device to limit the DC voltage to 600VDC max

Hi,

I am little bit knowing about solar technology5811.contentimage_1.png.

Operating range is 230-500VDC. And this is called MPPT voltage, isnt it?

Situatuion of cold temperature could rise voltage "drasticly" - usually about 0,35% pre °C = 3,5% pre 10 °C. In really cold condition (let say -25°C) is difference against test condition 50 °C (STC = 25°C) and this give 17,5% of voltage up. But this is still only 587,5 VDC.

Real problem is not operational range (MPPT), but open circuit voltage (Voc). It is usualy much higher then MPPT - more than 10%. And Voc use same temperature principle as above. This means "only" Voc could rise above 600 VDC.

One easy solution could be: when power output is below measurable level, you can give some dumy load to panels and they would not be in Voc (open circuit) stage. Because output is minimal, there would be minimal danger of overheating. When output power will rice above minimal limit, dumy load would be switched back to invertor. Panel would be in that circuitry continuously under (some) load and would not go to Voc stage.

That dumy load in some cases can be solved by invertor directly (in Slovakia is manufacturer Vonsch, they are improving their invertor if customer need it). Or you coul need add some additional gear.

Dear Pavel,

You re rigth the problem will appear in case of open circuit. I didn’t follow you very well with the dummy loads for low power…  as far as I understand , you suggest when the inverter is out of MPPT range, to switch to dummy loads in order to sink current from the modules. But the PV panels are almost perfect current or voltage source, it means the voltage will not change drastically when the dummy loads have the power.

On the top it means to add not only a device, but some electronics components and control them to accomplish the function.

What do you mean by an additional gear – I’m not sure about the translation,

Thanks,

HB

De : Pavel.Simon messages@element14.com

Envoyé : lundi 7 mai 2012 21:03

À : Hervé Blampey - SES

Objet : - Re: device to limit the DC voltage to 600VDC max

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Re: device to limit the DC voltage to 600VDC max

created by Pavel Šimon<http://www.element14.com/community/people/Pavel.Simon> in Alternative Energy Solutions & Technologies - View the full discussion<http://www.element14.com/community/message/51182#51182/l/re-device-to-limit-the-dc-voltage-to-600vdc-max>

Dear Herve,

I will try give example:

PV panel are in two stages: 1. under load, 2. without any load = open circuit.

If you put any non connected panel to Sun (or light), even with small intensity, panel imediately is producing voltage (without current) - Voc. Because there is no load, current is 0 this mean power is 0 too.

When you give any significant load voltage drops immediately, and current is rising - output power depends on many conditions: intensity of lights, type of PV panel, parameters of load, etc. If panels are connected to "smart" load (usualy invertors, or chargers with MPPT) then this load tries to maximise output power.

For our topic is not important all issues with load and power of MPPT, except one - recognising when PV panel are able give power to work with MPPT. The "smart" load usualy behave "smart": during disconected time it is workins as voltmeter = high resistance bettwen both poles (this is exactly open circuit). Load measure voltage from panels, and if there is not any (so dark condition), load only measure voltage - still is Voc (or 0 V). But when load recognise that PV panels are givin any voltage (in that case Voc) it tries to connect to panels as load (and making MPPT). If there is enough power on panel, load starts is duty - making real power = MPPT. This doing up to time when panel are not able give significat level of power. In that time load would make decision (remember - it is "smart" load) to change its behaving to voltmeter only = now is once again Voc (or 0 V).

6153.contentimage_174809.png

I hope my easy explaining with quick picture help.

And now to next level of question:

additional small device must be placed bettwen "smart" load and PV panels. It will measure voltage of panels instead "smart" load. 1. When device recognise night condition, it is OK, will only measure. 2. When devive recognise Voc voltage, it will put on panel small load (resistors, which must be pecisely calculated). Because PV panel are at that time so soft power source, there would be only small heating on that load. And voltage from panels would be realy small (not Voc) 3. When device recognise power that going through it (and its load) is over certain limit, it would disconect from panels and give power to "smart" load.

I hope that is self expainable. I try to explain it in easy way (sorry for my English, in Slovak would be easier, but for smaller audince), but realisation must be very carefull - there are so many power, and this power could kill some one.

And I did not make any survey on "smart" load manufacturer = invertors or chargers. But I know that some of them could include this additional device behaiving in to their products - for them could by almost "only" SW (software) work. As I said Slovak Vonsch (www.vonsch.sk) they have this possibility. I do not know of others.