I present my solution, in the circuit that you can see in the image, to be able to connect 2 Strings to a single MPPT input of any power inverter, in my case East string and West string with automatic switching at a time close to solar noon to the PV2 input of my inverter.
I have a Goodwe ES5048 inverter, for those who don't know, this almost 5Kw inverter has only 2 photovoltaic inputs (2 mppt) for 2 strings.
The idea arose a little over 2 (two) years ago when I wanted to expand the area of solar panels to increase my solar production but I found myself with the problem of not having more space in my house to install more panels to the North.
I was not interested in buying another inverter to have 2 more mppt, not only for a matter of physical space to install them, but because the Goodwe is not parallelizable with other equal ones and the fact of having to modify the electrical circuits of my house downstream of the inverters was a resounding "no" for me
I have very good roofs at home to the West and East, so I began to investigate if there was any commercial solution for this purpose and I did not find anything.
I designed this Circuit that I present to you now, but that I have been working at home since March 2021, exactly tested for 2 (two) years, working perfectly with the DC RELAY Values that appear in the photographs of the Real Board installed in my home. It has been tested in all temperatures and conditions.
Before March 2021 I tried other 40 amp relays but they got too hot and I was afraid they would burn out. That's why I bought these that are 150 Amps and with the suggested aluminum heatsinks, even in the middle of summer with temperatures of 40 degrees, the heat dissipated is not high.
It's a very simple circuit that anyone with an idea of elementary electronics/electricity can do.
I could comment much more about it, but I prefer that you ask the questions that may arise.
The relays and their heat sinks can be purchased on aliexpress. From 600 Volts DC they have up to 400 Amps.
The timer can be purchased at any electrical/electronics store for a DIN board. It only matters that you have a Normal Open contact and a Normal Closed contact.
The other thing that is needed is a low voltage DC source to open or close the relays, it can be from an old cell phone since it supports more than 5 Vdc.
Really for me it was a solution that I am very happy with. None of the relays have burned out in two years of use. with an investment of a few dollars, minimal compared to the benefit it brings.
The scheme is self-descriptive, so as not to lengthen this Post.

 

 

Edited March 11, 20232 yr by Cef
correct

Hi @Cefvery impressive work, well done.

I am going to ask a potentially very dumb question. I have very limited knowledge about PV, except when cleaning it. 

If you have two identical strings, but on different location/angles as per yours, what is the disadvantage of merely parralelling the two strings with steering diodes, as oppose to the way you doing it? 

 

My 1st question would be what the cost of the 4 solid state relays are. 

Then I have to say good for you to come up with a own design. 

I have to agree with BRG on this one. While East is working full tilt the west string will still contribute handsomely if but we're just in parallel. As east start to dwindle after say 13h west will pick up well and continue to sunset. Overall I would expect production to be about 25-30% down compared to a North facing string with the same panels making up the east or west string. 

There is one condition that I can think of why this is an important and even an essential reason for the switch. With both strings in parallel the current from them might exceed the maximum input into the MTTP and the inverter should switch off. Now I hear a number of us saying no it will only cap the input. This is normally so but good designs should see a 1.7 or more times the maximum current as a no-no and protect itself by switching off. 

Still an interesting approach. 

1 hour ago, Scorp007 said:

There is one condition that I can think of why this is an important and even an essential reason for the switch. With both strings in parallel the current from them might exceed the maximum input into the MTTP and the inverter should switch off. Now I hear a number of us saying no it will only cap the input. This is normally so but good designs should see a 1.7 or more times the maximum current as a no-no and protect itself by switching off. 

Good explanation, thanks. 

Yes, one would be tempted to argue that the MPPT should be equipped to merely current limit to its max spec.  

 

 

Edited March 11, 20232 yr by BritishRacingGreen

20 minutes ago, BritishRacingGreen said:

Good explanation, thanks. 

Yes, one would be tempted to argue that the MPPT should be equipped to merely current limit to its max spec.  

 

 

How do I know this? 

I do now and again during LS connect my panels in 2 parallel string for the Axpert low voltage MPPT. Now they must revert back to series for the grid tied other inverter. I forgot to change to series and after the sync period the inverter switched on and off immediately. I glanced up to my switch and saw the problem. Now my grid tied will happily throttle if the PV is far too high. It will even for short periods generate 20% more than spec. 

1 hour ago, BritishRacingGreen said:

Hi @Cefvery impressive work, well done.

I am going to ask a potentially very dumb question. I have very limited knowledge about PV, except when cleaning it. 

If you have two identical strings, but on different location/angles as per yours, what is the disadvantage of merely parralelling the two strings with steering diodes, as oppose to the way you doing it? 

