Over voltage problem with Grid-tie micro inverter

meetyg

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Hi there.
I have a 1200w no-name grid-tie micro inverter from Aliexpress (model GTB-1200).

According to specs, it should handle 22-50v input, on each one of its 4 inputs.
It has over-voltage protection.
The problem is that it seems to be out of spec, and accepts voltages upto around 36v.

For example, when I connect two 18v (etfe, 100w) panels in series, it works fine.
But I am trying to connect a pair of new glass panels (not in series, one to each port), and they seem to trip the over voltage protection, because each one outputs 40-45v, even when not in full sunlight.
I was able to verify this by manually shading the glass panel, bringing down the voltage to around 34v, and it worked.

These new panels are rated at around 450w STC each, which is a bit more than the 300w recommended input for each of the inverters 4 ports. I am skeptic that the actual output will be that, because of various conditions. Even if, I don't mind clipping of the AC output. But after doing some testing I'm pretty sure that the problem is over voltage, not over current.

Having that this micro-inverter was bought from Aliexpress, the seller hasn't been very responsive, giving me laconic answers.

Anyways, I opened up the inverter to see if there is anything I could adjust myself (like a potentiometer) but there isn't anything I could identify. Some similar Chinese micro-inverters have a potentiometer on them, as I have seen on YouTube, but this one doesn't.

I'm attaching a picture of it, if you guys have any ideas, I would greatly appreciate it.

Another thought I had was to somehow split the voltage, but I'm not sure of an affective way to do so. Some simple voltage divider circuits would require some resistors, I suppose I would need some seriously rated resistors to handle this kind of load. In any case, I wouldn't want to loose power turning it into heat with resistors.

Any ideas how to split the voltage in an affective way?

I know my use case is kinda odd, but any help would be appreciated.20210709_154450.jpg
 

snoobler

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Those 18V panels are actually 42V in series. You have to look at Voc.

You've already posted on "splitting the voltage":

 

meetyg

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Those 18V panels are actually 42V in series. You have to look at Voc.

You've already posted on "splitting the voltage":

I posted previously to understand what will happen if I split one panel into two inputs.
Since the replies suggested that voltage will stay the same, I need to split voltage somehow.
Another thing is that meanwhile I found that this inverter behaves out if spec. It was supposed work up to 50v.
 

GXMnow

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The microinverter you have is likely meant for 60 cell panels, just like my Enphase iQ7's. That makes sense for 300 watts per input. Your 450 watt panels are 72 cells (or even 96 cell) and a full 50% more power than recommended. That is over the top. 72 cell panels need microinverters rated for the higher voltage. 50 volt max VOC is certainly too close for a 72 cell panel, let alone if they are 96 cell ones.
 

meetyg

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The microinverter you have is likely meant for 60 cell panels, just like my Enphase iQ7's. That makes sense for 300 watts per input. Your 450 watt panels are 72 cells (or even 96 cell) and a full 50% more power than recommended. That is over the top. 72 cell panels need microinverters rated for the higher voltage. 50 volt max VOC is certainly too close for a 72 cell panel, let alone if they are 96 cell ones.
Yeah, If I recall correctly they are 72 cells.
The strange thing is, that I have a similar micro-inverter, GTB-600, which is rated at 600w, with two inputs.
If I connect the panel to that one, it seems to work fine, but of course it won't be able to handle two of these panels. It's also rated up to 50v, 300w per input.

That's why I have come to the conclusion that the GTB-1200 is out of spec, handling only around 36v per input, no matter how many amps go in, and even if the total wattage is still around 300w per input.
 

meetyg

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What would happen if I try this (I am asking before trying, to see if it sound reasonable, before I try and fry something...):

Connect one positive connector to one input, connect a negative to another inputs negative, and then jumper (connect) the remaining negative and positive with just a short cable.
Will this split the voltage or just create a short circuit and blow something out?
 

GXMnow

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Without knowing anything about the internal circuit, I would assume the separate inputs tie to a common buss in some way, so it would likely short one of the inputs. So it will likely still just run one input, while the other sees no power. It might not damage it, but it could. It really depends on the circuit. I don't recommend trying it. You just might let out some smoke.

Enphase has 4 different models now to cover 60 cell, 72 cell, and 96 cell panels, as well as a new one that can take 60 or 72 cell with a wider range MPPT section. The ones you have are most likely optimized for 60 cell panels. Even the one that is working, is out of it's range, and probably won't ever put out more than 300 watts per input channel. So you are tossing out efficiency. Over paneling can be helpful to a point, but 50% over is a bit much. IF you paralleled two 60 cell 200 watt panels, it would probably work fine and give you more time at peak output because the voltage would still be in range of the MPPT tracker. I looked up 450 watt panels, and the ones I have found have a VOC over 50 volts. And panels can easily exceed the STC VOC rating in some conditions. The 445 watt panels are rated at 49.9 volts. Most MPPT manufacturers say to allow 20% margin from VOC rating to maximum input voltage. So you should be looking at panels rated around 40 volts, or a microinverter rated at 60 volts. That will give you enough headroom for reflected light and colder temperatures.

