On 23 Jun 2016 at 19:56, RussellMc wrote:
> E&OE.
> No guarantee that this is fully correct.
> As it in part MAY disagree in some degree with Brent it's probably in som=
e
> degree wrong (probably for both  correlated and non correlated versions o=
f
> some)
> * I changed disagrees' to 'may disagree'  as I'm talking about V and he m=
ay
> have been talking about I.
> We appear to still not be in full agreement.  eg I say max power at V +/-
> 90 lead/lag of Vgrid. .

I don't know if we agree or disagree yet... more likely my degree of wrongn=
ess is=20
just waiting for me to grasp it :-)

I was trying to imagine (and not explaining well) current as seen by an imp=
ort/export=20
power meter (grid connection point). Say there is no PV generation, the met=
er=20
would see imported power according to whatever load is present, that is, cu=
rrent in=20
phase with voltage assuming a resistive load. Right so far? Then let's say =
PV=20
generation kicks in and exceeds load or load is disconnected, meter sees ex=
ported=20
power. Assuming unity power factor (bad assumption?) would I be correct in =
saying=20
the current is now flipped, eg. 180 degrees out of phase with the voltage? =
If not,=20
then how does an import/export meter determine direction of power flow?

> A simplistic but useful look at power transfer is given by:
>=20
> V1 =3D invertervoltage
> V2 =3D grid voltage
> X12 or just X is coupling impedance between inverter and grid. This usual=
ly
> includes a physical inductor or sustem inductance.
> Delta =3D d is phase angle between inverter and grid.
> sind =3D sin(d) , cod =3D cos(d)
>=20
> Real power transfer =3D V^2/X =3D (V1 x V2)/X x sind
> So may transfer occurs at d =3D 90 degrees.
> V1 leading V2 gives max real powerexport.
> V1 lagging V2 gives max real power import.
> Power is also controllable by varying V1.
> [Murphy suggests that getting V1 leading V2 may require V1 > V2 but that'=
s
> a technicality].
>=20
> Imaginary power transfer =3D V1^2/X - (V1 x V2)/X x cosd
>=20
> As cosd =3D max =3D 1 when sind =3D 0 and d=3D0, pur  reactive power max =
occurs
> when V1 and V2 are in phase BUT if V1 =3D V2 you then get zero reactive
> power. By then altering V1 relative to V2 you generate negative or positi=
ve
> reactive current in X and thus overall reactive power.

Ah, maths. I will have to think on it a bit more. It's more looking from th=
e viewpoint=20
of the inverter, which I'll be happy with if I can correlate it with what t=
he meter sees.

Thanks for the links too, reading...



--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.