Enphase Micro Inverter Operation

[czars]

Can anyone explain how the enphase micro inverter interacts with the utility? Does the output stop when the utility disappears?

 

[Smart $]

Can anyone explain how the enphase micro inverter interacts with the utility? Does the output stop when the utility disappears?

The simple (not necessarily accurate ) explanation is the inverter measures the voltage level required to "push" its current onto the connected system (utility or otherwise). When the utility power is disrupted, it takes less "push". At a preset threshold, the inverter disconnects its output. Required for grid-tie inverters.

 

[DirectGrid]

UL 1741

UL 1741

When a grid-tie micro inverter such as Enphase or Direct Grid is connected to the grid, it senses the voltage AND frequency, starts up and syncs with the grid. If the grid falls outside the voltage and frequency parameters set forth is UL 1741, the inverter shuts itself down. The main reason is to prevent electrocution of linemen (you can imagine what happens if a rogue electricity source back-feeds the grid). ANY grid tie inverter (including micro inverters) with such potential of back-feeding the grid has to comply with UL1741 or your utility will simply not allow it.
-Lou

 

[czars]

I found some info on the Enphase site about the criteria for disconnecting from the utility. However I could not find anything about the connecting criteria except a reference to a 5 minute wait. Anyone have more info on the connecting criteria?

 

[ggunn]

Can anyone explain how the enphase micro inverter interacts with the utility? Does the output stop when the utility disappears?

Yes, it shuts down when the grid goes down or is disconnected. It has to be that way for several reasons, only one of which is that it is required by law.

 

[jaggedben]

I found some info on the Enphase site about the criteria for disconnecting from the utility. However I could not find anything about the connecting criteria except a reference to a 5 minute wait. Anyone have more info on the connecting criteria?

The inverter measures that the voltage and frequency stay within the required ranges for 5 minutes. Then it starts operating. This is part of the UL standard for grid-interactive inverters and is true of all of them.

Enphase M215s (and D380s) are a little unusual in that they are designed to operate in both 240V and 208V systems. I'm not sure if the inverter knows which is which; it may only shut down if voltage falls outside the range for both types of systems.

Enphase can also extend the operating range of their inverters if your utility is not good at staying within the usual ranges.

The actual ranges for the M215 can be found here:
http://enphase.com/wp-uploads/enphase.com/2011/10/Enphase-Datasheet-M215-Microinverter.pdf

 

[jaggedben]

Enphase M215s (and D380s) are a little unusual in that they are designed to operate in both 240V and 208V systems. I'm not sure if the inverter knows which is which; it may only shut down if voltage falls outside the range for both types of systems.

As I thought about this a little more it occured to me that the inverter can probably calculate the ratio of the line-line and line-neutral voltages to tell if it is on a single phase or 3-phase supply, then look for the appropriate voltage range.

 

[tallgirl]

The simple (not necessarily accurate ) explanation is the inverter measures the voltage level required to "push" its current onto the connected system (utility or otherwise). When the utility power is disrupted, it takes less "push". At a preset threshold, the inverter disconnects its output. Required for grid-tie inverters.

That's simple, and wrong.

 

[tallgirl]

As I thought about this a little more it occured to me that the inverter can probably calculate the ratio of the line-line and line-neutral voltages to tell if it is on a single phase or 3-phase supply, then look for the appropriate voltage range.

Enphase MIs sense the line voltage and compare that against the valid ranges. They don't have access to all three phases, just two lines which may or may not be the same phase (for 240 volts systems).

Remember that the maximum range for system voltage is +/- 10% of the nominal value. So, for 120/208 systems, line-to-line is in the range 188 to 228 (more or less) and for 120/240 single phase, the range is 216 to 264. While there is 12 volts of overlap (from 216 to 228), that's not a whole lot of the total range (188 to 264).

It's a pretty stupid-simple thing to figure out, provided the power electronics and inverter firmware are up to the task.

 

[Smart $]

That's simple, and wrong.

How so?... and where's your "better" explanation. It seems to me, you're saying the same thing with different words...

Enphase MIs sense the line voltage and compare that against the valid ranges. They don't have access to all three phases, just two lines which may or may not be the same phase (for 240 volts systems).

Remember that the maximum range for system voltage is +/- 10% of the nominal value. So, for 120/208 systems, line-to-line is in the range 188 to 228 (more or less) and for 120/240 single phase, the range is 216 to 264. While there is 12 volts of overlap (from 216 to 228), that's not a whole lot of the total range (188 to 264).

