measuring current flow

[openspaceman]

I basically understand that when the PV outputting power the inverter output voltage rises to a bit more than the grid voltage and either the household load (and battery) consumes the electricity or AC current is driven back to the grid by the higher voltage but how is that current measured, after all the actual direction of flow changes every 100th of a second?

[marshman]

A current transformer (CT) will only tell you the size/value of the current and not the direction - and as you say with AC it is changing direction 100 times a second. So in order to determine whether the power is flowing to or from the grid you need to compare the phase of the current to the phase of the voltage. Whilst the current obviously needs to be measured at the incoming grid connection - i.e. right by your meter / incoming tails to your consumer unit, the voltage can be measured anywhere so if the CT is wired direct to the inverter the inverter will reference the phase of the signal from the CT with that of the voltage it sees on its grid connection.

The inverter should also measure / display the true power (including the power factor) rather than the "crude" apparent power (simple V x A) given by simple power meters that just rely on measuring the current and multiplying it by an assumed 230V.

[Tinbum]

I basically understand that when the PV outputting power the inverter output voltage rises to a bit more than the grid voltage

Not quite like that. It is to do with the frequency curve. The inverters curve will be very slightly shifted compared to the grid curve.

[openspaceman]

So the phase of the inverter voltage is shifted to slightly lead the grid as well as be at a slightly higher voltage, effectively making the grid look like a giant capacitor?

[Tinbum]

No, voltage is the same assuming no cable losses. The frequency curve is the same but one leads the other very slightly.

It's very complex. Think of paralleling 2 identical diesel generators together. They will both run at 1500 RPM. If you want to run 50% on each then the frequency curves of each will align. If you want to run one at 100% and one as standby you would run the standby one slightly behind the other but still at 1500 RPM..

[openspaceman]

I sort of get that Tinbum, it's about 53 years since I did any electrical engineering theory and that was well before IGBT existed but I think MOSFETSs were just about there.

[ClockmanFRA]

I use AC coupling with used toroid SB GTI's.

Our design of our OzInverter is the humble H BRIDGE and we wind the bare 28kg torroid to very exact specs so that the GTI's push up the 240vac Mini Grid created by the OzInverter and its battery pack of 48vdc 1300ah.

If power is not used by the Mini Grid, then the GTI's push up the Mini Grid voltage and this therefore goes back to the H BRIDGE at the OzInverter and anything above the AC 240v is pushed above our OzInverter control board settings, and hay presto the Battery is now getting charge.

The OzInverter is Bi-directional.

I just love those' light bulb coming on' moments when electrical engineers have trouble getting there heads around the whole concept. Its simple really.

A couple of things are important for a successful system. Battery pack should be no smaller than 800ah 48v. Voltage sag is now important between GTI installations. And a very accurate GTI shut down process should be installed on each GTI to give sequential shut down.

The above is just the basics.

[John_S]

A good way of looking at how different generators work together is to think of two cyclists on a tandem.

The one at the back can either enjoy their legs going up and down with minimal effort or can work harder and provide contribute to the power. Either way he would be pedalling at the same cadence as the front cyclist. To work harder, he has to try and pedal faster.

[Tinbum]

A good way of looking at how different generators work together is to think of two cyclists on a tandem.

The one at the back can either enjoy their legs going up and down with minimal effort or can work harder and provide contribute to the power. Either way he would be pedalling at the same cadence as the front cyclist. To work harder, he has to try and pedal faster.

That's a great way of visualising it.

[openspaceman]

A good way of looking at how different generators work together is to think of two cyclists on a tandem.

The one at the back can either enjoy their legs going up and down with minimal effort or can work harder and provide contribute to the power. Either way he would be pedalling at the same cadence as the front cyclist. To work harder, he has to try and pedal faster.

Yes I get all that.

As the cyclist slacks back the pedaling cyclist ultimately starts putting effort into lifting the slackers legs.