Inverter Maximum DC Current

[Intertechne]

Dear Andre,

Why the designing criteria, on the "System" window, does not consider the inverter maximum input current as a restriction, warning when the total array short circuit current is higher then the inverter DC current? Isn´t that should be consider a problem in the design?

Thank you in advance

[André Mermoud]

There are 2 input current limitations in PVsyst:

- At the sizing time: some manufacturers specify a maximum ISC current (or sometimes a maximum PV power) for the array to be connected on the inverter. To my understanding, this doesn't make much sense as the inverter is normally able to limit the input current (or power) by itself.

The only cases where this should be respected are when this is a contractual condition, which affects the warranty. This will provide an error message, and the prohect cannot be created as such.

- During operation, up to now there was a input parameter of the inverter "Minimum Voltage for getting Pnom". This corresponds indeed to an input current limiting, managed by the inverter.

Since recent versions of PVsyst, this current limit may also be explicitly defined, alternatively to VminPnom. These 2 varialbes are closely related: Imax * VminPnom = Pnom !

[Katinka]

Dear Andre,

I have another question about the maximum current per inverter input or MPPT input for multi-string inverters with multiple MPPT.

For my project I use Huaweis string inverters SUN2000-185KTL-H1 with 9 MPPT. Considering the maximum input current of this MPPT, 2 parallel module strings can be switched to each MPPT.

Due to the available module area and the module type 22 inverters are required.

In total, a maximum of (22*9*2=) 396 parallel strings are therefore possible.

The system part tells me instead that 391 to 418 parallel strings are possible. Where do these values come from?

I have tested two different variants:

1. variant without checking the box "Use multi-MPPT feature"

2. variant with ticking the box "Use multi-mppt feature".

The first one seems to take into account the maximum current per MPPT, as there is a "inverter loss over nominal inv power". Is this correct?

But why not consider it as "Inverter loss due to max input current"?

So the defined current limit in the inverter data is only taken into account to calculate the losses, but not to determine the maximum number of parallel module strings, right?

So the best way could be to check the box "Use multi-MPPT feature" and adjust the number of strings until the individual maximum desired power loss is reached?

Thanks a lot in advance!

[mauro.marchi]

Dear André,

I have the same problem described above by Katinka:

how to set PVsyst system for a correct simulation of Huawei 185 KTL string inverter, in case the number of input strings exceed 18 i.e. 2 strings for each of the 9 MPPTs of the inverter?

This string inverter is designed with 18 string inputs and have 1 MPPT each 2 inputs (total 9 MPPTs); Inverter multi MPPT feature cannot be disabled (unless you put in parallel all the strings before inverter input). Therefore in PVsyst system I tick the “multi MPPT” option. Everything is fine with the simulation when there are max 18 strings each inverter.

However, Huawei allows also connecting 3 strings (instead of 2) to each MPPT, using a Y connector on one of the two inputs of each MPPT. This way you can connect more than 18 strings to each inverter. But, according to inverter data sheet, each MPPT has a maximum input current of 26 A and with actual modules, often the Imp @STC of 3 strings exceed this limit.

In this case I am not sure what is the more correct PVsyst system setup:

- If I thick the “multi MPPT” option, in simulation results I have severe “over nominal power” losses.

- If I don’t tick the “multi MPPT” input instead the “over nominal power” losses are negligible.

I tried also defining two arrays, one having 2 strings each MPPT and the other having 3 strings each MPPT; then I used the “power sharing” option. In this case:

- If I click the “same DCAC ratio for all arrays” button, in simulation results I have negligible “over nominal power” losses (same as “no multi MPPT”).

- If I don’t click that button, in simulation results I have severe “over nominal power” losses (same as “multi MPPT” option).

The difference in simulation results in the two cases is really significant: about 3-4% losses using the “multi MPPT”, almost zero losses disabling the option.

What is the more correct PVsyst system setup for this string-inverter configuration?

I guess it is the “multi MPPT” option, since in inverter data sheet EACH inverter input has a current limitation. However I see the lower performances in PVsyst output are categorized as “over nominal power” and not as “over nominal current” losses, therefore I am not sure if the “multi MPPT” is really the correct option to use.

Can you please help me?

Thanks in advance

[dtarin]

It depends on your design. I dont know about those Huawei inverters specifically, but some string inverters have jumpers which share the power input across all inputs, instead of running independently. Using the jumper will distribute the power when inputs are loaded differently.

a) Do you have some inverters with a mix on the inputs? For example, there are 9 total inputs on an inverter, 7 of them are 2 strings each, and 2 of them are 3 strings each?

b) Or do you have entire inverters with 3 strings per input for all inputs (i.e. 27 strings per inverter), and others with 2 strings per input (i.e. 18 strings per inverter)

If it is a, separate the mixed inverters from non-mixed inverters (if it is the case): for the mixed inverters, count the total number of inputs with 2 strings per input, and set as one array. Count the number of inputs with 3 strings per input, and set as another array. These two arrays will use the multi mppt feature. The non-mixed arrays will not, and you will simulate normally. If your inverter operates the mppt channels independently, and does not power share between inputs, do not set the Pnom ratio to be the same. If they do power share, set Pnom ratios to be equal for the multi mppt arrays.

If it is b, dont use the mppt option, and have two arrays with the total number of inverters for each design (2 strings/input, 3 strings/input)

[m.marchi]

Thank you very much.

I am in situation "b". However, it is not clear to me (also after a conversation with Huawei) if the MPPT inputs of this inverter share or not share power.

Someone has a clear understanding of Huawei 185KTL internal architecture? Possibly something "official", from Huewei (that's unlikely, but... asking costs nothing :)

[dtarin]

Since the inputs are equally loaded in all inverters, it may not matter much if it power shares or not. There will be some variation between inputs on a single inverter, but it wont be very drastic. Modeling as two arrays with multi mppt off should be sufficient. If you're skeptical about the clipping losses, model a single inverter with 18 strings, and then in a separate run one inverter with 27 strings. In any case, you will not want to use the mppt feature with setting Pnom identical, since power will not be shared between 27 string inverters and 18 string inverters.

[m.marchi]

It does not work.

I simulated one invereter with "2 x strings x 9 mppt" and "3 x strings x 9 mppt", in both cases not checking multi-MPPT option nor power sharing,

in the 1st case give negiglible losse, in the 2nd case give huge losses.

Are you suggesting to do a weighted average of the two results based on the number of MPPT inputs with 2 strings and with 3 strings?

In my case, 2 strings x 7 mppt + 3 strings x 2 mppt

Doing this, the weighted averaged result (obviously) "in the middle" between the 2 extreme cases.

But the result of the weighted average is not equal to the result of simulation with "2 strings x 7 mmpt +3 strings x 2mppt, multi MPPT option, shared power with all Pnoms identical", but it is lower.

Interestingly, it is almost exactly the straight average between the above configuration and the same, but using shared power but NOT all Pnoms identical

The truth is in the middle? For sure, but it is not a satisfying answer.

André, what are your suggestions?

[dtarin]

It does not work.

I simulated one invereter with "2 x strings x 9 mppt" and "3 x strings x 9 mppt", in both cases not checking multi-MPPT option nor power sharing,

in the 1st case give negiglible losse, in the 2nd case give huge losses.

 

This was to show that your design with 27 strings per inverter will have high losses and unless you change your design, it should be expected to have high clipping losses. You havent mentioned DC AC ratio yet, but I presume it is high with 27 strings.