Hypothetical: Generator into grid-tie inverter

[keaton85]

Yes, I know, not recommended or allowed via most manufactures recommendations. This is more of a thought on design then anything else.

I have a small diesel gen that I can run on cheap/free fuel. My thought was take a bridge rectifier off the L1, L2 and N to output DC into a GTI, which in theory should work.

am I missing something here? It seems to simple and that’s why I ask.

[organic farmer]

Are we discussing you generating power and putting that power onto the grid?

[keaton85]

Yes, gen—>diode bridge—>GTI-->grid/house load

[organic farmer]

In my state any power that is going to be put onto the grid, must first have the schematics pre-approved by Utility company engineers [which the customer pays for], and then you hire them for inspections during the installation.

[Mike 134]

Yes, I know, not recommended or allowed via most manufactures recommendations. This is more of a thought on design then anything else.

I have a small diesel gen that I can run on cheap/free fuel. My thought was take a bridge rectifier off the L1, L2 and N to output DC into a GTI, which in theory should work.

am I missing something here? It seems to simple and that’s why I ask.

K.I.S.S. keep it simple why run it through an inverter? why make a rectifier? buy a simple metal interlock for your main breaker and 2 pole breaker and just backfeed your electric panel. You said free fuel, run it till you get tired of listening to it then switch back to the grid.

HOM200IUL Square D Homeline Generator interlock kit 200 Amp Listed | eBay

[keaton85]

I totally understand that part of the install, this would be an already approved GT solar install. The Gen would be hooked up to one of the MPPT inputs.

this is more the theory behind the concept not the code/UL listing/shore side issues.

[Mike 134]

I totally understand that part of the install, this would be an already approved GT solar install. The Gen would be hooked up to one of the MPPT inputs.

this is more the theory behind the concept not the code/UL listing/shore side issues.

If your rectified input falls within the specs the inverter MPPT is rated at should work on paper. Now time test let us know the results.

[SunEagle]

I totally understand that part of the install, this would be an already approved GT solar install. The Gen would be hooked up to one of the MPPT inputs.

this is more the theory behind the concept not the code/UL listing/shore side issues.

I believe most GTI's require a current source which would be solar panels. A generator putting out DC voltage may not be accepted by the MPPT input. Just a thought

[keaton85]

K.I.S.S. keep it simple why run it through an inverter? why make a rectifier? buy a simple metal interlock for your main breaker and 2 pole breaker and just backfeed your electric panel. You said free fuel, run it till you get tired of listening to it then switch back to the grid.

HOM200IUL Square D Homeline Generator interlock kit 200 Amp Listed | eBay

it’s actually much simpler to run it into the GTI then run to the main panel with an interlock. The diode bridge is cheap and easy. Also when hooked directly to the house, this would allow for varying loads and at times no load which would waste fuel.

Running a gen at a constant load is a lot more efficient and better for the gen in a diesel application. On a side note I already have an ATS for my main panel yet I’m reluctant to install it with a GTI on the house since the GTI could sync with the gen and back feed. So I have to figure that one out as well.

I guess the diode bridge is cheap enough thst I can tests it’s output. I’m just wondering if I need a 2 or 3 diode bridge. As I can run L1/L2/N for 240vac or just run L1/N for 120vac on a 2 diode bridge.

as for the MPPT input, I can slow the gen down to compensate for that if need be but most GTIs would take 240vdc.

[keaton85]

I believe most GTI's require a current source which would be solar panels. A generator putting out DC voltage may not be accepted by the MPPT input. Just a thought

how so? DC is DC, how would it know that it’s solar DC?

[jflorey2]

how so? DC is DC, how would it know that it’s solar DC?

Impedance characteristics are different. For example, older MPPT trackers would periodically short the input to determine Isc and/or open the input to determine Voc. Both of those are hard on generators, but are fine for solar panels because they are happy with both of those conditions.

In addition they can change with time. Even better MPPT trackers will "hunt" and vary the operating point to find the highest power point. If you do that to a generator, and the generator is ALSO trying to regulate its throttle setting to maintain a specific voltage/frequency, then the two control loops can fight each other and result in odd operation (generator surging, power changing rapidly.)

[keaton85]

Impedance characteristics are different. For example, older MPPT trackers would periodically short the input to determine Isc and/or open the input to determine Voc. Both of those are hard on generators, but are fine for solar panels because they are happy with both of those conditions.

In addition they can change with time. Even better MPPT trackers will "hunt" and vary the operating point to find the highest power point. If you do that to a generator, and the generator is ALSO trying to regulate its throttle setting to maintain a specific voltage/frequency, then the two control loops can fight each other and result in odd operation (generator surging, power changing rapidly.)

Thanks! That really helps actually. This gen is pretty dumb so the throttle issue wouldn’t be a problem but the shorting would not make anything happy. This would be into a SunnyBoy 7.7kw most likely.

yet the other option is using a cheap GTIL the blue Chinese units. Those should be dumb enough to not care about input much. Also, I already have one on hand.

[jflorey2]

yet the other option is using a cheap GTIL the blue Chinese units. Those should be dumb enough to not care about input much. Also, I already have one on hand.

The "cheap blue ones" seem to work OK with a battery as a source, so if you used the gen to charge a battery (or even a large capacitor) you might get it to work.

[keaton85]

The "cheap blue ones" seem to work OK with a battery as a source, so if you used the gen to charge a battery (or even a large capacitor) you might get it to work.

I don't think I would need a battery to get it running though as you can current limit on the GTIL2 units. The other issue is having both the GTIL2 and the SMA inverter connected to mains at the same time. I guess they should sync to mains and no care about eitherother.

[SunEagle]

how so? DC is DC, how would it know that it’s solar DC?

Then go ahead and try it. I know that others have tried to use a DC power supply which failed to connect to a solar panel Inverter. IMO a generator that puts out DC is no different then a DC power supply.

Again remember that while a solar panel does produce DC voltage it also provides a current output that the MPPT is looking for. I don't think the generator will do that.