Max inverter wattage

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electrofelon

electrofelon

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Cherry Valley NY, Seattle, WA
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Electrician
Its the AC current that seems to be the "hard stop" on an inverter output. Take for example a sunny boy 7.0 which has a max voltage of 264V and current of 32A. Does that mean I can get 264x32=8448 watts out of it providing it doesnt need to thermally protect itself by restricting output? The manual doesnt give a "maximum power" value.
 
Yes, I have seen inverters go over the nominal wattage in this fashion, but some have other internal 'stops' below max current*max voltage. For example, the Enphase M series stopped at 10watts over nominal. (Remember those are 215-250W micros, so that's like the equivalent of 300W for something like a 7KW, percentage wise.) This was listed on the data sheet as the 'peak power'. I've seen it for other brands, too. For an SMA you'd probably have to ask them what the limit is, if they'll tell you.
 
In general for switching mode designs the output current and the maximum voltage would be the hard limits for the output stage, and the actual input power would probably also be a hard limit for the MPPT input stage.
 
This question arises from my system, so Ill have to play around with it. Ill post any findings. Right now I am getting 7200 watts and the voltage is 248. I have transformer taps so I can play with the AC system voltage and see what happens to the power. Of course the available power from the array is a limit that I dont know and of course will vary with temp and irradiance.
 
I have not yet tried changing the AC voltage to see what happens, but after several days of observing, it seems that the inverters dont want to go above 7200 watts. It also seems the array has more power available and the Max AC current figure is not being reached. The reason I know this is all 3 strings are at about 355V, despite one being 11 modules, and the other two being 12. Both inverters are doing the same thing. Out of curiosity Ill still try bumping the AC up 2.5%, but my prediction is there is something they like about that 7200 figure.

Here is an interesting article on "hyper clipping". I always knew the part about the inverter changing the DC voltage to shed power, but never considered in extreme cases that that voltage needed may be outside the inverters mppt window causing the whole string to drop out (that is not happening to me, just mentioning it).

https://www.solarpowerworldonline.com/2018/06/too-much-of-a-good-thing-inverter-hyper-clipping/
 
I tried the higher voltage today. With "normal" setting AC was 250V, about 7180 watts. Bumped up 2.5% to 256V, wattage basically unchanged, if anything maybe 10-15 LOWER. I cant try 244V as I am out of tap range in that direction.
 
electrofelon said:
Its the AC current that seems to be the "hard stop" on an inverter output. Take for example a sunny boy 7.0 which has a max voltage of 264V and current of 32A. Does that mean I can get 264x32=8448 watts out of it providing it doesnt need to thermally protect itself by restricting output? The manual doesnt give a "maximum power" value.
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Is this specifically a photovoltaic limit?
I can't see why it could/would/should be?
We have designed and built DC/AC inverters up to a few thousand Amps and a couple kV at fixed frequency. I'm sure they are not by any means the largest in the world. HV DC transmission comes to mind.
Why do solar panels have the "hard stop" mentioned?
 
Besoeker3 said:
Is this specifically a photovoltaic limit?
I can't see why it could/would/should be?
We have designed and built DC/AC inverters up to a few thousand Amps and a couple kV at fixed frequency. I'm sure they are not by any means the largest in the world. HV DC transmission comes to mind.
Why do solar panels have the "hard stop" mentioned?
Click to expand...

For grid tie PV inverters, the AC output is documented and will never be exceeded. The wattage however seems to not be as definite and can be exceeded. In my case its a 7kw inverter, and I am seeing ~7200. My array is capable of more during ideal production times, so its the inverter limiting to 7200 not the array. Most systems have more DC wattage than the inverter can handle because its not worth paying for inverter capacity that you only occasionally use.
 
Besoeker3 said:
All of the DC to DC to AC inverters we did were into the grid. A few hundred kW to several thousand. I still don't see the justification for the limit on photovoltaics.

https://en.wikipedia.org/wiki/List_of_photovoltaic_power_stations
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UL listed inverters have a max current output. Since this is used to size all the output wiring, it can't be exceeded. This is particularly important for small mass produced inverters, but I'm sure that somewhere in the course of the projects you worked someone stated the max expected output and it was verified that the conductors going onto the grid were adequate.
 
electrofelon said:
You don't see the justification for current being a limiting characteristic for PV inverters? Is that correct? I just want to be sure, I am having a hard time understanding your position.
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I don't see why it is a "hard stop". I posted an example of a PV installation in CA that is several hundred MW.
 
jaggedben said:
It is a hard stop for any one listed inverter.
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That I can understand. You wouldn't expect a 10kW motor to output 100 kW. That doesn't mean you can't get a 100 kW motor. Or 1,000 kW for that matter.
 
Besoeker3 said:
I don't see why it is a "hard stop". I posted an example of a PV installation in CA that is several hundred MW.
Click to expand...


There is certainly no 11th commandment saying thou shalt not have inverters bigger than 7kw, however there are some cases where one may be limited to a certain size. We were doing a 2 MW PV project and the utility wanted the inverters "dialed down" a bit so their wattage didnt exceed the transformer size. Also you may be limited in size if you want to do a load side connection and need to comply with the 120% rule for panelboard busbar protection. In the first case, that is a wattage issue, in the second that is just about current.
 
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