One work around might be to use a zener diode shunt. As long as the voltage is being pulled down below say 45 volts, the circuit will not do anything, and all of the power of the solar panel will be going to the microinverter. If the voltage creeps up too high, then the zener diode will begin to conduct and drive the base of a power transistor. The transistor will pull current from the solar panel and hold the voltage down so it does not hit the shutoff of the microinverter. The 450 watt panel that I found can put out up to about 11 amps. So the circuit should be designed to be able to handle that, just in case the microinverter shuts off and all of the power ends up going to the shunt. So you need transistors that could take 500 watts to be safe. In normal operation, they won't need to handle much though. The 450 watt panel at maximum power should be down to just 42 volts. That is still likely on the edge of the MPPT tracker, but should work. As a quickie experiment, do you have an old filament 60 watt or larger light bulb? If you connect that across the solar panel output, how low does it pull the voltage? If it is below 45 volts, try connecting to the microinverter and see if it works. If you remove the light bulb, it will likely shut down again. A 120 volt 60 watt bulb at 45 volts DC will be pulling about 0.2 amps, so it will only be wasting about 9 watts out of your 450 watt panel. If that works, then a zener shunt regulator might do the trick.
 

meetyg

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Without knowing anything about the internal circuit, I would assume the separate inputs tie to a common buss in some way, so it would likely short one of the inputs. So it will likely still just run one input, while the other sees no power. It might not damage it, but it could. It really depends on the circuit. I don't recommend trying it. You just might let out some smoke.

Enphase has 4 different models now to cover 60 cell, 72 cell, and 96 cell panels, as well as a new one that can take 60 or 72 cell with a wider range MPPT section. The ones you have are most likely optimized for 60 cell panels. Even the one that is working, is out of it's range, and probably won't ever put out more than 300 watts per input channel. So you are tossing out efficiency. Over paneling can be helpful to a point, but 50% over is a bit much. IF you paralleled two 60 cell 200 watt panels, it would probably work fine and give you more time at peak output because the voltage would still be in range of the MPPT tracker. I looked up 450 watt panels, and the ones I have found have a VOC over 50 volts. And panels can easily exceed the STC VOC rating in some conditions. The 445 watt panels are rated at 49.9 volts. Most MPPT manufacturers say to allow 20% margin from VOC rating to maximum input voltage. So you should be looking at panels rated around 40 volts, or a microinverter rated at 60 volts. That will give you enough headroom for reflected light and colder temperatures.

One work around might be to use a zener diode shunt. As long as the voltage is being pulled down below say 45 volts, the circuit will not do anything, and all of the power of the solar panel will be going to the microinverter. If the voltage creeps up too high, then the zener diode will begin to conduct and drive the base of a power transistor. The transistor will pull current from the solar panel and hold the voltage down so it does not hit the shutoff of the microinverter. The 450 watt panel that I found can put out up to about 11 amps. So the circuit should be designed to be able to handle that, just in case the microinverter shuts off and all of the power ends up going to the shunt. So you need transistors that could take 500 watts to be safe. In normal operation, they won't need to handle much though. The 450 watt panel at maximum power should be down to just 42 volts. That is still likely on the edge of the MPPT tracker, but should work. As a quickie experiment, do you have an old filament 60 watt or larger light bulb? If you connect that across the solar panel output, how low does it pull the voltage? If it is below 45 volts, try connecting to the microinverter and see if it works. If you remove the light bulb, it will likely shut down again. A 120 volt 60 watt bulb at 45 volts DC will be pulling about 0.2 amps, so it will only be wasting about 9 watts out of your 450 watt panel. If that works, then a zener shunt regulator might do the trick.
Wow, thanks for that informative reply !

I like your idea about the shunt.
I was looking for something along the lines this:


It's a dc-dc step down, that can handle upto 1000w, and 50-55v input.

What do you think, is this a reasonable solution (even if efficiency is around 90%, I can live with it) ?

It's fairly cheap too, at around $25.
Maybe this could solve my problem. I would probably get two of them, one for each 450w panel, just to be on the safe side.
 
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GXMnow

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The problem with this is it will totally confuse the MPPT tracking.

MPPT works by making small adjustments to the current draw, and checking the voltage to calculate the power. IF it pulled more current and saw an increase in power, it will then pull even more current on the next adjustment cycle. If the power drops off, it will then reduce current and try again. By doing this a few times a second, it will find the current draw that produces the most power. Maximum Power Point Tracking.

When you add the step down buck converter, the MPPT is no longer seeing the real current/voltage of the solar panel. When the MPPT tries pulling more current, the buck converter will pull even more current from the solar panel to hold the same output voltage. So the MPPT will just keep seeing the current increase with the voltage holding the same, so it does not know the solar panel voltage is falling like a rock until the buck converter just can't hold voltage any more and it shuts off. It will most likely just start pulsing and not hold a steady current, or it could just lock up and quit.
 

meetyg

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From the video I posted (at about 30:00), it seems that when maximum current set is reached, it does bring down the voltage. So I suppose that if I set it to around 36v output and an appropriate maximum current (may need some trial and error, or measurements in various conditions), it should work OK with the MPPT. Again, maybe not ideal or best efficiency, but better than nothing :rolleyes:
 
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meetyg

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Well just to update, I have given up on that cheapo microinverter, and got a Deye 2000w microinverter. So far it's working well.
I really like the Solarman App and website that it uses for monitoring. It has alot more info than the cheap microinverter has.
For example now I can see current and historical data for each if the 4 MPPT inputs separately, or aggregated as a whole.

Also, the Deye has an internal temperature sensor, so I can monitor if it's overheating.

It cost me twice as much (around
$400) as the cheap 1200w one, but it seems like alot better quality.
It may be a bit overkill for my 1200w setup (inverter probably not running at maximum efficiency) but it leaves room for future expansion.
 
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