piclist 2016\06\22\224851a >
Thread: Solar PV panels
picon face BY : Brent Brown email (remove spam text)

On 22 Jun 2016 at 18:25, John Gardner wrote:

> If I've got it right (& I probably don't),  the PV system,  after converting
> the DC output of the panels to AC grid specs,  adjusts the phase angle
> between grid power & PV power to suit the needs of the moment?

Perhaps, kind of, but not really(?). At times you will see 0 degree phase angle (give or take a bit) and other times 180 degrees phase angle (give or take a bit), and ideally not much in between (reality is real life capactive/inductive loads will mess up the phase angle somewhat, but the general idea remains valid I think). Hope the following helps to explain...

To keep it simple, assume perfect power factor ~ that's what we're aiming for anyway. From AC theory 101, in a resistive load the (sine shaped) current waveform preciesly follows the voltage waveform, i.e. it's in phase. Power factor is +1. Power flows from supply (grid) to load (resistor). As the load draws more current it tries to pull down the voltage (which it won't do very much, because the grid has very low impedance).

Now change from perfect load to perfect generator. The current is now required to be completely out of phase with the voltage, 180 degrees. Power factor is -1. Power flows from generator to grid. The generator tries to push up the grid voltage (which it won't do very much, because the grid has very low impedance).

The grid connected inverter is your generator. It will try to keep power factor at unity, -1, and will alter current as required according to power to be transferred. Additionally it will try to maintain a sine waveform (current and/or voltage? - not sure). Additionally additionally it will (should) avoid pushing up the supply voltage beyond a programmed limit. For example, my lines company required evidence the inverter(s) would "throttle back" at 247.5V (nominal 230V line voltage).

I see this post is far too long, and I risk sounding like Russell ;-)

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