Mppt voltage range vs max DC input voltage

[Sterling9250]

what’s the difference between max MPPT voltage range and max DC input voltage?

My inverter max dc input is 600V and the max range goes up to 550V. I’m wanting to use 14 panels that have a 45.16 open circuit voltage using Nominal Operation Cell Temperature (49.37 open circuit voltage using standard test conditions). Basic math puts me at a peak of 632 volts, which is obviously over either limits. 12 panels would put me at 541 volts. Should I keep it at 12 or can I push it to 13?

 

[BentleyJ]

12 x 49.4V (25C) = 592.8V The issue will be cooler temperatures causes Voc to go up. What is the coldest temperature you will experience at your geographic location. It may be necessary to reduce the panel count to 11.

Take a look at the data sheet and find Vmp or Vnmot. This is a better representation of the operating voltage when current is flowing.

 

[Sterling9250]

12 x 49.4V (25C) = 592.8V The issue will be cooler temperatures causes Voc to go up. What is the coldest temperature you will experience at your geographic location. It may be necessary to reduce the panel count to 11.

Take a look at the data sheet and find Vmp or Vnmot. This is a better representation of the operating voltage when current is flowing.

Coldest is about 0° C. I see a vmpp (rated voltage) of 41.60V (standard testing) and 38.05V (nominal operation).

 

[BentleyJ]

For simplicity lets say the Vmpp is 40V under normal operating conditions.

40V x 11 Panels = 440V which should be well within the MPP range of the charge controller.

49.4Voc x 11 Panels = 543.4V (at 25C)

At 0C, Voc will go up to 584.1V which is still below the 600V max limit. (calculation based on estimated temp coefficient of -0.3%/deg C)

11 panels per string is a much more conservative way to design the system.

 

[timselectric]

Coldest is about 0° C.

Look up the record coldest for your area.
This is not something that you want to guess about.

 

[Davo_100]

Guys, I'm really not getting a good understanding as to what the inverter does (or happens to it) if the MPPT voltage of the string is exceeded for short periods, but never exceeding the Overall Max String Voltage at any time. My Fronius Primo has 800V & 1000V levels respectively. I know all about the 600V Aussie String regulation and I'm not asking about that. That is a matter for insurance & licences. Surely a regular operating range for the MPPT voltage from 750-820V will not break the unit? Surely exceeding this 800V MPPT level is just an issue related to the inverter & panel efficiency? Thanks guys in advance.

 

[Luk88]

Guys, I'm really not getting a good understanding as to what the inverter does (or happens to it) if the MPPT voltage of the string is exceeded for short periods, but never exceeding the Overall Max String Voltage at any time. My Fronius Primo has 800V & 1000V levels respectively. I know all about the 600V Aussie String regulation and I'm not asking about that. That is a matter for insurance & licences. Surely a regular operating range for the MPPT voltage from 750-820V will not break the unit? Surely exceeding this 800V MPPT level is just an issue related to the inverter & panel efficiency? Thanks guys in advance.

I believe so too. Giving MPPT range or sometimes "standard MPPT voltage" also gives you an idea what kind of panels to match for the inverter. If you don't match them well, but you don't ever exceed VOC you're likely leaving some energy "on the table" instead of slurping it.

For example, my Y&H inverter has 500V VOC and 90-450V MPPT range, also 360V "standard MPPT voltage" which means if I take my panels (585W Jinko bifacial) that have 42V max power voltage and 52V VOC as well as -0.25%/℃ temperature coeffiecient of VOC which means on a cold winter morning (-30C or 55C difference between the standard conditions of 25C and the operating conditions) the VOC will be ((55*-0.0025)*52)+52 = 59.15V ! so I can't use more than 8 panels which means if I use the same string in the peak of spring my power will be at best 42V*8 * 16.5(max current of the MPPT) = 5544W for a 8S2P setup. Which means I'm leaving almost 4kW "on the table" in the sunny season, but the bonus is I get these 5kW from sunrise to sundown (almost). Also if I have only one string (not two in pralell) I'm using their full potential.

Then there is option two, different setup for summer and winter. Lets take my max VOC of 500V and say I wanted to use 9 panels, which gives a VOPC cutoff of 55.5V. which is 55.5/52=1.0673 or 6.73% above the 25C VOC. 6.74/0.25 is 26 so I'm fine with my 9S2P until the temperature stays above 25C-26 = -1C which is a reasonable assumption during summer here....

 

[JBertok]

The 600V rating on the PV side is absolute maximum electrical limit. Voltage beyond that is potentially (likely) damaging and most manufactures say it voids warranty. The MPPT operating range is always going to be a range with a lower upper limit. With the equipment I've got for example, PV over 140V will throw an error and shut down the MPPT requiring manual intervention to restore, even though it's a "150V" MPPT.

The temperature compensation mentioned above refers to a characteristic of silicon photovoltaic where the open circuit voltage is inversely proportional to the cell temperature with a coefficient correction factor that should be used when determining how many panels you should put in series for your MPPT. There is a chart available here: https://diysolarforum.com/resources/adjusting-solar-panel-voc-for-temperature.219/

 

[DIYrich]

Guys, I'm really not getting a good understanding as to what the inverter does (or happens to it) if the MPPT voltage of the string is exceeded for short periods, but never exceeding the Overall Max String Voltage at any time.

The purpose of the MPPT is to set the resistance of the circuit to maximize power flow from the panels. The min/max resistance corresponds to max/min voltage ratings listed as its operating range. Below and above that, it passes through whatever is presented by the panels (MPPT has no benefit).

Max Voc is the max voltage the system can handle. Above that, and you risk frying circuits.

Note: there is something between the MPPT and inverter that converts the voltage from the panels to a constant voltage that the inverter can use. Some convert to a high voltage the inverter uses directly, or to battery voltage that the inverter boosts to make 240v split phase ac. That "something" has a min voltage that it can boost to the constant voltage. That is the min voltage of the unit.