208/1 ph service with 240v residential inverter

[pendexter]

I came across a new-to-me scenario that's aparently common for condo's and townhomes.

SoCal, SDGE, 208/3 ph service to an 800a switchgear that goes to a 4 gang meter pack. EZM314225 (pic of label attached) if that makes a difference. There are (4) utility meters, each with a 150a breaker at the meter, feeding a main lug only panel for each unit. Each subpanel at the units get (2) lines + a neutral, for 208 single phase service. AB or AC or BC

I've read that most appliances are able to run on 208/1 ph, and it's typically not that big of a deal.

However, when trying to install solar, the standard residential inverters run on 240/1 ph. Let's say that you were forced to install a standard residential inverter, how would that be accomplished?

The only thing that I could think of is to put a step up transformer in the garage, upstream of the unit's main panel (between the 150a breaker near the utility meter and the main lug only panel). I'd need to install a main breaker in the MLO subpanel so that there is protection on both sides of the transformer, and this would provide 240v service to the residence. In that scenario, once the transformer was installed it would be no different than any other residential install.

Am I missing anything? I found a quote for a single phase step up transformer, 45 kva for $5,000. I'd hopefully like to find a more affordable solution.

 

[LarryFine]

Can you wire the transformer between the inverter's output and the tie-in point, so it only has to be sized for the inverter current, and not the entire residence, which would remain with a 208v service?

 

[electrofelon]

However, when trying to install solar, the standard residential inverters run on 240/1 ph. Let's say that you were forced to install a standard residential inverter, how would that be accomplished?


Am I missing anything?

Many/most single phase inverters will connect to 208 grids: SMA, fronius, enphase, just to name a few......

 

[synchro]

Can you wire the transformer between the inverter's output and the tie-in point, so it only has to be sized for the inverter current, and not the entire residence, which would remain with a 208v service?

Many/most single phase inverters will connect to 208 grids: SMA, fronius, enphase, just to name a few......

And if a transformer is used it will draw excitation current all of the time, including when the inverter is not outputting any power.
If 240V operation is wanted and the inverter doesn't require a neutral, then an autotransformer would take significantly less excitation current than an isolation transformer.

I suspect that the only real benefit of running 240V vs. 208V (assuming that the inverter under consideration can run on either one) is perhaps a 15% higher maximum kVA output, but I haven't checked into this.

 

[PWDickerson]

You don't need a transformer. Just use an inverter that can connect to 208V 3ph. There are lots available like Electrofelon said. I just checked the SolarEdge spec sheet, and their 3.8, 6, and 11.4 inverters can all operate at 208V with a slight reduction in capacity. You don't want to saddle the customer with the high initial cost of the transformer, and the ongoing cost of the transformer inefficiency during the day and excitation losses at night.

 

[Electrical Geek]

Most inverters that I deal with can connect to 208v single phase. You just have to change the country code in the settings.

 

[ggunn]

You don't need a transformer. Just use an inverter that can connect to 208V 3ph. There are lots available like Electrofelon said. I just checked the SolarEdge spec sheet, and their 3.8, 6, and 11.4 inverters can all operate at 208V with a slight reduction in capacity. You don't want to saddle the customer with the high initial cost of the transformer, and the ongoing cost of the transformer inefficiency during the day and excitation losses at night.

If I understand the OP correctly, the point of interconnection will only have two of the three 208V phases, but as others have stated, many/most single phase inverters can connect to either 240V or 208V. Don't make this more complicated than it has to be.

 

[LarryFine]

Don't make this more complicated than it has to be.

Awww. But that's what we live for.

 

[BackCountry]

Most inverters that I deal with can connect to 208v single phase. You just have to change the country code in the settings.

The question is going to be whether your utility supports an interconnection application at 208v.

@electrofelon ; is absolutely right that most single phase inverters on the market can operate at 208v, the challenge will be is the grid profile you set.

Start with the utility app first and see what they say. 208/120 is extremely common in condos and apartments.