It's a pretty stupid-simple thing to figure out, provided the power electronics and inverter firmware are up to the task.

 

[jaggedben]

While there is 12 volts of overlap (from 216 to 228), that's not a whole lot of the total range (188 to 264).

Yes, so if an M215 encounters a line to line voltage within that 12V overlap, how does it know if it is supposed to be staying in the 220V range or the 208V range?

My theory is that they could compare the ratio of the line-to-line and line-to-neutral voltages. I don't know for sure if that's what they do, but it seems like they could.

 

[BillK-AZ]

At an Enphase training session a few years ago it was explained that the inverter measures the voltage with respect to the neutral and does not measure voltage line to line. To determine if the inverter is on two legs of 3-phase 120/208, the inverter measures the phase angle and sets the voltage limits accordingly.

 

[jaggedben]

At an Enphase training session a few years ago it was explained that the inverter measures the voltage with respect to the neutral and does not measure voltage line to line. To determine if the inverter is on two legs of 3-phase 120/208, the inverter measures the phase angle and sets the voltage limits accordingly.

There you go, thanks Bill. Of course measuring phase angle is another option.

The only thing that's a loose end with me with this is that inverters report voltages to Enlighten that are e.g. 220. I wonder if they just multiply by 2. In that case the voltage measurement may be a little inaccurate.

 

[Smart $]

There you go, thanks Bill. Of course measuring phase angle is another option.

The only thing that's a loose end with me with this is that inverters report voltages to Enlighten that are e.g. 220. I wonder if they just multiply by 2. In that case the voltage measurement may be a little inaccurate.

Correct me if I'm wrong, but don't these inverters only connect L-L? So I'm wondering how a L-N measurement is possible. With no direct neutral connection, perhaps they use the EGC for reference...???

 

[mivey]

Correct me if I'm wrong, but don't these inverters only connect L-L? So I'm wondering how a L-N measurement is possible. With no direct neutral connection, perhaps they use the EGC for reference...???

4 conductors: Line 1, Line 2, Neutral, Ground.

 

[mivey]

There you go, thanks Bill. Of course measuring phase angle is another option.

The only thing that's a loose end with me with this is that inverters report voltages to Enlighten that are e.g. 220. I wonder if they just multiply by 2. In that case the voltage measurement may be a little inaccurate.

Why would they do that if they could combine the 120 V voltage vectors? FWIW, traditional utility residential meters meter Line-Line and thus have slight metering errors when the voltage is unbalanced.

 

[mivey]

They don't have access to all three phases, just two lines which may or may not be the same phase (for 240 volts systems).

They would not need access to all three. With access to the grounded conductor, you can measure the phase angle differences between the 120 volt signals with a neutral reference. A 120/240 system will show 180? difference and a 120/208 system will show a 120? difference. THEN it is stupid-simple.

 

[mivey]

How so?... and where's your "better" explanation. It seems to me, you're saying the same thing with different words...

She really did not say anything. The basic test is voltage and frequency. These are not adequate by themselves and there are many other tests that can be added. Many inject a small signal distortion (like a frequency shift, etc) and check to see if the response is dominated by the inverter or something else (the grid). You can even go so far as to have the grid send a SCADA signal to let the inverter know the system has tripped but that is probably beyond the scope of what you would do with MIs.

 

[Smart $]

Correct me if I'm wrong, but don't these inverters only connect L-L? So I'm wondering how a L-N measurement is possible. With no direct neutral connection, perhaps they use the EGC for reference...???

4 conductors: Line 1, Line 2, Neutral, Ground.

I see... and additionally Line 3 for 3? system connection.

http://enphase.com/wp-uploads/enphase.com/2011/06/Enphase_M215_InstallationOperation_Manual.pdf (page 16)

They don't have access to all three phases, just two lines which may or may not be the same phase (for 240 volts systems).

They would not need access to all three. With access to the grounded conductor, you can measure the phase angle differences between the 120 volt signals with a neutral reference. A 120/240 system will show 180? difference and a 120/208 system will show a 120? difference. THEN it is stupid-simple.

First, mislead to believe connected only to two lines...

Regarding your reply, while you may not need access to all three phases (though the M215 does have access to all three... when wired correctly ), you still need access to three points (e.g. L1, L2, N) of the system to directly establish any difference in phase angle.

 

[Smart $]

She really did not say anything. ...

What do you mean? She said my "simple" explanation in post#2 was "wrong", with nothing presented to explain her comment. Let's experiment... you are wrong?