest substation ?

Thanks.

Parents
• 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.

• 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.

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.

• 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

• 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.

• 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?
Exactly that happens -- this is the whole basis of the active power factor correction.

It does need a deliberate  inverter/ "active rectifier"  design that allows the switching transistors to conduct in either direction, not just from DC bus towards the output (the inverter bit), but also to recharge the DC bus from the supply (the rectifier bit). This in effect means that in some designs  flywheel diodes need to be replaced with transistors as well, so perhaps a bridge with 2 diodes and 2 transistors becomes 4 transistors,  or pairs of anti-series transistors allowing separating of switching in each direction.
However, with modern devices and the ability to control the individual switching times under software control, it is not  anything like as as hard as it would have been a few years ago to switch active devices at close enough to the right point in the cycle to conduct like  rectifier diode would have if required.

Mike.

• 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?
Exactly that happens -- this is the whole basis of the active power factor correction.

It does need a deliberate  inverter/ "active rectifier"  design that allows the switching transistors to conduct in either direction, not just from DC bus towards the output (the inverter bit), but also to recharge the DC bus from the supply (the rectifier bit). This in effect means that in some designs  flywheel diodes need to be replaced with transistors as well, so perhaps a bridge with 2 diodes and 2 transistors becomes 4 transistors,  or pairs of anti-series transistors allowing separating of switching in each direction.
However, with modern devices and the ability to control the individual switching times under software control, it is not  anything like as as hard as it would have been a few years ago to switch active devices at close enough to the right point in the cycle to conduct like  rectifier diode would have if required.

Mike.

Children
• It does need a deliberate  inverter/ "active rectifier"  design

So possibly not what we have with a typical domestic PV inverter?

- Andy.

• Most newer domestic ones, and all those models that support the addition of a storage battery have been made that way for a while, and in Germany at least you cannot  install one that doesn't allow active power factor correction without a load of justification and paperwork, Not sure about the UK legal position - I suspect that it becomes the standard fitment.

Mike

• allow active poetry factor correction

Has you smell checker gone a bit Pete Tong?

• yep 'poetry factor' should read 'power factor', well spotted. Fixed now.

Not so sure about VDE approach to German poetry - mind you there probably is a DIN standard somewhere. 'The Poet shall ....'

if so it would have to be approved in conjunction with  the Gesellschaft für deutsche Sprache

M.