 

Hi @BritishRacingGreen
If you put 2 strings in parallel on a single mppt, it's a particularly dangerous condition for power inverters that run on high DC voltages and low amperage, like Goodwe. When placing 2 chains in Parallel, the Amperages of each of them are added. For the particular case of Goodwe, the specifications say a maximum for each MPPT of 14 Amps (17.5 A short-circuit current) and 580 Volts.
The operating amperages of the current configuration of the Brand and Model of the Panels that make up my two strings that alternately go to the same mppt at maximum power range from 10.5 amps to 12 amps. Near solar noon about 9 amps each. If they were in parallel, the inverter input burns.
What this circuit does is automatically ensure that the inverter only takes one string at a time.
Since the voltage drops for these relays at full power are negligible (less than 2 volts per relay), put a second relay in SERIES to act as a backup to ensure that if one burns out in an exceptionally closed condition, the other cut the current. when it remains in the condition determined by the TIMER. The literature on electronic RELAYS abounds that the mean life between failures is enormous. By placing 2 relays in series, the probability of a situation where both strings are placed in parallel is statistically insignificant.

Hi @Scorp007

Attention since they sell them for different voltages too. I use the one rated for 600 Volts DC

200 Amper (Rated)

https://www.aliexpress.com/item/4000295498163.html?spm=a2g0o.detail.0.0.107a688e1jm7rh&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.291025.0&scm_id=1007.13339.291025.0&scm-url=1007.13339.291025.0&pvid=ecfc4a56-c200-44cf-b38a-6a7f814b43f7&_t=gps-id%3ApcDetailBottomMoreThisSeller%2Cscm-url%3A1007.13339.291025.0%2Cpvid%3Aecfc4a56-c200-44cf-b38a-6a7f814b43f7%2Ctpp_buckets%3A668%232846%238110%231995&pdp_ext_f={"sku_id"%3A"10000001227318743"%2C"sceneId"%3A"3339"}&pdp_npi=3%40dis!USD!7.1!5.68!!!!!%402103222816785598342477844efa45!10000001227318743!rec!AR!

 

Edited March 11, 20232 yr by Cef

Cef, there is a more interesting application where switching PV string (as you did) is must.

MPPT inverters have different efficiency. There are so many and so bad documented  solar inverters sold nowadays that basically you do not know what you buy only when you measure it's parameters. Nowadays high voltage MPPTs have 120V-500V ( or even higher 800V-1000V) inputs. Some of them are claiming to run with 50V input AFTER it receives an input above 120V and the sun light dropped down. Actually this power input is fully consumed by the MPPT switching MOSFETS, almost nothing goes in battery for sure. You just have to measure how much the MPPT drawns from PV for it's own working, early in the morning or immediately after/during sunset.

Switching is must if you want to use the unused string with a small power, high efficiency MPPT charger *in the same time* in which the other string is producing highest energy. The small power MPPT can charge batteries even below 120V inputs and lower than 0.3A...0.5A available on string ( this is below 50w which usually is lost by most "efficient" solar inverters). Actually you will not find easily such MPPTs which works at 50V but is not damaged at full 500V input only if you DiY.

I'm using classic( and cheaper) 30A DPDT relays with coil at 24V named JQX 30.

Edited June 29, 2024Jun 29 by rowesley

On 2024/06/29 at 3:53 AM, rowesley said:

Cef, there is a more interesting application where switching PV string (as you did) is must.

MPPT inverters have different efficiency. There are so many and so bad documented  solar inverters sold nowadays that basically you do not know what you buy only when you measure it's parameters. Nowadays high voltage MPPTs have 120V-500V ( or even higher 800V-1000V) inputs. Some of them are claiming to run with 50V input AFTER it receives an input above 120V and the sun light dropped down. Actually this power input is fully consumed by the MPPT switching MOSFETS, almost nothing goes in battery for sure. You just have to measure how much the MPPT drawns from PV for it's own working, early in the morning or immediately after/during sunset.

Switching is must if you want to use the unused string with a small power, high efficiency MPPT charger *in the same time* in which the other string is producing highest energy. The small power MPPT can charge batteries even below 120V inputs and lower than 0.3A...0.5A available on string ( this is below 50w which usually is lost by most "efficient" solar inverters). Actually you will not find easily such MPPTs which works at 50V but is not damaged at full 500V input only if you DiY.

I'm using classic( and cheaper) 30A DPDT relays with coil at 24V named JQX 30.

Very good @rowesley ! It would be good if once correctly tested you could share your circuits.
Regards !