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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

My guess, it is just a slightly higher voltage so it isn't pushing against anything.

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

Yes but if it peaked at the same time, it would be ahead of the grid.

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

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.

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

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.

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.

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.

Tesla's explanations ring true to me. That scope picture actually shows both traces cross 0 together. the difference I see is magnitude not time and phase shift.
Adrian I assume you agree that AC can supply linear loads? So if your dam was to only supply 200,000,000 amps of heat across simple carbon resistors the load and voltage would be in phase? only if the load is inductive or capacitive would a phase shift occur between the voltage and the current. Yes I know this is not a real world case as most grids are inductive and current lags the voltage. Any load sharing is related to the capacity of the sharing device and a 100 amp solar array is certainly a higher impedance than the 200 MA dam. Load sharing is simple Ohm's law if the load is 100 ohms, it is 100 ohms to both sources the voltage created in the inverter and the dam must be synchronized to cross 0 at the same time with neither leading or lagging the other in the example of real power only. so the base is the grid and we must sync all input sources to the grid at the location they are interconnected. which ever source has the higher voltage and lowest internal impedance will deliver the amps. Now if my entire load is within the capacity of the inverter then I would want the output voltage to be higher than the grid but not much higher, just enough that the 1st 100 amps comes from my source rather than the grid. As we reach the maximum current output of the inverter it's internal impedance rises a little as the voltage sags a little so that once we tip over it's output capacity any excess demand comes from the grid.
I can tell you it is possible to parallel many generators of many sizes but as soon as one gets out of sync it drops off or quickly becomes a load or fault. In sync definitely means crossing for 0 volts happens at the same instant. How many voltage sources are on the same North American Grid? Thousands! If they are connected together they are in sync and the inverter of DC sources must sync to the same grid.

Actually the generator speed does not change in theory as that would get the generator out of sync and trip the protection. As load is added so is power from the prime mover and that is controlled by a governer. I had a big project where 2 generators could not be synced. they would run up and parallel but because their governors were different as soon as 1 started to speed up as you say the sync was broken and they would trip off line. A simple governor change cured it. They must absolutely run at 60 Hz and cross 0 volts at the same time 120 times per second. One generator was 600 KVA and the other was 1000 KVA. After the project was complete the smaller was rewound for 750 KVA and paralleled to two 1 mva with another 1 MVA added some 4 years later. They all share load according to their ratings but when parallel they must rotate at 60 hz which is related to the number of poles wound in the alternator so a 6 pole and a 12 pole machine would have different synchronizing speeds. The governor works to ensure the alternator speed is constant not speeding up or slowing down with load but adding or removing power according to load. Once overloaded the engine slows too and the electrical protection trips on frequency which is speed of the alternator.

Larger three-phase alternators control output via their SALIENT POLES EXCITATION CURRENT.

Google the terms: Salient Pole, Excitation Current ( it's DC, BTW )

The Poco -- all of them -- run Infinite Busses -- i.e. you can consider their output as coming from one alternator -- or thousands of perfectly tuned alternators.

Tesla was the inventor of this system. His Alternating Current 3-phase machines solved Edison's Number One Headache: DC generators would keep 'hunting' up and down voltages -- swinging from generation to motoring.

Edison couldn't follow Tesla's theories. Edison then screwed Tesla over a mere $50,000 -- a bonus promised if Tesla could solve this back breaking problem. Until Tesla solved it, Edison couldn't built an infinite bus. He couldn't scale up.

The very best load following prime movers are hydroelectric dams. Between their excitation and water flow -- they can stay load-matched.

Nuclear power stations are the WORST load followers. So they are always base loaded prime movers.

Iran's new nuke at Bushehr is taking weeks to come up to full power. That is entirely typical.

If Reactive Power is needed the traditional Poco solution was a Synchronous Capacitor. This is always the latest state-of-the-art Synchronous Alternator -- with Zero Loading. It's sole function is to inject rotary capacitance by way of OVER EXITED SALIENT POLES.

This provides Reactance in mega-quantities. ( Think giga-watts-reactive )

The Canadian - New England/ New York inter-tie famously has a monster Synchronous Capacitor at the border. By the terms of the deal -- the Canadians must provide juice with virtually 100% power factor. This, then, shunts the maximum amount of real power possible over the Very High Voltage long distance power lines.

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Rather than propound novel electrical theories -- use Bing/ Google to find what the inventor-brains have said.

BTW, it wasn't just Edison -- until Tesla Wrote His Book -- and worked up the mathematics of AC power -- no one else could follow it.

( Witness this forum. )

To understand AC you need to get up to speed on Complex Numbers, Trig and Vector Calculus. That's what's taught to EE's in college.

McGraw-Hill has a fantastic summary of the electrical engineering art -- try and get a copy -- even second hand.

You can study homopolar generators, railroad traction motors, and complex double-headed power delivery to major industrial plants.

It still details technology that is now out to pasture.

EE is so big a field you can spend your entire life in it and not reach the end.

Cheers.