Temperature Effect on MPPT Voltage

[GreenFields]

One of the recurring solar topics here is what the impact is of temperature on MPPT Voltage, and choosing your panels' Voc with some spare gap in theory to the inverter's maximum input limit.

I'm just wondering whether the current cold snap could bring a surge of failures in some parts of SA. For example, Johannesburg's forecast for Tuesday morning is showing freezing temperatures overnight, with early morning sunshine around 7am-9am. Are there any folks around those areas with longer strings of around 450Voc that could share their real Voltage measurements and/or experience in these conditions?

Edited July 8 by GreenFields

[Scorp007]

On 2024/07/08 at 2:14 PM, GreenFields said:

One of the recurring solar topics here is what the impact is of temperature on MPPT Voltage, and choosing your panels' Voc with some spare gap in theory to the inverter's maximum input limit.

I'm just wondering whether the current cold snap could bring a surge of failures in some parts of SA. For example, Johannesburg's forecast for Tuesday morning is showing freezing temperatures overnight, with early morning sunshine around 7am-9am. Are there any folks around those areas with longer strings of around 450Voc that could share their real Voltage measurements and/or experience in these conditions?

Let's look at this question. Some time ago I raised the statement that early cold mornings the irradiation is low thus we normally do not get close to the 1000W/m² Voc. 

This morning with panels at - 12° my panels were at Voc=185V at 06h45 during low irradiation. 

Yesterday @ 0° around the same time but not Voc the level was about 200V.

This morning at around -2to-4° the loaded voltage was about the same. 

Not ideal as the string is short but one can at least get an idea. 

 

Edited July 9 by Scorp007

[GreenFields]

25 minutes ago, Scorp007 said:

Let's look at this question. Some time ago I raised the statement that early cold mornings the irradiation is low thus we normally do not get close to the 1000W/m² Voc. 

This morning with panels at - 12° my panels were at Voc=185V @7h45 during low irradiation. 

Yesterday @ 0° around the same time but not Voc the level was about 200V.

This morning at around -2to-4° the loaded voltage was about the same. 

Not ideal as the string is short but one can at least get an idea. 

 

Thanks. If I may ask, what's the nominal Voc and Vmp based on the panel spec sheet for your string? I guess the values at 25 degrees, and based on STC and/or NOCT, before doing temperature adjustments?

Trying to get a sense whether the temperature issue is overblown for our SA conditions. I mean, will you ever get a case where the daytime temperatures hover around freezing, and the sun then emerges behind a cloud at noon? Sutherland's looking promising for the week, but I don't expect a massive wave of inverters failing coming from that area.

[Scorp007]

1 hour ago, GreenFields said:

Thanks. If I may ask, what's the nominal Voc and Vmp based on the panel spec sheet for your string? I guess the values at 25 degrees, and based on STC and/or NOCT, before doing temperature adjustments?

Trying to get a sense whether the temperature issue is overblown for our SA conditions. I mean, will you ever get a case where the daytime temperatures hover around freezing, and the sun then emerges behind a cloud at noon? Sutherland's looking promising for the week, but I don't expect a massive wave of inverters failing coming from that area.

I doubt that even in Sutherland It would be a problem from behind a cloud because at that point the panels will be connected to a MPPT and not under Voc. 

So only a problem if the panel isolator was opened and during closing. 

Another example this morning. Panel with STC of 48V was at Voc=34.3V and once connected to MPPT that started it dropped to 33.8V under very light load. 

I have a string of 6. Also a great test site as 3 makes of mono and 1 poly. Production is the same per panel as string started life as 2 series. Added over a few years. 

STC values of the 6S string. 

Edited July 9 by Scorp007