Utility Scale w/ string inverters and First Solar mods

E

electro7

Senior Member
Location
Northern CA, US
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Electrician, Solar and Electrical Contractor
Looking to do a quick design for bidding purposes on design build utility scale sites. These sites average around 3.4MW.

250kw CPS string inverters are shown. I see by the spec sheet they only have 36 inputs- 12 mppts with 3 inputs each. Each mppt max current is 26A.

530W First Solar series 7 modules with a high Voc (226V) which allows for only 6 modules max per string. Isc is 3.05A.

I don't see how this configuration will work with a 1.3 ratio. A 1.3 ratio would be around 613 modules per inverter. 613/6 modules per string = 102.16 strings per inverter. 103 (102.16 rounded up)/12 mppt inputs = 8.58 strings per input.
Isc 3.05 x 9 (8.58 strings rounded up)= 27.45A per MPPT.

Any thoughts on this math and how it could work?

Also, when the MPPT says 26A max does that include the 1.25 continuous load multiplier? I'd assume it would which in this case would only allow for 6 strings per MPPT. 6 x 3.05A x 1.25 = 22.88A. 7 strings takes it over 26A.

Appreciate the help. I think I must be missing something.
 
I'm not sure what data sheet you are looking at but the CPS 250kW 600V inverter has a single MPPT input and can be used with either a string wire box with 30 inputs or a centralized wire box with one input for use with a remote DC combiner. The max dc short circuit current is 450A which easily allows a DC/AC ratio of 1.3.

The typical MPPT current spec is the maximum current that the MPPT will accept with no adjustments. If it says 26A then that's the maximum current you will see if you put an ammeter on it, if the PV array could provide more then the MPPT shifts the voltage to lower the current. It's not a limit on the possible current that the PV array can provide. That will typically be given by a max short circuit current rating or the max DC/AC ratio allowed.
 
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electro7 said:
Looking to do a quick design for bidding purposes on design build utility scale sites. These sites average around 3.4MW.

250kw CPS string inverters are shown. I see by the spec sheet they only have 36 inputs- 12 mppts with 3 inputs each. Each mppt max current is 26A.

530W First Solar series 7 modules with a high Voc (226V) which allows for only 6 modules max per string. Isc is 3.05A.

I don't see how this configuration will work with a 1.3 ratio. A 1.3 ratio would be around 613 modules per inverter. 613/6 modules per string = 102.16 strings per inverter. 103 (102.16 rounded up)/12 mppt inputs = 8.58 strings per input.
Isc 3.05 x 9 (8.58 strings rounded up)= 27.45A per MPPT.

Any thoughts on this math and how it could work?

Also, when the MPPT says 26A max does that include the 1.25 continuous load multiplier? I'd assume it would which in this case would only allow for 6 strings per MPPT. 6 x 3.05A x 1.25 = 22.88A. 7 strings takes it over 26A.

Appreciate the help. I think I must be missing something.
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I did not go through your math, but if your calculated max Voc adjusted for low temperature shows a maximum string length of 8.58 you cannot round up to 9; your maximum is 8.
 
electrofelon said:
In addition fo to what PV noob said, you typically use I max power not Isc for that value.
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Some inverter spec sheets will show that they have already included the 1.25 factor in their maximum connected Isc number, but some do not. If they don't say that they do, then you have to assume that you must do it.
 
If i am looking at the correct spec sheet, they say "max PV input current - 26A" and "max PV short circuit current 36A". I always feel that they often need to be more clear on this, but i assume the 36A is the "code value" and 26A is the "design value" that the inverter will limit its input to. Do others agree?
 
electrofelon said:
If i am looking at the correct spec sheet, they say "max PV input current - 26A" and "max PV short circuit current 36A". I always feel that they often need to be more clear on this, but i assume the 36A is the "code value" and 26A is the "design value" that the inverter will limit its input to. Do others agree?
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I do, and since Imp is always less than Isc it makes sense. The point I was trying to make earlier is that sometimes the inverter spec sheet states explicitly to use Isc and sometimes it doesn't, in which case you should use 1.25*Isc instead of Isc for maximum short circuit current.
 
