EG4 18kPV - String Calculations

[FilterGuy]

I searched for Ioc limit on this forum but this posting and another looking for specs on PV panels are the only ones that mention it.
I am not aware of the term Ioc limit, so would like to know more!

Sorry. Ioc was a typo. It should have said Isc and I corrected it above.

Note: Ioc would always be zero. (You can't have current in an open circuit).

So if an mppt input is rated for 3000 watt but you oversize it with 5000 watt but not get near the max current limit, it will not be a problem.
But being able to pull more current than the mppt has specified is a problem ?

We need to be specific.

There are two different kinds of specs

1) Specs that are limits of what the system can be exposed to
2) Specs that describe the capabilities of the system.

Limit Specifications:
For the 18K pv there are a few limits to what the MPPTs can be exposed to:

• Isc (31/19/19A)
• Voc (600/600/600V)
• Total array PV power for all three MPPTs. (21KW).

The solar array(s) must not exceed any of these limits or the unit can be damaged. Conversely, as long as the solar arrays do not exceed these specs, you are good to go.

Capability Specifications:
For the 18Kpv there are several capability specifications:

• Usable IMP current: (25/15/15A)
  The Imp current spec tells you how much current the MPPT will use but does not specify a limit on the solar array. Even if the solar array is capable of producing a higher Imp, the MPPT will not use it.
• Usable PV power of all three MPPTs: (18kw)
  This spec tells you the total power the 3 MPPTs can generate. Even though the the PV arrays might be able to generate more power, the system will not use it. If the total generated power reaches this limit, the MPPTs will start limiting the amount of power they will harvest from the array.
• Full power MPPT Operating Range: (230V - 500V).
  This tells you that the MPPT will do power point tracking in this range. However, the system can still work below and above that range (up to the 600V Voc limit).

EDIT: Added this:
Unfortunately, it can sometimes be an art to understand what is a limit spec and what is a capability spec. EG4 has been getting better at clarifying this but *all* solar equipment manufacturing companies need to improve.

 

[technobob]

Hello! Thank you for this great information! I am also trying to do the calculations for my EG4 – 18kPV and have a couple questions.

I am using the Canadian Solar C56W-530MB-AG bifacial panel.

Watts: 530W

Vmp: 40.9V

Imp: 12.96A

Voc: 48.8V

Isc: 13.80A

Here is a link to all of the specs of that panel: https://static.csisolar.com/wp-cont...2/CS-Datasheet-BiHiKu6_CS6W-MB-AG_v2.1_EN.pdf or see the attachment

I have 40 panels but it appears I can only use 38 or 39. (530W x 38 = 20,140W) I would like to configure as follows:

MPPT 1: 9s2p
MPPT 2: 10s1p
MPPT 3: 10s1p

I think that the total voltage of MPPT 2 and MPPT 3 should work at 488V each and MPPT 1 looks good with MPPT 1 having 440V for each string. This should give me the cushion needed during cold temps. However the amperage on MPPT 1 will be above the 25A usable limit but still below the 31A max. Do you think this would work?

The other question deals with the bifacial calculations. I am not sure how to figure that.

Thank you for your help!

 

[FilterGuy]

I think that the total voltage of MPPT 2 and MPPT 3 should work at 488V each

Hmmm.... 488Voc leaves plenty of room to prevent cold weather damage. That is fine. However, with a beta of -.26%/c it does not have to get very cold for the Voc to drift above 500V. I have tried to get more info on what the inverter does between 500V and 600V, but have not succeeded. I have heard that the MPPT will taper the current down as the voltage goes higher than 500V. I suspect that is to keep the total power (V*I) in a range it can handle. What I don't know is how the tapering affects the Power out of the MPPT.

 

[technobob]

Thank you for your quick response! Should I decrease MPPT 2 and MPPT 3 to 9 panels? And then what is you feeling about the amperage on MPPT 1: 9s2p?

 

[Quattrohead]

You should be fine to use 10s with all the strings, the voltage is under the 600 volt maximum that the inverter can handle and it will only pull as many amps as it can use.

 

[FilterGuy]

Thank you for your quick response! Should I decrease MPPT 2 and MPPT 3 to 9 panels? And then what is you feeling about the amperage on MPPT 1: 9s2p?

You should be fine to use 10s with all the strings, the voltage is under the 600 volt maximum that the inverter can handle and it will only pull as many amps as it can use.

Yes, 10 on the strings will work fine. My only question (and I don't know the answer) is what happens to the energy output above 500V.

 

[technobob]

I take it that I am okay with 27.4A on MPPT 1 using a 9s2p configuration?