Utility interactive inverters

hhsting

Senior Member
I am not sure which inverter is utility tie grid or not. I have SE43.2 and 14.4k. Which UL listing would indicate inverter is utility grid tie only? UL 1741 or some other listing?
 

jaggedben

Senior Member
Those are both grid-tie only.

The UL 1741 listing does not indicate whether it is interactive, stand-alone, or multi-mode. You need to consult the product literature.
 

Carultch

Senior Member
Location
Massachusetts
I am not sure which inverter is utility tie grid or not. I have SE43.2 and 14.4k. Which UL listing would indicate inverter is utility grid tie only? UL 1741 or some other listing?
The 43.2 kW unit is three of the 14.4kW units ganged-up on a common input/output wiring box. The 33.3kW and 100kW units work in an analogous manner to this, for the 277/480V grid. These are all grid-tie only inverters, and therefore either need to be connected to the utility grid, or connected to some other grid-forming source. Such as a system of generators that is large enough to accept the inverter source, or has a system to control the inverters. Or a battery-backup inverter system.

Grid-tie, grid-interactive, utility interactive are all terms that mean the same thing. It means that the inverter's power source is subordinate to another source that establishes the grid. The inverter looks for a grid within the voltage and frequency tolerance of a "healthy grid". It will then produce a synchronized voltage waveform that is slightly higher in amplitude in order to "push" its current onto that grid. If the grid ceases to be within the voltage and frequency tolerance programmed in to the inverter's settings, the inverter will disconnect and cease operation.

What product literature? Datasheet? I don’t see it in their. Isn’t grid tie only to mean interactive?
The manual will describe it in more detail. Unless otherwise specified to be grid forming/batttery backup, inverters are grid interactive.
 

hhsting

Senior Member
The 43.2 kW unit is three of the 14.4kW units ganged-up on a common input/output wiring box. The 33.3kW and 100kW units work in an analogous manner to this, for the 277/480V grid. These are all grid-tie only inverters, and therefore either need to be connected to the utility grid, or connected to some other grid-forming source. Such as a system of generators that is large enough to accept the inverter source, or has a system to control the inverters. Or a battery-backup inverter system.

Grid-tie, grid-interactive, utility interactive are all terms that mean the same thing. It means that the inverter's power source is subordinate to another source that establishes the grid. The inverter looks for a grid within the voltage and frequency tolerance of a "healthy grid". It will then produce a synchronized voltage waveform that is slightly higher in amplitude in order to "push" its current onto that grid. If the grid ceases to be within the voltage and frequency tolerance programmed in to the inverter's settings, the inverter will disconnect and cease operation.



The manual will describe it in more detail. Unless otherwise specified to be grid forming/batttery backup, inverters are grid interactive.
Please correct to me if I am incorrect or correct. Utility Interactive inverter can power buildings loads not just put energy back to grid. Let’s say meter usage indicates 80kwh for one of the months and solar produces 70kwh then that energy goes to powering building loads Not the grid. However if solar produces 90kwh then 80kwh goes to powering building loads and 10kwh to the grid.

Correction or incorrect?
 

jaggedben

Senior Member
Please correct to me if I am incorrect or correct. Utility Interactive inverter can power buildings loads not just put energy back to grid. Let’s say meter usage indicates 80kwh for one of the months and solar produces 70kwh then that energy goes to powering building loads Not the grid. However if solar produces 90kwh then 80kwh goes to powering building loads and 10kwh to the grid.

Correction or incorrect?
Correct. An interactive inverter typically outputs as much power as is available from the solar array, within limits proscribed by its listing and programming (e.g. max current, voltage window, frequency window, power factor). The position of a utility meter or any other meter at any point is arbitrary to its operation. The power and energy will go wherever there is a load to consume it. However if there is no primary source detected, an interactive inverter will stop outputting power in that mode.
 

hhsting

Senior Member
Correct. An interactive inverter typically outputs as much power as is available from the solar array, within limits proscribed by its listing and programming (e.g. max current, voltage window, frequency window, power factor). The position of a utility meter or any other meter at any point is arbitrary to its operation. The power and energy will go wherever there is a load to consume it. However if there is no primary source detected, an interactive inverter will stop outputting power in that mode.
So let’s say there are no transformers in between building loads where the power produced by the solar inverter is produced and those loads consume that power from solar are unbalanced then that unbalanced current what’s to stop it not return back to solar inverters neutral?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Often a grid-tie-only inverter is a strictly balanced source. In which case if you have unbalanced loads, the unbalance will be drawn from the grid, not the inverter.

