Tesla Powerwall setup- autotransformer

[greenspark1]

Just read an interesting JLC article on a Tesla Powerwall installation:
http://www.jlconline.com/how-to/electrical/tesla-powerwall-not-just-for-solar_o

It's not a very technical article, but it does a good job. What caught my attention was the 'StorEdge' autotransformer:

Wired to the inverter is a small transformer serving the backed-up circuits (this component is necessary to provide consistently regulated 120-volt current and grounding). In backup mode, the transformer feeds regulated power through the “load center”—a small breaker box similar to a subpanel, with circuit breakers for the individual backed-up circuits in the home.
​

I don't really understand what this autotransformer is doing or why it is required. For comparison, as far as I know, a grid-tied PV system (without battery) doesn't need an autotransformer as the inverter does a fine job of regulating voltage, current, and frequency.

Thought I'd share and see if others have experience or thoughts on why this component is required. I'm sure we'll be seeing many more of these types of installations in the coming years. (And the NEC will have to keep pace)

 

[ggunn]

Just read an interesting JLC article on a Tesla Powerwall installation:
http://www.jlconline.com/how-to/electrical/tesla-powerwall-not-just-for-solar_o

It's not a very technical article, but it does a good job. What caught my attention was the 'StorEdge' autotransformer:

Wired to the inverter is a small transformer serving the backed-up circuits (this component is necessary to provide consistently regulated 120-volt current and grounding). In backup mode, the transformer feeds regulated power through the “load center”—a small breaker box similar to a subpanel, with circuit breakers for the individual backed-up circuits in the home.
​

I don't really understand what this autotransformer is doing or why it is required. For comparison, as far as I know, a grid-tied PV system (without battery) doesn't need an autotransformer as the inverter does a fine job of regulating voltage, current, and frequency.

Thought I'd share and see if others have experience or thoughts on why this component is required. I'm sure we'll be seeing many more of these types of installations in the coming years. (And the NEC will have to keep pace)

What the autotransformer does is balance the 120V loading across the two legs of the 240/120V output of the inverter. Exactly how it does it is something I and my colleagues have been puzzling over, but that's what it's there for.

 

[Jraef]

What the autotransformer does is balance the 120V loading across the two legs of the 240/120V output of the inverter. Exactly how it does it is something I and my colleagues have been puzzling over, but that's what it's there for.

It's essentially just there to create that grounded center point of a split phase circuit. The output of the inverter is 240V single phase, just like the utility transformer, but without that center tap.Remember, when you are on total inverter power you are isolated off of the utility neutral grounding point, so if you just fed the 240V into your panel, the neutrals would all end up floating.

It's only an "autotransformer" because isolation is unnecessary since it's a separate source anyway, so an autotransformer is cheaper.

 

[ggunn]

It's essentially just there to create that grounded center point of a split phase circuit. The output of the inverter is 240V single phase, just like the utility transformer, but without that center tap.Remember, when you are on total inverter power you are isolated off of the utility neutral grounding point, so if you just fed the 240V into your panel, the neutrals would all end up floating.

It's more than that, according to SolarEdge. It is unlikely that the 120V loading in the protected loads panel would ever be equally split between L1 and L2; the autotransformer (again, according to SE) equalizes the loads between the two legs coming from the inverter.

 

[GoldDigger]

It's more than that, according to SolarEdge. It is unlikely that the 120V loading in the protected loads panel would ever be equally split between L1 and L2; the autotransformer (again, according to SE) equalizes the loads between the two legs coming from the inverter.

The two half windings of the autotransformer force the L1-N and L2-N voltages to be equal. Since the main reason for voltage offsets will be unbalanced line to neutral currents on the two legs it has the effect of conducting matching current on the two legs to balance so that the net current flows only from L1 to L2.
I find it easier to just look it as an autotransformer that creates a stable neutral point for unbalanced loads.

Sent from my XT1585 using Tapatalk

 

[greenspark1]

Neat stuff. Good ideas, thanks for the discussion all.

 

[ducks1333]

Inverter schematic

Inverter schematic

There should be a schematic attached that shows the inverter that is supposed to be used with a Powerwall 1. It is a SolarEdge StorEdge inverter. Interesting that the neutrals are solidly connected to the source and that the one leg of the autotransformer is also connected to the neutral. I'm still not sure how the autotransformer is working when connected like this. They say the autotransformer is only needed if it is going to operate a backup panel. Is anyone aware of other installations that use an autotransformer like this?

 

[GoldDigger]

I suspect that the inverter itself is designed to deliver power between L1 and L2, i.e. to feed 240V loads only, and that the inverter by itself cannot supply unequal currents to L1 and L2.

If you intend to connect any (or many) Line to Neutral loads, the autotransformer is required to convert some L1 current to L2 current, for example, for delivery to the unbalanced loads in the backup panel.

Contrary, perhaps, to expectations an inverter which is connected to L1, L2, and N may not be inherently capable of supplying unbalanced L to N loads.

The inverter in the StorEdge is not, I believe, a pure grid tie inverter.
FWIW a typical grid tie inverter at 240V is also not capable of supplying unbalanced L to N loads. The difference is made up in that situation by exporting extra to POCO on, say, L1 and importing a corresponding amount from L2.
Or by exporting different amounts of power to L1 and L2
Or by importing different amounts of power from L1 and L2.

 

[jaggedben]

I suspect that the inverter itself is designed to deliver power between L1 and L2, i.e. to feed 240V loads only, and that the inverter by itself cannot supply unequal currents to L1 and L2.

If you intend to connect any (or many) Line to Neutral loads, the autotransformer is required to convert some L1 current to L2 current, for example, for delivery to the unbalanced loads in the backup panel.

Contrary, perhaps, to expectations an inverter which is connected to L1, L2, and N may not be inherently capable of supplying unbalanced L to N loads.

The inverter in the StorEdge is not, I believe, a pure grid tie inverter.
FWIW a typical grid tie inverter at 240V is also not capable of supplying unbalanced L to N loads. The difference is made up in that situation by exporting extra to POCO on, say, L1 and importing a corresponding amount from L2.
Or by exporting different amounts of power to L1 and L2
Or by importing different amounts of power from L1 and L2.

I don't know if I'm agreeing or disagreeing. I guess just clarifying...

I believe that the core of the inverter on the StorEdge is the same as the standard grid-tie inverter. It can act as grid-tie or as stand-alone, but that's largely about firmware and ensuring that anti-islanding works when it's supposed to. In any case, the inverter core does not deliver current on the neutral - it only switches L1 and L2 - which is why the autotransformer is necessary when the utility is disconnected. If the neutral connection shown to the inverter core were a dashed line, to represent instrumentation only, it would be more accurate.

Essentially the auto transformer plays a similar role as the utility transformer winding when the latter is connected.

There's a bit of assumption here on my part, but I'm making educated guesses based on various sources of info, including conversations with SolarEdge reps. Among other things, they at one point stated that the standard 7600W grid-tie inverter would be convertible to the StorEdge version.