Hello all – my first post on this forum and any forum for that matter, so please excuse me if I make any school boy errors, or simply asking a silly question!

I wanted to gain an understanding into the phase differences between my domestic voltage and current supply as I have a 4KWsolar PV installed. I hooked up a current clamp round the live cable going into the fuse box to observe the current sine wave on the oscilloscope. When the sun is down I notice that both the voltage and current are very much in phase. As expected, during sunshine (as long as supply > demand) I see that the two sine waves are 180 degrees out of phase, thus producing negative power again as expected when supplying the grid. The current sine wave’s amplitude grew and shrank according to sun light intensity, and if I turned on the kettle for example to draw all solar power available as well as extra from the grid then I see that the sine wave shifts back to be in phase with the voltage. All this is as expected, and I understand the principles (to a very basic degree!).

BUT!! When I look at the voltage and current waveform supplied to a specific load e.g. my 2KW heater, then I don’t understand why the voltage and current are always both in phase regardless of whether or not the source of power is from the grid or from the solar PV?
i.e. how is it that a 180 phase shift PV supply, then appears in-phase at the load?

(I guess this begs the question therefore, is the grid voltage and current out of phase but appears in phase at the load i.e our homes for example)

 

There is no 180 degree shift with a grid-tie inverter. The power being feed into the grid is exactly phase-locked to the voltage source. (grid-power) If your inverter is supplying normal 240 split-phase power and you are looking at the other 120vac cable (from ground ref) this might explain the confusion.

http://en.wikipedia.org/wiki/Split-phase_electric_power

 

I'm having a bad time tracking your logic until you get to the voltage and current at the kettle. (My ignorance.) However, I can tell you that a resistive load has no phase shift and it responds to voltage. The idea that you are applying a phase shifted current is simply not true. You are applying a voltage. E/R = I. No sin or cos something or other.

You might have a phase shifted current load when driving power into the grid, but that is a symptom of the load, not the cause. You are not driving a phase shifted current, the grid is accepting a phase shifted current.

 

Thank you both,

I am positive that the voltage and current are out of phase with each other when delivering from the panels to the grid. This is called negative power as P=VI and either V or I will always be negative at any instant. I can observe this on the scope. See two attachments, the first with both V and I in phase when drawing from the grid, and the second sees the current shifted 180 degrees (when delivering power from the solar panels to the grid).

Do you agree with the above to start with, or have a miss understood some fundamentals here?

Thanks again

 

That makes no sense to me, and doesn't jive with conventional wisdom, which states:
"The grid tie inverter must synchronize its frequency with that of the grid (e.g. 50 or 60 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage. A high-quality modern GTI has a fixed unity power factor, which means its output voltage and current are perfectly lined up, and its phase angle is within 1 degree of the AC power grid. The inverter has an on-board computer which will sense the current AC grid waveform, and output a voltage to correspond with the grid."

 

If you were monitoring the main AC active line coming onto your property from your electricity utility provider then I would expect a current phase reversal depending upon whether your were drawing power from the utility or supplying PV power to the utility.

At the PV inverter bus active output line I would not expect such a phase reversal.

 

I agree with t_n_k. A current monitor at the fuse box would show a reversal of the current phase (polarity) when the current reverses direction as it changes from the house accepting power to supplying power to the grid. (Note that the current monitor output polarity is determined by the current direction through it). So I believe the OP's phase observations are correct.

 

See the end of this page under the sub heading
Determining the direction of power flow.

http://openenergymonitor.org/emon/buildingblocks/ac-power-introduction

This better explains what I fail to get across maybe.

Wayneh: the frequency has to match to keep all in phase certainly, and the voltage is in phase......but although I am a big fan of Wikipedia - there is a message there claiming that the page you reference has a number of issues, lacking verification/citations, and does not represent all technologies.

t_n_k: yes I am monitoring the live going from my grid meter to the fusebox, and interestingly (but understandably) I observe no current flow in the wire if e.g. the solar PV produce 2KW and I consume 2KW through appliances. Voltage stays the same in amplitude and phase. Current varies in amplitude according to solar generation and domestic demand from appliances, and phase switches between exactly in-phase or 180 out.
t_n_k: I think I agree with "At the PV inverter bus active output line I would not expect such a phase reversal" ....I take it you mean that we won't see the phase change from being in-phase to 180 out?.. but do you agree it would be 180 at this output line also?

I live in the UK by the way, not that that makes any difference to the fundamentals regarding our discussion I believe.

Thanks

 

If the PV inverter is always supplying power to the common bus the current delivered on the active line must always be in phase with the bus voltage. This might need some qualificationif the PV inverter is capable of providing reactive power but the phase difference would always be less than 90 degrees. I suspect any reactive power may come from the utility incoming bus. I don't know if a typical PG inverter generates reactive power.

If you imagine a situation with the utility supply disconnected and run a purely resistive load from the PV system the current & voltage must be in phase. With the utility bus connected the bus either accepts excess power or makes up any deficit in PV power. The conditions at the inverter output should only reflect the overall domestic power demand and the excess or deficit in PV capacity at any instant.