 

[AlbertL57]

One issue to keep in mind is that there may be problems with production metering. Some blocks in Washington, DC have this type of service (120/208V from two phases and a neutral off a wye-connected transformer bank) for rowhouses. I work mostly with Enphase, and their IQ7/IQ7+ microinverters will work (IQ8/IQ8+ will not) , but the Envoy revenue-grade production metering gives incorrect measurements. The Envoy uses a single CT (on L1), and I think it calculates power as (L1 Amps x L1-N volts) + (L1 Amps x L2-N volts). Because the phase angle between L1 and L2 is 120 degrees instead of 180 as it would be on a 120/240V split-phase system, if the Envoy measures the voltages at the same instant, the L1 and L2 voltages will be significantly different, resulting in an incorrect power reading.

 

[ggunn]

Because the phase angle between L1 and L2 is 120 degrees instead of 180 as it would be on a 120/240V split-phase system, if the Envoy measures the voltages at the same instant, the L1 and L2 voltages will be significantly different, resulting in an incorrect power reading.

Does the Envoy use the neutral? If it doesn't, phase angle is irrelevant.

 

[AlbertL57]

Does the Envoy use the neutral? If it doesn't, phase angle is irrelevant.

The Envoy monitoring device has a neutral connection, and based on the results I'm seeing, it apparently is used for metering. The microinverters themselves don't have a neutral - they generate power line-to-line. My guess is that the error comes from multiplying the instantaneous L1 current (which is the only current measurement) by the instantaneous L2 voltage, since the two will be out of phase.

Enphase may have chosen line-neutral voltage measurement because the Envoy can also do consumption metering, where there's a CT for each line so the line-neutral voltage would be the correct input to the calculation.

I've also worked with Solaredge inverters in that environment, but they have a specific Country Code setting to accommodate it: "US 208V No Neutral", which apparently forces line-line voltage measurement.

 

[ggunn]

The Envoy monitoring device has a neutral connection, and based on the results I'm seeing, it apparently is used for metering. The microinverters themselves don't have a neutral - they generate power line-to-line. My guess is that the error comes from multiplying the instantaneous L1 current (which is the only current measurement) by the instantaneous L2 voltage, since the two will be out of phase.

Enphase may have chosen line-neutral voltage measurement because the Envoy can also do consumption metering, where there's a CT for each line so the line-neutral voltage would be the correct input to the calculation.

I've also worked with Solaredge inverters in that environment, but they have a specific Country Code setting to accommodate it: "US 208V No Neutral", which apparently forces line-line voltage measurement.

In any event, if the Envoy uses the neutral then the 120 degree phase angle will be an issue, but it seems to me that the math to correct for it would be pretty simple.

 

[jaggedben]

One issue to keep in mind is that there may be problems with production metering. Some blocks in Washington, DC have this type of service (120/208V from two phases and a neutral off a wye-connected transformer bank) for rowhouses. I work mostly with Enphase, and their IQ7/IQ7+ microinverters will work (IQ8/IQ8+ will not) ,

Some IQ8s will do 208V. But they are separate SKUs (unlike IQ7s where the same micro can do either one). And this is not available in the smaller sizes.

but the Envoy revenue-grade production metering gives incorrect measurements. The Envoy uses a single CT (on L1), and I think it calculates power as (L1 Amps x L1-N volts) + (L1 Amps x L2-N volts). Because the phase angle between L1 and L2 is 120 degrees instead of 180 as it would be on a 120/240V split-phase system, if the Envoy measures the voltages at the same instant, the L1 and L2 voltages will be significantly different, resulting in an incorrect power reading.

Per the Enphase 'Planning a Commerical IQ Microinverter System' Technial brief :"The production CT ... is not used when installed on a three-phase system."

There is also a separate SKU for a commercial Envoy that uses CTs on all three phases.

I actually don't know why the single phase Envoy can't measure correctly, as it can measure power factor and phase angle. Probably just a feature they haven't bothered to develop. If the design just assumes no phase angle difference then yes the measurement will be off for the reason you say.