Grid Tie inverter AC output must be greater than grid voltage?

Mosquito CA

I'm considering a grid tie solar sytem for our home. I measure 243.5Vac coming into breaker box. This divides down into two 121.75Vac legs.
The inverters I've looked at state a nominal 220Vac output with a max. of 240Vac. Do they typically produce more than stated?
I understand that the grid tie inverter's ac output must exceed the grid voltage in order to source current into the grid.
Looking at 4x Sharp 235W panels to start with a central inverter.
My background is 25 years as an electrical engineer, however the operational side of grid tie inverters is new to me.
thanks.

Cariboocoot

Re: Grid Tie inverter AC output must be greater than grid voltage?

Actually a GT inverter's output matches Voltage to the grid and pushes current. Grid frequency has to be within range too.

North American Voltage standard is 240 VAC, but the 'old' standard was 220 VAC. Functionally there is little difference between the two.

When you go with a GT system the whole thing has to be done with approval of the utility and local AHJ. The plan has to be set before you buy equipment and try to put it together. Four 235 Watt panels is only 940 Watts, and most central GTI's are much larger than that. Trying to power a large inverter from a small array will be a waste, and switching out inverters later likewise.

If you want to start small and grow big consider using micro-inverters instead.

BB.

Re: Grid Tie inverter AC output must be greater than grid voltage?

It depends on the particular GT inverter... You can get GT inverters that run at ~40 VDC input (typically micro inverters like Enphase), you can get GT inverters from EBay that take ~12-48 volt input, and you can get GT inverters that need ~200-600 VDC input...

More or less, the typical GT inverter will output upwards of 264 VAC RMS (or srt(2)*264=373 volts peak).

My guess, it would appear that a typical (modern) pure GT inverter operates in Current Source mode and outputs current to the grid based on Parray=Iarray*Varray and the output AC current is something like Iac=Parray/Vsine-AC ...

In general, it is difficult to "cost effectively" grow an off grid or even a Grid Tied AC inverter system.

For GT inverters, many locations require utility approval and building permits--And adding some array/inverters a few at a time is not very cost effective (in my town, it was something like $800 for every permit pulled and, in theory, an inspection by the utility).

For central inverters, there is a (sometimes) relatively narrow range of acceptable solar array configurations. You have to meet the arrays Vmin-Vmax requirements (remembering that Vmp--voltage maximum power--falls with increasing panel temperature; and Voc--voltage open circuit--which rises with decreasing temperatures). So, you are limited to a range of (for example) 8-12 panels in series, and 1-2-3 or so parallel strings...

One way around this is to use Enphase or other "micro inverters"... Typically one inverter per panel (or per two panels)--And you connect to 240 VAC branch circuit. With micro inverters, you usually have to get a power line network box (something like ~$600) so you can monitor the output of each micro inverter. Getting a network box for a few micro inverters may be on the expensive side...

There are the EBay GT inverters (for solar, wind, etc.)--But these are usually not UL/NRTL LISTED devices and also have been known to not be very reliable (overheat/fail in months sometimes). You will frequently see these advertized as "plug and play" solar kits--But, as far as I know, there is no truly "plug and play" LISTED GT inverter in the US (in US, Canada, North America), you need a dedicated branch circuit to your GT inverter to prevent issues with multiple power sources/loads on a branch circuit bypassing the protection of the branch circuit breaker.

-Bill

bill von novak

Re: Grid Tie inverter AC output must be greater than grid voltage?

Mosquito CA wrote: »

The inverters I've looked at state a nominal 220Vac output with a max. of 240Vac. Do they typically produce more than stated?

More power? No, power out is determined by input power and inverter efficiency.
More voltage? No, they won't exceed their output voltage rating. (Specifically they will cease operation outside their legal operating range.) However during operation the voltage seen at the output of the inverter will be slightly higher than the voltage measured at your service entrance due to wiring and breaker resistance. This is typically a very small voltage and is normally negligible.

I understand that the grid tie inverter's ac output must exceed the grid voltage in order to source current into the grid.

Yes, but since the grid's impedance is low, the amount it must exceed grid voltage is small.

ggunn

Re: Grid Tie inverter AC output must be greater than grid voltage?

Mosquito CA wrote: »

I understand that the grid tie inverter's ac output must exceed the grid voltage in order to source current into the grid.
...
My background is 25 years as an electrical engineer, however the operational side of grid tie inverters is new to me.
thanks.

