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Using an Autotransformer for PJ with PV


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Just making sure my math is right...

When running a PJ inverter in parallel with a solar array (off grid), I am assuming that the PJ's transformer will split the 240 VAC coming from the PV into 120+120v, eliminating the need for a separate autotransformer.

That said, the PJ transformer power ratings will obviously still apply.  As an example, let's say I am using a AS-9 transformer (rated at 20 kW, for the sake of argument) with a 10 kW PV array:

- if the PJ is pushing out 10 kW, then the transformer will still have enough room to split the 10 kW (240v) coming from the PV into 120+120v, assuming that the load balance is not terribly off.

- Since the PV string inverter is MPPT, it will step up the voltage high enough so that the PV is doing all the heavy lifting. 

So, if your house needs 10 kW, it should all come from the 240v PV array, and the PJ inverter itself will probably throttle down close to 0%.  However, the PJ transformer will still be at 50% load, splitting the 240 into 120+120, while the inverter is idling.  At least in theory.

I just want to make sure that the theory is correct before I flip the switch and let her rip!!!

 

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17 hours ago, Paul Z. said:

When running a PJ inverter in parallel with a solar array (off grid), I am assuming that the PJ's transformer will split the 240 VAC coming from the PV into 120+120v, eliminating the need for a separate autotransformer.

Yes, providing you're connecting to L1 and L2; the transformer will "generate" the Neutral.

17 hours ago, Paul Z. said:

- if the PJ is pushing out 10 kW, then the transformer will still have enough room to split the 10 kW (240v) coming from the PV into 120+120v, assuming that the load balance is not terribly off.

Correct.  If the load is quite balanced, your theoretical limit would be your wiring between the solar array and the load--as the transformer wouldn't be doing much work (due to the balanced load).

 

17 hours ago, Paul Z. said:

- Since the PV string inverter is MPPT, it will step up the voltage high enough so that the PV is doing all the heavy lifting. 

So, if your house needs 10 kW, it should all come from the 240v PV array, and the PJ inverter itself will probably throttle down close to 0%.  However, the PJ transformer will still be at 50% load, splitting the 240 into 120+120, while the inverter is idling.  At least in theory.

I just want to make sure that the theory is correct before I flip the switch and let her rip!!!

Here is the major "gotcha" that you have to look out for: Grid-tie inverters are designed to blast as much power as possible back into the grid.  While I personally don't have any to test, they will have to push the AC voltage higher than the "grid" in order to get power to flow the opposite direction.  Not a problem for the grid.  Now, for an inverter...especially one that doesn't know squat about grid-tie systems, we have a major issue.  Let me explain:

If your loads exceed the ability of the grid-tie inverters, everything should be perfect.  HOWEVER...

...if your loads do NOT exceed the grid-tie inverters' power output (which changes with the sunshine/clouds/atmospheric conditions, etc.), there's a serious problem awaiting.  The grid-tie inverters will continue pushing as much power out as they can...which will back through the PJ inverter, and start charging the battery (I've seen this to a smaller extent on my house system when testing parallel mode.  Yes, the master inverter's battery current will go negative.)  This uncontrolled/unregulated charging may damage the batteries, as the inverter is completely unaware of this phenomenon.

Yes, the PJ inverter throttle will come downwards if the grid-ties end up backfeeding power into the PJ inverter...but the saving grace will be the voltage drop in the internal PJ chassis wires--as if the PJ inverter throttle DOES reach 0% (due to the grid-tie units pushing power into it) will result in the FETs basically putting a dead short across the low voltage side of the transformer.  Not exactly good.  (The inverter runs a push-pull driver on the transformer; the transformer is never "freewheeled", thus 0% throttle = shorting both transformer leads to battery positive.  Yes, the PJ H-bridge is upside-down from what you might expect.)

 

To summarize: it might work if you are aware of how it will work 😉.

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