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#203349 09/23/11 05:47 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline OP
Member
A student asked me a question about the inverter output that I could not answer to my own satisfaction.

Why does the PV inverter (connected to the service panel) feed loads inside the building first then send excess power out into the grid?

Or, to put it another way, how does this dinky little 3 kw inverter push power out into the grid working against all those megawatts?

I could take a S.W.A.G. on this & I might even get it right, but I would like to know for sure.

Tom


Few things are harder to put up with than the annoyance of a good example.
Tom #203350 09/23/11 06:07 PM
Joined: Jul 2004
Posts: 9,662
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G
Member
My guess, it is just a slightly higher voltage so it isn't pushing against anything.


Greg Fretwell
Tom #203351 09/23/11 07:20 PM
Joined: May 2011
Posts: 10
A
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Alas, with synchronised AC systems it just isn't that simple! (as it would be with DC) Just having a higher voltage would cause the inverter to export reactive power (VAr) but not real power (kW). To export real power it must create a sinewave which is very slighly ahead of the sinewave from the grid....ie its trying to pull the grid faster, this causes it to export power.

Adrian

Tom #203354 09/23/11 10:53 PM
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Yes but if it peaked at the same time, it would be ahead of the grid.


Greg Fretwell
Joined: May 2011
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No, it wouldn't. If it peaked at the same time it would be in-phase with the grid - there would be no phase angle difference & no power would transfer. Even if the magnitude of the sinewave were bigger it would still only export reactive power.

The inverter sinewave must be a few degress ahead of the grid sinewave & attempting to "pull it faster" in order to export real power.

Think about generator theory. Remember that when a generator is operating in parallel with the grid controlling excitation influences reactive power & the torque from the prime mover determines the real power - the prime mover is trying to pull the generator rotor out-of-step with the magnetic field created in the stator by the grid. This ripping apart of the two magnetic fields is what causes the generation of real power. A similar process has to happen with the inverter.

Adrian

Tom #203366 09/25/11 02:55 AM
Joined: Jun 2004
Posts: 1,273
T
Member
Adrian...

Nope.

Only voltage superiority injects power.

The frequency can't be tampered with -- such would create crazy harmonics.

The inverter sends power to the system by staying entirely in synch...

But at a trivially higher voltage. That's all it takes.

Because of voltage drop over distance -- the local loads will 'feel' the PV first.

Only after they are saturated by the PV array will enough 'over-voltage' remain for energy ( joules ; watt-seconds ) ever make it out into the grid.

Leading the Poco's frequency absolutely DOES NOT WORK. Instead, you would impose a harmonic upon the grid...

Something that would draw finger wags from the Poco.


Tesla
Tom #203375 09/25/11 04:28 PM
Joined: May 2011
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No, sorry I can't agree.

For a synchronised AC system, changing the voltage can only ever change the REACTIVE power exported. It cannot influence the REAL POWER.

Your ideas would work with a DC system - but the grid is not DC. Life in the AC world is far more complicated than just offering a higher higher voltage.

Adrian

Tom #203376 09/25/11 04:49 PM
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I am still not sure how you can say that a 100% in phase level that is higher does not lead the lower signal.

If you look at this picture you clearly see the higher voltage is there earlier and leaves later.

[Linked Image from gfretwell.com]


Greg Fretwell
Tom #203377 09/25/11 05:59 PM
Joined: Jun 2004
Posts: 1,273
T
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Greg...

That's a beautiful snapshot off a scope.

It illustrates that the inverter is ALWAYS beyond the voltage of the Poco -- if it is to drive energy back up the line.

Frequency deviations simply do NOT work.


Tesla
Joined: Jul 2004
Posts: 783
L
Member
Gentlemen,

I believe in a paralleled generator situation like Adrian is talking about, he is correct.

When synchronizing generators to the grid, you increase the speed control to take on more real load. The actual generator speed does not change because it is locked to the grid frequency due to the circulating currents, however it will pick up more of the real power load.

To increase the paralled generator's reactive loading, you try to increase the generator's output terminal voltage.

I believe the grid tie inverters are designed to try to vary their output frequency to control how much real power they output. Obviously a KW sized unit will not pull the utility units off frequency, but they will try to unload them.

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