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How many panels can be installed with Axpert VM-3000 plus?


Taleb

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Hello brothers.
I have some unclear points regarding choosing solar panels compatible with MPPT charge controller,I have Axpert VM-3000 Plus and TrenaSolar panels 425W.
Based on the technical specifications of this solar panels and MPPT charge controller:
The first question : How many TrenaSolar 425W panels can be installed? is it two or three panles?! 
Second question: What does operating voltage Range depend on? does it depend on Vmp or Voc?
The third question: What does "Isc:11A" mean? Is this the maximum ampere that the MPPT can handle?! Does this mean that two panels cannot be connected in parallel on this MPPT?! I have dealt with a some of Axpert inverter
It's the first time I've seen "Isc" written in the MPPT specification. I don't understand why "Isc:11A" is written in the technical specifications
Although the ability of the MPPT is 60Amp!

The last question: What will happen if these numbers are exceeded? MAX. Solar voltage(Voc), Operating Voltage Range, Does the MPPT will be damaged?

Thanks

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Edited by Taleb
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  • Taleb changed the title to How many panels can be installed with Axpert VM-3000 plus?
1 hour ago, Taleb said:

How many TrenaSolar 425W panels can be installed? is it two or three panles?! 

As far as panel power is concerned, you could fit 4. 4 would total 1700 W. The rule of thumb I use is a maximum of 120% of nominal PV power can be added. So that would be 1.2 x 1500 = 1800 W. But see 

But these are high voltage panels; you can only connect two in series for a 25°C Voc of about 100V. 3S would produce over 150 V which will damage the solar charger; it would be worse in colder temperatures. So you would connect them 2S2P (two strings each of two panels in series).

1 hour ago, Taleb said:

What does operating voltage Range depend on? does it depend on Vmp or Voc?

You have to consider both, but Voc is the damaging one. 2S Vmp will be around 84 V, which is plenty for a 24 V system, and well under the MPPT maximum voltage of 115 V.

1 hour ago, Taleb said:

What does "Isc:11A" mean?

Normally, that's the current that the panels produce in very good sun when short circuited. It's close to the maximum current that the panels will ever produce on their own. These panels just squeak under that limit. But 2P connection means that you will end up with over 21 A. I agree that the Isc rating of this inverter is unusual and confusing. In order to produce 1500 W at 11 A, the panel voltage would need to be over 136 V, which is way above the maximum MPPT voltage. Perhaps they are expecting 2P, and the limit would then be 11 A per string. The whole solar charger circuit has to be rated for the 60 A output current; there is very little in the buck converter that is limited to the panel current.

1 hour ago, Taleb said:

I don't understand why "Isc:11A" is written in the technical specifications
Although the ability of the MPPT is 60Amp!

The solar charger is a buck converter, so it bucks the voltage but boosts the current. Say the panel voltage is 75 V and the battery voltage is 25 V, so the panel voltage is 3.0 times the battery voltage. For 2/3 of the time, the buck switch is open and panel current flows into the input capacitor. During this time, energy stored in the buck inductor capacitor charges the battery. For 1/3 of the time, the buck switch is on, so panel current plus twice the panel current stored in the previous cycle flows to the battery and fluxes the buck inductor. The cycle then repeats.

But since the maximum MPPT panel voltage is 115 V, taking the battery voltage as 25 V, the the ratio of input voltage to output voltage is a maximum of 115:25 or 4.6:1. So the ratio of output current to panel current is also a maximum of 4.6:1, so the input current has to be at least 60 A / 4.6 = 13 A for full output. It would require a panel with an Isc of about 15 A to achieve this. And of course, that's at the extreme edge of operation; normally you would operate with a lower panel voltage and hence higher input current. So I agree that the Isc rating is confusing and contradictory.

2 hours ago, Taleb said:

What will happen if these numbers are exceeded? MAX. Solar voltage(Voc), Operating Voltage Range, Does the MPPT will be damaged?

Yes. Exceeding the 145 V maximum under any conditions even for a brief time could lead to permanent failure of the solar charger.

In simple theory, the solar charger should limit the current it draws to what it needs, but in practice there are limitations of the control system. Adding too much solar panel power will cause the control system to overshoot and possibly harm itself. That's why it's important not to exceed the 120% rule of thumb.

My guess is that the 11 A figure is actually for a different model with a lower power solar charger. It's very disappointing and not confidence inspiring to see this sort of blunder.

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My one inverter 3kw MKS  plus  has similar specs also 1500w 30v to  115v ,  145v max  , 60amp and I have been running 2s 3p PV , 4 330w and 2 370w panels  making 2060Wats

The inverter max at 1550 under loads and been running like this for 2 years plus with out issues . 

Your panels being 425w best is 2s 2p

Edited by GMAC
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At the outset, I would like to thank you, Mr. Coulomb, for this beautiful reply, which took so much of your time.

13 hours ago, Coulomb said:

You have to consider both, but Voc is the damaging one. 2S Vmp will be around 84 V, which is plenty for a 24 V system, and well under the MPPT maximum voltage of 115 V.

