Max MPPT Current

[bendesa]

Hi all,

A 50 kw grid tied inverter has the specs according to the attached picture. It's a Huawei 50 KW

On the Huawei 50KTL-M3 inverter datasheet
Max current per mppt 30A,
Max current per input 20A.
Max short circuit per MPPT 40A

What if we have two strings connected with the two inputs of 1 MPPT.

The specs of the PV Module are
Impp 17.4 A
ISc 18.52 A
550 Wp mono

2 x 17.4 A = 34.8 A

What do you guys think about this?

 

[pv_n00b]

That looks like an acceptable setup, but you may have clipping at peak PV production.

 

[GoldDigger]

The peak clipping may be greater on a cold clear day when the panel termperature is lower and therefore the Impp is higher,
If your panel orientation is not ideal for your peak power time, there may not be a clipping problem at all.

 

[ggunn]

The peak clipping may be greater on a cold clear day when the panel termperature is lower and therefore the Impp is higher,
If your panel orientation is not ideal for your peak power time, there may not be a clipping problem at all.

Temperature has more effect on voltage than on current, but yes, cold and sunny will maximize power output.

 

[bendesa]

Hi all,

Thanks for participating in this topic. Highly appreciated.

I also asked the question on the technical forum of Huawei. Perhaps you guys are interested in it.





Any comments ?

Kind regards

Ben

 

[jaggedben]

Ask them to try a different translation for whatever they meant by 'it is not so superimposed.' Whatever they meant, it's not coming through.
Also what is the 40A spec whose description is cut-off at the bottom?

 

[GoldDigger]

Ask them to try a different translation for whatever they meant by 'it is not so superimposed.' Whatever they meant, it's not coming through.
Also what is the 40A spec whose description is cut-off at the bottom?

That corresponds to the "Maximum Short Circuit per MPPT" mentioned in the OP.
The 37A parallel combination of the two strings meets this limitation.

I wonder whether they are trying to say that the real limit per MPPT is the input power and so if the string voltage is less than the MPPT Max a higher current is allowed. But I am not sure the MPPT input limit works that way.
Or they are thinking that the two strings will be in series OR will be facing in different directions.
If it is true that the two strings do not have the same azimuth, then the peak outputs for the two strings will not occur at the same time. But with just the input given by the OP there is no reason to assume that.

 

[bendesa]

Ues I agree, I will ask Huawei some extra questions to clearify this.

What I also didn't know is the fact that the Simulation Software PVSyst is not checking on this directly. Its even impossible to give in the max current per mppt in the software for the inverter model. So they must make some other kind of calculation.

 

[ggunn]

Ues I agree, I will ask Huawei some extra questions to clearify this.

What I also didn't know is the fact that the Simulation Software PVSyst is not checking on this directly. Its even impossible to give in the max current per mppt in the software for the inverter model. So they must make some other kind of calculation.

It doesn't matter to the inverter warranty what PVSyst says; if the inverter specifies a maximum connected Isc per MPPT you risk voiding the warranty if you connect more than that.

 

[bendesa]

It doesn't matter to the inverter warranty what PVSyst says; if the inverter specifies a maximum connected Isc per MPPT you risk voiding the warranty if you connect more than that.

Not according to Huawei.

 

[pv_n00b]

The manufacturer can say that the total current into an MPPT input cannot exceed a given value or they can say that the MPPT can only process a given amount of current and anything more will be lost to clipping. They usually state if it's the former and if nothing is said you can assume the latter.
Since they give an Isc maximum I would interpret this as a hard limit. The sum of the string Isc can't exceed this value for input to a single MPPT. There is no other reason to provide this value other than to specify a not to exceed value. The internal protection is designed assuming that the available fault current will not exceed the maximum Isc provided.

 

[bendesa]

The manufacturer can say that the total current into an MPPT input cannot exceed a given value or they can say that the MPPT can only process a given amount of power and anything more will be lost to clipping. They usually state if it's the former and if nothing is said you can assume the latter.
Since they give an Isc maximum I would interpret this as a hard limit. The sum of the string Isc can't exceed this value for input to a single MPPT. There is no other reason to provide this value other than to specify a not to exceed value.

Yes, I agree with you. I also send them someadditional questions after I received the answer from Huawie.

When they reply I will share it here.

The explanation they gave is fussy for me

Kind regards

 

[pv_n00b]

My guess is that the person who wrote you wrote the email in their native language, ran it through something like Google Translate, and sent you the result. You can't do that with this kind of tech support.

 

[bendesa]

My guess is that the person who wrote you wrote the email in their native language, ran it through something like Google Translate, and sent you the result. You can't do that with this kind of tech support.

It certainly looks like that

 

[electrofelon]

The manufacturer can say that the total current into an MPPT input cannot exceed a given value or they can say that the MPPT can only process a given amount of current and anything more will be lost to clipping. They usually state if it's the former and if nothing is said you can assume the latter.
Since they give an Isc maximum I would interpret this as a hard limit. The sum of the string Isc can't exceed this value for input to a single MPPT. There is no other reason to provide this value other than to specify a not to exceed value. The internal protection is designed assuming that the available fault current will not exceed the maximum Isc provided.

I just had this question/issue with SMA. I ran it thru their design tool because the destructions we're vague. The software says it is an acceptable configuration for the the module Imp to exceed the "max current per input" but it says not acceptable for module Isc to exceed "max SCC per input"

 

[ggunn]

I just had this question/issue with SMA. I ran it thru their design tool because the destructions we're vague. The software says it is an acceptable configuration for the the module Imp to exceed the "max current per input" but it says not acceptable for module Isc to exceed "max SCC per input"

That has always been my understanding as well. My thinking is that there is a limit to how much current the inverter's MPPT circuitry is capable of clipping.

 

[GoldDigger]

The manufacturer can say that the total current into an MPPT input cannot exceed a given value or they can say that the MPPT can only process a given amount of current and anything more will be lost to clipping. They usually state if it's the former and if nothing is said you can assume the latter.
Since they give an Isc maximum I would interpret this as a hard limit. The sum of the string Isc can't exceed this value for input to a single MPPT. There is no other reason to provide this value other than to specify a not to exceed value. The internal protection is designed assuming that the available fault current will not exceed the maximum Isc provided.

To me, clipping indicates that the input will not accept any additional current beyond that point and will raise the operating voltage to keep the current lower.
The additional current may also not be lost to clipping, just the voltage at that current may not necessarily be the experimentally derived Vmp voltage. Running out of input acceptance (clipping) and running out of control range (shaving?) have two different consequences.
The simple statement of the MPP operating range does not tell us unambiguously what happens when that range is exceeded.

 

[pv_n00b]

To me, clipping indicates that the input will not accept any additional current beyond that point and will raise the operating voltage to keep the current lower.
The additional current may also not be lost to clipping, just the voltage at that current may not necessarily be the experimentally derived Vmp voltage. Running out of input acceptance (clipping) and running out of control range (shaving?) have two different consequences.
The simple statement of the MPP operating range does not tell us unambiguously what happens when that range is exceeded.

I think you are talking about the difference between the MPPT tracking range and the maximum input current. The maximum input will be above the MPPT tracking range since once you start reducing current to keep it under the maximum allowed you are no longer doing MPPT tracking.
Also, MPPT tracking ranges are often artificially defined by the inverter manufacturer to get higher efficiency numbers. The MPPT will still track outside of the published range but at lower efficiency. It's all a game.