# 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.

• It's all sorted by physics - more precisely Kirchhoff's current law.

Consider the way my inverter was installed.  There's a Henley block.  It has 3 wires going into it - the grid mains, the feed to the main house CU, and the new feed to the inverter's mini-CU.

Kirchhoff's current law says that the total current going into a point (in this case a Henley block) must equal the total current going out.  It rather makes sense when you think about it.

Imagine a case when it's moderately sunny, and the inverter's producing 10A.  Suppose I'm cooking, and using 15A in the kitchen.  It follows that the current being drawn from the grid must be 5A, to make the currents in and out of the Henley block add up.

If I finish cooking, and the load in the house drops to 2A, then the inverter must export 8A out to the grid as that's the only place it can go.

Nobody is deciding to do anything.  It just works.

• how does the main board decide to supply the loads via the inverter and not the incoming fuse cutouts

I think that you mean get its supply from the inverter or cut out.

If the load is high, both sources will push the wiggly amps through it, rather like two batteries wired in parallel. If the load is low (or nil), the inverter will supply a neighbour instead, but if the neighbour is closer to the transformer, it may have to raise the voltage a little.

I doubt that the current would have to go all the way back to the transformer, but it is an interesting question. What if the whole street has PV, the sun is shining brightly at midday, and everybody is out at work and, accordingly, consuming nothing?

• It might be bit silly but how does a main board/busbar etc know when to draw power from the grid or an inverter?

It doesn't, as Simon Barker says it's all done by the laws of physics.

The PV inverter, along with Ohm's Law and what we know as "Kirchhoff's voltage and current laws" decide that for us.

Basically, the inverter provides a slightly higher voltage than the grid, which means (using the above "laws") that if local loads are using less power than the grid, current is delivered to the grid, but if local loads are using less power, then there is a net "import" of power.

To prevent unnecessary raising of the grid voltage, the inverter tracks the voltage it's connected to, and lowers or raises its own output voltage accordingly.

•  how does a main board/busbar etc know when to draw power from the grid or an inverter?

You could equally ask, in a simple conventional setup, now does the DB know how much power to send down to the individual outgoing circuits? Clearly the outgoing protective devices can't regulate the power flowing through them (they just switch off if something goes wrong) and the power must vary as individual loads are switched on/off (or dimmed). Or how a plumbing system knows how to send the right amount of water down a pipe when a tap is turned on. The answer's the same - Ohm's & Kirchhoff's Laws (for electrics) (basically the connection of a load causes a small voltage (or pressure) drop which allows a current to flow until it all reaches a dynamic equlibrium, change things and a new equlibrium if found.

Now just think of PV inverter in the same way, other than the currnt is flowing in the other direction - i.e. it's a negative load - so exacty the same principle, but with -ve numbers (i.e. you get a small voltage increase from the DB to the inverter).

- Andy.

• I doubt that the current would have to go all the way back to the transformer, but it is an interesting question. What if the whole street has PV, the sun is shining brightly at midday, and everybody is out at work and, accordingly, consuming nothing?

It can indeed go all the way back to the transformer (and often does where you have PV farms or medium sized wind turbines) - and yes the power then flows backwards through the transformer and reinforces the HV system.

- Andy.

• further to the above, which is right, but part of the story.

The way to tell which way the current is flowing.is by looking at the voltage gradient - if the voltage at the transformer  at the street end is higher than the voltage in the house power flows in the direction that the voltage is lost towards the house, and the current is in phase with the voltage. ()

If there is excess power in the house then the voltage at the house end is raised and the voltage drop, and the phase of the current, is reversed. It is the phase relation ship between voltage and current that determines if the meter goes backwards or forwards, and the voltage drop or rise (and street cable resistance) that determines how much current flows in a given 'direction'. (*)

In a traditional network, the voltage starts high (near 250V) at the transformer, and falls towards the far end off the village. If the houses are generating this is reversed, and indeed solar panel inverters must be set to trip out if the voltage rise is too much;  to  protect other users of the network.

It is the same with the batteries in parallel thing - the current flows out of the full one with the higher voltage, and into the flatter one with the lower voltage.

Mike

(here by current flow direction, as really mains is AC and reverse every cycle, we use "in" and "out" to mean "in time with" our "in antiphase to", the alternating voltage relative to the direction of interest.)

• 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.

• indeed - but once solar generation exceeds local demand, the voltage on the load side of the meter will be pushed up higher than on the supply side, and then the power flow will reverse, and then you are propping up the street main with home generated power.. Only then does the meter  "reverse".
M.

• 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.