Look at it as a parallel circuit with an AC voltage source (the utility) on one end and an AC current source (your inverter) on the other, with the loads (those on the grid as well as those in your home) all in parallel between them. The current source is perfectly happy to supply current to the loads with the voltage the same all the way across the top of the circuit, which is what the utility does, discounting voltage drop issues. Your inverter puts out the current it wants to and the the utility supplies whatever current it needs to to keep the voltage constant across the loads. The only reason the voltage across the terminals of the inverter is higher than the grid voltage is due to the voltage drop between it and the grid; if the resistance were zero, the voltage would be the same and the inverter would work just as well.

Mosquito CA

Re: Grid Tie inverter AC output must be greater than grid voltage?

ggunn wrote: »

Look at it as a parallel circuit with an AC voltage source (the utility) on one end and an AC current source (your inverter) on the other, with the loads (those on the grid as well as those in your home) all in parallel between them. The current source is perfectly happy to supply current to the loads with the voltage the same all the way across the top of the circuit, which is what the utility does, discounting voltage drop issues. Your inverter puts out the current it wants to and the the utility supplies whatever current it needs to to keep the voltage constant across the loads. The only reason the voltage across the terminals of the inverter is higher than the grid voltage is due to the voltage drop between it and the grid; if the resistance were zero, the voltage would be the same and the inverter would work just as well.

Thanks for the clear answer.

vtmaps

Re: Grid Tie inverter AC output must be greater than grid voltage?

ggunn wrote: »

The only reason the voltage across the terminals of the inverter is higher than the grid voltage is due to the voltage drop between it and the grid; if the resistance were zero, the voltage would be the same and the inverter would work just as well.

Mosquito CA wrote: »

Thanks for the clear answer.

It's not clear to me... I guess I don't understand superconductors. If the resistance between two points is zero, then I think they must have the same voltage.

What bothers me is how can current flow in a superconductor?... I was under the impression (perhaps wrongly) that current ALWAYS flows from higher potential to lower potential.

--vtMaps

stephendv

Re: Grid Tie inverter AC output must be greater than grid voltage?

vtmaps wrote: »

It's not clear to me... I guess I don't understand superconductors. If the resistance between two points is zero, then I think they must have the same voltage.

What bothers me is how can current flow in a superconductor?... I was under the impression (perhaps wrongly) that current ALWAYS flows from higher potential to lower potential.

--vtMaps

Not a trained EE, but the way I understood it was that the voltage from the GTI has to be slightly more than the grid at that moment in the AC waveform. So instead of thinking that the GTI has a higher voltage, you can think of it as the 60Hz AC wave leading the grid's 60Hz wave by a hair. The wave in front will always have a slightly higher voltage.

Cariboocoot

Re: Grid Tie inverter AC output must be greater than grid voltage?

Usually you get current flow as a result of Voltage differential. This is what most people are used to.

The GTI matches Voltage to the grid so that the differential is zero. If there were superconducting circuits between the two there would be no potential difference so the Voltage would be 'ideal' at all points in the circuit.

In many ways a grid-tie inverter is more like an MPPT charge controller is to a battery (rather than a conventional battery-based inverter). With a battery controller so long as the V IN is above battery V level it can flow current to the battery because the panels are a current source: always trying to produce current and allowing Voltage to fluctuate - the opposite of a battery which tries to maintain Voltage against current fluctuations (but does not always succeed; Voltage sag). If you place loads on the battery the controller doesn't know the difference; the battery is a load to it as well. The GTI flows current because the utility appears as a load to it, not a power source.

inetdog

Re: Grid Tie inverter AC output must be greater than grid voltage?

stephendv wrote: »

Not a trained EE, but the way I understood it was that the voltage from the GTI has to be slightly more than the grid at that moment in the AC waveform. So instead of thinking that the GTI has a higher voltage, you can think of it as the 60Hz AC wave leading the grid's 60Hz wave by a hair. The wave in front will always have a slightly higher voltage.

Not true. With just a phase shift the magnitude will be larger for the first half of half cycle, then lower for second part of the positive half cycle, then repeat the same for the negative half cycle.

The voltage will be higher at the output of the inverter, but only and exactly by the relatively small amount needed to overcome the resistance of the wire on the way out to the POCO service drop.

If there is truly zero resistance in a circuit (as with superconductors) current will continue to flow once started, but will not start on its own.
Two ways to get the current moving are to
break the superconducting circuit, pump current in from a battery or DC generator and then close a superconducting short to complete the superconducting circuit, or
use a change in an applied magnetic field to induce current into the superconducting loop.

vtmaps

Re: Grid Tie inverter AC output must be greater than grid voltage?

inetdog wrote: »

If there is truly zero resistance in a circuit (as with superconductors) current will continue to flow once started, but will not start on its own.
Two ways to get the current moving are to break the superconducting circuit, pump current in from a battery or DC generator and then close a superconducting short to complete the superconducting circuit, or use a change in an applied magnetic field to induce current into the superconducting loop.