ggunn said:
I do, and since Imp is always less than Isc it makes sense. The point I was trying to make earlier is that sometimes the inverter spec sheet states explicitly to use Isc and sometimes it doesn't, in which case you should use 1.25*Isc instead of Isc for maximum short circuit current.
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I do see it as a bit of a gray area as to whether you need to use the 125% value, if the inverter doesn't require it.
 
electrofelon said:
I do see it as a bit of a gray area as to whether you need to use the 125% value, if the inverter doesn't require it.
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I don't see it as gray. If the inverter calls out maximum Isc per 2 input MPPT, I use Isc to decide if I can connect two strings. If it calls out maximum short circuit current, I use 1.25*Isc. It usually doesn't make a difference to the design, but sometimes it does.
 
ggunn said:
I don't see it as gray. If the inverter calls out maximum Isc per 2 input MPPT, I use Isc to decide if I can connect two strings. If it calls out maximum short circuit current, I use 1.25*Isc. It usually doesn't make a difference to the design, but sometimes it does.
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Okay but what is requiring you to add the 1.25 multiplier? I assume 690.8, but I am not sure that applies to this. It seems to me it's up to the inverter manufacturer to say what the current is.
 
electro7 said:
Looking to do a quick design for bidding purposes on design build utility scale sites. These sites average around 3.4MW.

250kw CPS string inverters are shown. I see by the spec sheet they only have 36 inputs- 12 mppts with 3 inputs each. Each mppt max current is 26A.

530W First Solar series 7 modules with a high Voc (226V) which allows for only 6 modules max per string. Isc is 3.05A.

I don't see how this configuration will work with a 1.3 ratio. A 1.3 ratio would be around 613 modules per inverter. 613/6 modules per string = 102.16 strings per inverter. 103 (102.16 rounded up)/12 mppt inputs = 8.58 strings per input.
Isc 3.05 x 9 (8.58 strings rounded up)= 27.45A per MPPT.

Any thoughts on this math and how it could work?

Also, when the MPPT says 26A max does that include the 1.25 continuous load multiplier? I'd assume it would which in this case would only allow for 6 strings per MPPT. 6 x 3.05A x 1.25 = 22.88A. 7 strings takes it over 26A.

Appreciate the help. I think I must be missing something.
Click to expand...
Don't forget, you need to use corrected temperature when calculating max string length based on Voc and Max input voltage. With the 226.7 Voc and -.28%/C Temp coefficient of Voc, if the extreme min temperature in the array local reaches -12C, the voltage would exceed 1500 at that record low with 6-module strings.

On the CPS data sheet I see for the current 250/275kW string inverter with 12mppt's, the max PV input current per mppt is listed as 30A. That is the max current before clipping, so you should be using Imax of 2.84A from the module data sheet. With 3 strings per input x 3 inputs per mppt, , that is 3 x 3 x 2.84A = 25.56A per MPPT. If you have a data sheet for an older model or something that has a 26A limit, it is still ok.

In the CPS data sheet I have, it lists the max PV Short-circuit current as 50A. The 3.05A Isc from the data sheet should be temperature corrected for highest expected local temperature if there is any chance of being close to the limit, but I would doubt this would ever be the case. The temp coeff of Isc is +.04%/C. If Isc corrected for temp is 3.2A, then 9 strings per mppt would give 9 x 3.2A = 28.8A, which is far below the 50A limit. If the data sheet you have gives a different PV Short-ckt current, then what is it?

So 3 strings per input, 3 inputs per mppt, 6 mods per string, 12mppt's, gives 9 strings per ppt, 108 strings total, 648 modules total, 343.44kW total, and a 1.37 DC to AC inverter ratio. I don't see any issues with that.
 
electrofelon said:
It seems to me it's up to the inverter manufacturer to say what the current is.
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I agree. If the inverter spec sheet says "maximum short circuit current" it means 1.25*Isc, the same as what you use to size the DC conductors. If it calls for the maximum Isc, Isc is what it means because the effects of excess solarization have been taken into account.
 
pv_n00b said:
I'd be interested in knowing what data sheet you are looking at. This is the one from the CPS website for the only 250kW inverter they make and it does not match the spec you are looking at.
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That data sheet is for their single MPPT inverter, which I think is the newest model. They have another model that has 12 mppts with the current limits I stated above, but they deviate slightly from the OP numbers.
 
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