In contrast, an inverter that supports standalone mode has to be able to handle the unbalanced loads on the microgrid. So either the inverter itself can support being an unbalanced source, or the system incorporates a neutral-forming transformer to change the unbalanced load into a balanced load for the inverter.

Cheers, Wayne
 

jaggedben

Senior Member
So let’s say there are no transformers in between building loads where the power produced by the solar inverter is produced and those loads are unbalanced then that unbalanced current what’s to stop it not return back to solar inverters neutral?
The inverter. The inverter is controlling the voltage at its terminals. Just like a load is not going to consume current on a neutral if it is not configured to do so, neither will the inverter. Think of an oven that only has electronic controls connected to the neutral, and the heat element is only connected to the line terminals. The inverter can be engineered to only connect the actual inverter power electronics to the line terminals.

The transformer on the utility side will balance the load neutral currents.
 

Carultch

Senior Member
Location
Massachusetts
So let’s say there are no transformers in between building loads where the power produced by the solar inverter is produced and those loads consume that power from solar are unbalanced then that unbalanced current what’s to stop it not return back to solar inverters neutral?
The inverter measures the voltage of the respective lines. For simplicity, let's consider just a single phase inverter that operates on a 120/240V grid.

It measures line 1 at 121V. It measures line 2 at 123V. This slight difference in voltage is what is existing, as a consequence of the existing imbalance of loads. Different amps drawn on the two lines, means different ohmic drops on the service conductors that deliver power to the premises. And therefore, slightly different voltages on each line at the point of interconnecction.

Based on DC power available to the inverter, suppose it "wants to" provide 6 kW or 25A of current onto the building distribution system and utility grid. The resistance between the inverter and grid at the point of interconnection is 0.08 Ohms. Through a trial and error feedback loop that solves the algebra problem, it detects this resistance and determines that it must exceed the grid voltage by 2V on each line in order to output 25A. It then produces a voltage on line 1 of 125V, and an opposite phase voltage on line 2 of 123V. This voltage difference produces equal and opposite currents of 25A.

The inverter only offsets equal and opposite currents of the premises load. Any imbalance in the loads on the premises, must be supplied by the grid-forming device, in this case the utility's transformer with a centertapped secondary. Grid-forming battery-backup inverters are commonly built to work with autotransformers, that convert their single phase output into a split phase grid that can supply the imbalance as well.
 

hhsting

Senior Member
The inverter. The inverter is controlling the voltage at its terminals. Just like a load is not going to consume current on a neutral if it is not configured to do so, neither will the inverter. Think of an oven that only has electronic controls connected to the neutral, and the heat element is only connected to the line terminals. The inverter can be engineered to only connect the actual inverter power electronics to the line terminals.

The transformer on the utility side will balance the load neutral currents.
What if let’s say the inverter is configured to Wye because SE43.2KUS 208V three phase can be configure to wye and you connect neutral output to grid utility neutral then would neutral going to inverter be balanced or unbalanced?
 

jaggedben

Senior Member
What if let’s say the inverter is configured to Wye because SE43.2KUS 208V three phase can be configure to wye and you connect neutral output to grid utility neutral then would neutral going to inverter be balanced or unbalanced?
My guess is that the neutral will have no current that is not used for detection or instrumentation, and that the line-line currents will be near perfectly balanced. That's because these types of inverters these days are usually engineered to invert the DC source onto the AC conductors between 208 and 277V nominal. So in a 208/120 wye it will not be inverting onto the neutral.

For a 480/277 wye inverter, it may be that the neutral sees a little unbalanced output current because the inverter puts 277 onto the line-neutral terminals from the DC source. So any slight imbalances between the phases would be seen on the neutral. But again, only the inverter manufacturer can confirm this. And the imbalance has essentially nothing to do with other loads on the system, just as it wouldn't for, say, a wye connected three-phase motor.

But ask Solaredge to send you the doc that Carultch mentioned. Only they can tell you how their inverter is designed to behave.
 
Top