Section 2.1.2 on DTI Guide - Voc/Isc mulitpliers

[SibertSolar]

Guys,

As we know, for switches/cables/isolators etc, you need to multiply your Voc values by 1.15 and Isc values by 1.25 to calculate the relevant max values for suitable protection. However, does this apply to MPPT or Max DC voltage/current input values on inverters? Any thoughts?

Thanks

Andy

 

[SolarCity]

We work it on the Voc and Isc values of the arrays. I don't think the max value of the inverters should come into it, should it?

 

[SibertSolar]

No, you're right Chris. I was just curious whether anyone out there actually multiplied their total Voc per string by 1.15 and then checked whether that still fell into the "MaxDC or MPPT range" details of the inverter.....

I've just checked with a certain Mr Cotterell and he confirmed that it doesn't apply to inverters per se, with the caveat that the inverter ratings must therefore still apply at -15degreeC....makes sense to me.

Cheers

 

[SolarCity]

No, you're right Chris. I was just curious whether anyone out there actually multiplied their total Voc per string by 1.15 and then checked whether that still fell into the "MaxDC or MPPT range" details of the inverter.....

Ah, I'm with you now. I've not thought about that before. It certainly isn't something that I've considered when designing a system.

with the caveat that the inverter ratings must therefore still apply at -15degreeC....makes sense to me.

At the risk of sounding stupid again. What does this mean?

 

[SibertSolar]

Don't worry, I was looking at it slightly wrong I think. The MaxDC rating of the inverter input is not intrinsically linked to temp. The module's Voc temp coefficient dictates its response to temp drops in terms of voltage output. I need to run some simulations before I'm certain as to whether manually adding 15% to the Voc is roughly the same as the resulting increase in module voltage output at -15degreeC based on the module's temp coefficient for Voc (typically around -0.33/degreeC). A 40C drop in temp will result in approx 13% increase in voltage....

 

[SolarCity]

Good point. Am I right in saying that generally, overvoltage on inverters is main culprit when it comes to damaging them? Over-current not so much of an issue?

 

[dansk]

On the point of temp coeffcients, we check with PV-SOL - our is set to check VOC -15d as this will give a realistic high voltage, plus you know its going to start up on a cold day and keep running.

 

[SibertSolar]

Yes, the -15C is the critical temperature and it's the initial over-voltage periods for some modules that can cause a problem. Also, most inverter manufacturer's specify their max Voc input values @ -15C for this reason. It's a "worst-case" scenario I guess.