Voc string sizing reference temperature

[tecnun]

Hello,

I have encountered a problem when it comes to design long string sizes in warm locations. In project settings you can set the temperature but it cannot be higher than 30ºC so in system you are obliged to limite the string size (as it turns "red" if, for instance, Voc > 1500 V). I suggest to widen the temperature range to 40-45ºC (now this ranges from -40 to +30) bcs more and more utility installations are requiring for ex. 31-32 modules per string in very warm locations (like Colombia)

Looking forward to it in next versions, my best regards Jose

[Lazare Fesnien]

Hi,

The number of modules in series has to match the following conditions:

- The minimum array operating voltage (i.e. at max. module operating temperature, 60°C by default) should be above the minimum inverter's operating voltage (Vmin of MPPT range).

- The maximum array operating voltage (i.e. at min. module operating temperature, 20°C by default) has to stay below the maximum inverter's operating voltage (Vmax of MPPT range).

- The maximum array absolute voltage (i.e. Voc at min. temperature, -10°C by default) has to stay below the absolute maximum inverter's input voltage.

- The maximum array absolute voltage (i.e. Voc at min. temperature, -10°C by default) should not overcome the maximum system voltage specified for the PV module.

When the desired array configuration doesn't match these requirements, the system is usually not properly sized.

Please see the diagram in the "System" definitions, button "Show sizing", which summarizes all these constraints:

Design temperatures :

These conditions involve design temperatures, which are part of your project and may be changed according to your climate in the definition of the project, option "Albedo and settings".

The default values (for each new project) may be redefined in the Hidden Parameters, topic "System design parameters".

These are:

- Maximum cell temperature in operating conditions, default 60°C,

- Summer usual operating conditions, not used for sizing constraints, default 50°C,

- Winter minimum cell temperature in operating conditions, default 20°C,

- Absolute Cell lower temperature for determining the Maximum possible voltage of the array. The default is set to -10°C for most European countries (best practice rule).

For this limit, the cell temperature is considered as the ambient temperature (worst case when the sun suddenly appears on the field).

You should define the lowest temperature ever observed during the day for this site.

[tecnun]

Thanks Lazare,

Just as an example (SANDIA):

 

 

It´s common to find the Voc peak in the range of 600-1000 W/m2 of POA, so in cold days the lowest daytime temperature does NOT determine the highest Voc. Let´s bring this scenario to a warm equatorial site like Colombia: There exists the real possibility of designing with 34 modules per string, and the maximum Voc (1.464 V < 1.500) happens at 650 W/m2 and a cell temperature of 43ºC.

If we are limited to cell temperature of 30ºC, this would lead us to set the ambient temperature in this simulation to 17ºC, and the peak Voc now would rise to 1.512 V, so we would have to limit to 33 units per string.

This is only an example. The main problem is that in PVsyst you fix the cell temperature, but it must be a combination of temperature. wind and radiation, so my suggestion is to build internally what i have made in an excel, taking into account METEO yearly data not just a cell temperature, dont know if i ve explained...

BR Tecnun

[Bruno Wittmer]

The calculation of the maximum expected voltage at the inverter input is a safety issue, meaning that a single event might already cause damage to the equipment.

Therefore, it is necessary to check the worst case scenario for the question:'For my PV system, which is the lowest possible cell temperature with some significant amount of irradiance at the same time?'

As your plot nicely shows, the Voc curve is quite flat from 300 W/m2 onward, meaning that already rather low irradiance brings you close to the maximal voltages.

If you think of the worst case scenario, there are two points that are not taken into consideration in your argument:

• It is not advisable to look for the worst case in the meteo data of a typical year, since it is quite likely that the worst case happens rather in an exceptional year.
• If you would like to take the worst case from measured data instead, you should probably look at decades of measurements, in order to be sure to find a situation coming close to the worst possible case.
  In any case, I think it is better to just estimate the worst possible conditions, and work with generous safety margins.

• The worst case scenario is not a stationary situation, where the PV modules are in thermal equilibrium with the surroundings.
• I think the worst case is on a cold day, when the PV modules are roughly at ambient temperature, because there are many clouds and very little irradiance. If then the clouds open quickly enough, such that the PV modules do not have time to heat up, you will get the highest possible values for the PV module voltages.

 

Please note also, that the calculation uses Voc, which is higher than the normal operating voltage Vmpp. Also here, the idea is to consider the worst case, and not a standard situation.

As you can see, the reasoning here is to completely exclude the possibility of the voltages at the inverter input exceeding the safety threshold. For this purpose, we consider it to be reasonable to constrain the lowest possible cell temperature to values of 30°C or lower.

If however you think this is too limiting for your simulation, you can still increase the maximum voltages in the OND and PAN files, but be aware that you are working very close to real safety limitations.

[tecnun]

Hello Bruno, and thanks for you analysis,

In my opinion, we are overprotecting our system at the expense of a superconservative design. Just a few thoughts:

- We are assuming that in the event of a really cold cold day, assuming that the cells have not heated yet, during a daytime and under an irradiation over 500 W/m2 an event of electrical shutdown and reconnection (only at those events there exist open circuit conditions)

- We are assuming that at abovementioned event, there exist no quality, mismatch, soiling, DC ohmic etc losses

- We are assuming that the inverter does not protect itself from avobe 1500 V events. We have being chatting about this issue with the main inverter suppliers

- We are also assuming that this "fatal" event is befalling at the very beginning of the PV Plant COD. We must take into account in my opinion LID and natural degradation

PVsyst is, so far, accounting in Design Conditions (Project Settings) CELL temperature at this worst case event. Our proposal is to limit string sizes, but taking into account the "real" worst case. Is it too difficult to program this algorithm? Changing module parameters in my opinion is too risky and complicated to bring to a measure.

Looking forward to your feedback

BR Tecnun

[Ubers83]

An interesting discussion, especially as industry often accepts a longer string length than PVsyst allows and it is difficult to enter this value into PVsyst.

I agree that the string length calculation should factor in the very worst case cell temperature (by using the lowest historical site ambient temperature), however, the formula does not correct Voc with a realistic worst case plane of irradiance value for the start of the day. 1000W/m2 @ STC is overkill as the irradiance in the morning won't be anywhere near this when the central inverters self check prior to closing DC feeders in the morning.

The problem is you will need years of historical meteo data to justify this lower 'worst case' irradiance values in the morning are well below 1000W/m2. What do you think of this approach PVsyst admin?