Sir, it seems that I did not explain the question well.
In that MPPT There is two paramter:
Operating Voltage Range:30~115Vdc
Max. Solar Voltage(Voc):145VDC
And i thought the Operating Voltage Range:30~115Vdc depends on Vmp
And Max. Solar Voltage(Voc):145VDC depends on Voc
If the Voc is used to calculate the compatible "Operating Voltage Range" this means that Voc can not be exceeded 115Vdc
And here I must ask!
What's the point of saying Max. Solar Voltage(Voc):145VDC While Voc can not exceed 115Vdc in "Operating Voltage Range" paramter?!

Thanks Alot

 

 

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Also, allow me to thank you, Mr. GMAC for your reply.

9 hours ago, GMAC said:

My one inverter 3kw MKS  plus  has similar specs also 1500w 30v to  115v ,  145v max  , 60amp and I have been running 2s 3p PV , 4 330w and 2 370w panels  making 2060Wats

It's weird!
How does MPPT designed to work with 1500 watts still work with 2060 Watts.

I think that there are some influences such as the weather condition, the geographical location, and the electrical installations that were affecting one way or another on your system and blocking some of the ability of solar panles to reach the MPPT.

9 hours ago, GMAC said:

Your panels being 425w best is 2s 2p

The presence of "Isc: 11A" in the MPPT specification confuses me. How can I connect 4 solar panels as 2S2P?! The Amp will be more than 21A and This may be damaged the MPPT. 

Thanks Alot.

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5 hours ago, Taleb said:

Sir, it seems that I did not explain the question well.
In that MPPT There is two paramter:
Operating Voltage Range:30~115Vdc
Max. Solar Voltage(Voc):145VDC
And i thought the Operating Voltage Range:30~115Vdc depends on Vmp
And Max. Solar Voltage(Voc):145VDC depends on Voc
If the Voc is used to calculate the compatible "Operating Voltage Range" this means that Voc can not be exceeded 115Vdc
And here I must ask!
What's the point of saying Max. Solar Voltage(Voc):145VDC While Voc can not exceed 115Vdc in "Operating Voltage Range" paramter?!

If I may try to expand on this.

Voc is the Open-Circuit voltage of your panel. This is when there's no current flowing through the panel, zero power being produced yet.

Once the inverter begins generating, and once current begins to flow through the panel, the Voltage across the panels will drop down to the operating Voltage.

Best to look at the datasheet for your panel and see the graphs of P-V and I-V curves to get a feeling for this.

In your case the maximum Voc that your inverter can handle is 145V, but each panel has an output of 50.2Voc. If you place two panels in series you will get 100.4V across the panels, which is within spec, but if you put three panels in series that will be 150.6V, which is too much.

Now once current begins to flow, if you have two panels in series, the maximum power will be generated when the Voltage drops down to 42.2V over each panel, or 84.4V across the two panels. Up to peak power at a current flow of 10.08A This is comfortably within the specification of 30V-115DC operating Voltage. Three panels in series will again be too much.

It is common that people add more rated panel power than what the MPPT is rated for. Normally the MPPT will only allow a limited amount of current to flow to prevent damage, but you get the benefit of more power during poorer conditions, with lower light. Normally this can only be done up to a certain limit, for which Coulomb uses a rule of 1.2 times rated power, but GMAC has experience of 2060W on a 1500W MPPT, or nearly 1.4 times over.

It is not common for the maximum Isc that an inverter can handle to be stated prominently with an inverter's specs. I'd rather not comment here because as Coulomb said, there's room for interpretation on the 11A Isc versus the 60A MPPT input current.

Hope this makes sense.

 

 

Edited by GreenFields
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The mppt will only except and supply to the load a max of 1500w with my inverter it 1550w the rest is  capped .

With PV you will get the rated Wats in perfect condition which mite be ones a year .

So by going 2060w help to make the day longer as the panels start to generate power earlier in the morning and later in the day and on cloudy days still has the ability to carry the load . 

I also don't have all the panels facing the same direction so I will never get 2060wats out the setup as I have setup the panels to benefit from mid day to afternoon sun .

With my small setup I am able to use the max the mppt can give which is 1500w  longer through the day making the setup more efficient .

 

 

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8 hours ago, Taleb said:

And i thought the Operating Voltage Range:30~115Vdc depends on Vmp
And Max. Solar Voltage(Voc):145VDC depends on Voc

That's pretty much right, except that you should multiply the 25°C panel voltage specifications by about 1.07 to allow for operation down to 0°C. Allow more or less if your panels happen to have unusual temperature coefficients (silicon tends to behave like silicon though), or your climate regularly goes down way below 0°C or never below 10°C.

8 hours ago, Taleb said:

If the Voc is used to calculate the compatible "Operating Voltage Range" this means that Voc can not be exceeded 115Vdc

Perhaps I did not explain myself well. 145 V is the hardware limit, the "never exceed under any circumstances" limit.

The 115 V value is possibly a marketing figure to stay under the 120 V Extra Low Voltage definition (in some countries). In reality, the firmware doesn't behave any differently above 115 V, but starts ramping down the maximum solar power from 130 V. At 140 V for example, power is throttled to 33% of rated. At 145 V, assuming the silicon doesn't explode, power is reduced to zero in an attempt to save the hardware.

 

 

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