When inverters hit their peak output but the PV/DC side still has ability to produce more, what actually happens in the inverter to enable the clipping? Does the inverter just increase its input impedance on the DC side so much that it limits the current from the PV? Is it shunted somehow and lost as heat? Perhaps it depends on manufacturer, but what's the common method?
Inverter Clipping
- Thread starter bwat
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
I believe they operate the modules at a less than ideal point in the voltage/current curve by changing their effective input impedance. I have heard that a DC/AC ratio can be too high where the inverter can't move far enough off the peak to clip the power and must shut down.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
Yes. It moves the point on the IV curve of the array by raising the voltage until the power is down to the maximum for the inverter.
synchro
Senior Member
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- Chicago, IL
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- EE
Agree. As the voltage is allowed to rise, the forward biased diode will start to conduct and shunt a portion of the generated photocurrent, thereby reducing the cell's output current.
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
Presumably as a result of the answers above, more of the incident solar energy gets absorbed by the panels, so it ends up as heat in the panels. Just by conservation of energy.
A related conclusion is that if you have two PV panels on the roof with identical insolations, and one is hooked up to an inverter and producing energy, while the other is disconnected, the working panel will have a lower temperature. Is this correct, and is the difference significant enough to be observable?
Cheers, Wayne
I actually do know the answer to this one. Yes, you'll be able to see this with IR scan.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Does it ever lower the voltage? Looking at a generic power curve, it does look like increasing voltage drops the power more quickly than lowering.
synchro
Senior Member
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- Chicago, IL
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- EE
The output current drops much more quickly when increasing the voltage because the current shunted by the diode junction of the PV cell rises exponentially with voltage according to the equation:
ID = IS ( e (VD / nVt) -1), where VD is the voltage across the photodiode.
Lowering the voltage will only decrease the output power in an approximately linear manner, which makes it more difficult for the switching supply circuitry to accommodate the required reduction in power.
But the most important issue for the inverter is that lowering the voltage does not reduce the current output from the PV cells, and the components in inverters have limitations on how much current that they can sustain reliably.
PWDickerson
Senior Member
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- Clinton, WA
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- Solar Contractor
I have seen IR ariel imagery of a large grount-mount array with a couple strings of modules that weren't operating, and their higher temperatures were immediately obvious in the scans. The photos were shown in a NABCEP CE conference presentation a few years back.
electro7
Senior Member
- Location
- Northern CA, US
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- Electrician, Solar and Electrical Contractor
On this subject, I know it's been normal to have a DC to AC ratio of anywhere from 15%-35%. Some inverters allow 100% DC overage.
On standard string inverters or even micro-inverters, is there any other benefit than cost savings? Do the inverters operate better at full capacity, pushing their limits?
I've heard that before but still have not been 100% confident in that theory.
I know the STC rating of the modules is rarely ever met in real-life conditions, so I understand that side of it. But is it possible the STC rating is hit a few times a year in perfect conditions? Or is it never?
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On standard string inverters or even micro-inverters, is there any other benefit than cost savings? Do the inverters operate better at full capacity, pushing their limits?
I've heard that before but still have not been 100% confident in that theory.
I know the STC rating of the modules is rarely ever met in real-life conditions, so I understand that side of it. But is it possible the STC rating is hit a few times a year in perfect conditions? Or is it never?
Sent from my SM-G998U using Tapatalk
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I have a DC/AC ratio of 1.18. I clip quite a bit and the panels put out STC often, so it is definitely not accurate to say something like "you will pretty much never get STC". Now my system is facing due south, set at the ideal (year round) tilt, and I am in the north east where it is cooler.
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
In this thread we talked about all the different ways that designing with clipping can be an advantage. It's a lot more than just saving on what you pay for inverters.
Another one I don't think was mentioned is if you have an off-grid system and want to ensure a minimum average energy harvest in winter.
Another one I don't think was mentioned is if you have an off-grid system and want to ensure a minimum average energy harvest in winter.
This is a big driver in off-grid systems and the thing that people who have only worked on grid-tied systems have trouble with. With an off-grid system, you have to build for the worst-case time of the year where the combination of load and PV production drives the PV and BESS sizes to the peak. That means that the system overproduces most of the year, sometimes by a lot. If you have a grid to be your lossless battery you don't have to worry about this. Production in June can be used in December in a grid-tied system, can't do that off-grid. Off-grid you have to use it in a few days or lose it.
Outside of a few rough rules of thumb, like the 1.2 DC/AC ratio the only way to dial in a system to optimize clipping is to use software to model it. Then you can play around with DC/AC ratio and see where it maxes out financially. There is a point where adding that next module is just not going to pay off.
There are downsides. Running the inverter at 100% capacity is going to age it faster.
There are downsides. Running the inverter at 100% capacity is going to age it faster.
electro7
Senior Member
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- Northern CA, US
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- Electrician, Solar and Electrical Contractor
good read, thanks- Status