Thanks for the clear answer :-) Can you also explain monopoles in 100 words? :roll: --vtMaps

ggunn

Re: Grid Tie inverter AC output must be greater than grid voltage?

vtmaps wrote: »

It's not clear to me... I guess I don't understand superconductors. If the resistance between two points is zero, then I think they must have the same voltage.

What bothers me is how can current flow in a superconductor?... I was under the impression (perhaps wrongly) that current ALWAYS flows from higher potential to lower potential.

--vtMaps

Current flows from higher potential to lower potential, but current flowing can cause that difference in potential. It's a chicken or the egg situation. Current can flow from voltage sources or current sources, but virtually all the sources we deal with are voltage sources, like batteries or AC from the wall, which is where the thinking of setting up a voltage differential to cause current to flow comes from. GT inverters are basically current sources, not voltage sources, and they push current irrespective of differential in potential, i.e., they cause it rather than respond to it.

Pure (ideal) current and voltage sources do not exist in the real world; all real sources fall on a continuum between them, but if you mathematically short out an ideal voltage source with an ideal (zero resistance) conductor, the voltage stays the same across its terminals and the current goes to infinity (Ohms Law, dividing by zero), and in an open circuit condition the current is zero and the voltage is still the same. With an ideal current source, if you short it out, the voltage across the terminals goes to zero and the current stays the same, but in an open circuit condition the current stays the same and the voltage goes to infinity (dividing by zero again).

Batteries are way over toward the voltage source end of the continuum and GT inverters are much closer to the current source end. What keeps the voltage constant on a GT inverter is its tie to the grid, which acts a voltage source across the loads and varies its current to keep that voltage constant. A constant current source with its output voltage externally clamped is a constant power source, and that power has to have somewhere to go for the whole shebang to work.

BTW, it's not the inverter itself that dictates this, it's the PV array. That flat region of a PV module's IV curve over to the left of the knee means that over there the current from the module is constant irrespective of the voltage across its terminals, e.g., it's a current source. Batteries don't have this characteristic, which is why inverters fed by batteries can run off grid. They can vary the load across the terminals of the battery and sip as much current as they need to supply the AC loads.

Clear as mud?

ggunn

Re: Grid Tie inverter AC output must be greater than grid voltage?

vtmaps wrote: »

Thanks for the clear answer :-) Can you also explain monopoles in 100 words? :roll: --vtMaps

Isn't that a game designed by Parker Bradley?

Ken Marsh

Re: Grid Tie inverter AC output must be greater than grid voltage?

inetdog wrote: »

Not true. With just a phase shift the magnitude will be larger for the first half of half cycle, then lower for second part of the positive half cycle, then repeat the same for the negative half cycle.

The voltage will be higher at the output of the inverter, but only and exactly by the relatively small amount needed to overcome the resistance of the wire on the way out to the POCO service drop.

I don't follow you here intetdog.
Forget the superconductivity.
Make your voltage exactly the same as the grid.
If your phase leads you will be delivering power to the grid.
If your phase lags you will be absorbing power from the grid.

BB.

Re: Grid Tie inverter AC output must be greater than grid voltage?

Ken Marsh wrote: »

If your phase leads you will be delivering power to the grid.
If your phase lags you will be absorbing power from the grid.

No--AC power does not work that way... If the current is "in phase" with the grid voltage, and the current flow is in the correct direction (against the grid--Or the AC voltage of the inverter is higher than the grid voltage--voltage drop from wiring resistance), then 100% of the energy is transferred to the grid. Cos (0 degrees)=1.0

Power = V * I * Cos (phase angle between V and I).

If the current leads the voltage by 90 degrees, the net power transfer is zero. Cos (90) = 0. This is the equivalent of plugging in a Pure Capacitor to the AC outlet.

And if the current lags the voltage by 90 degrees, the net power transfer is also zero. Cos (-90) = 0. This is the equivalent of plugging in an inductor to the AC outlet.

Think of peddling on a tricycle... If you peddle correctly (in phase), your legs are adding energy to move the trike forward.

If your legs are "back peddling" or resiting the motion of the peddles, you are slowing the trike down.

And if you are "out of phase" with the motion of the peddles, you are not (on average) helping or retarding the motion of the trike.

-Bill