Connector Box for series connected solar panels

[TheIntern]

Hi guys,

I am an working with a team that will be deploying a microgrid in about a year from now. Right now we are building a prototype for testing.

What we have:

4 250W 24V Solar panels
1 Ideal Power 30kW hybrid converter

There is a rather large battery coming later, and a transformer as we will be connected to the grid as well. But 1st things 1st, I want to set up the solar panels into the Ideal Power...We will be connecting the solar panels in series to meet the minimum required DC voltage of 100V (the panels actually go a few volts over 24V).

My question is: Do I need a combiner box? And if so, what type of fuse/circuit breaker do I need?

-I have looked up how to make/connect a combiner box, but it seems like this is only needed for parallel connections? Also, do I need a special fuse since the voltage will be over 100 Volts?

Thanks for the help guys! I hope I'm given enough info to make a coherent question.

[Mike90250]

panels in series do not need a fuse. A DC circuit breaker is handy to use as a switch and a protective device if something goes catastrophically wrong.

And so you have noticed the issue of Voc (voltage open circuit) vs Vmp (voltage @ max power). When PV panels get cold, their voltage can rise, and often, the Voc can rise high enough to damage things. So be aware of this issue and don't BBQ your controller.

[SunEagle]

Hi guys,

I am an working with a team that will be deploying a microgrid in about a year from now. Right now we are building a prototype for testing.

What we have:

4 250W 24V Solar panels
1 Ideal Power 30kW hybrid converter

There is a rather large battery coming later, and a transformer as we will be connected to the grid as well. But 1st things 1st, I want to set up the solar panels into the Ideal Power...We will be connecting the solar panels in series to meet the minimum required DC voltage of 100V (the panels actually go a few volts over 24V).

My question is: Do I need a combiner box? And if so, what type of fuse/circuit breaker do I need?

-I have looked up how to make/connect a combiner box, but it seems like this is only needed for parallel connections? Also, do I need a special fuse since the voltage will be over 100 Volts?

Thanks for the help guys! I hope I'm given enough info to make a coherent question.

While you will not need a combiner box if you wire those 4 panels in series you will need on later one if you start to utilize even half of that 30kw inverter system.

So plan ahead if you are thinking about putting in a much bigger solar array in the future.

[TheIntern]

panels in series do not need a fuse. A DC circuit breaker is handy to use as a switch and a protective device if something goes catastrophically wrong.

And so you have noticed the issue of Voc (voltage open circuit) vs Vmp (voltage @ max power). When PV panels get cold, their voltage can rise, and often, the Voc can rise high enough to damage things. So be aware of this issue and don't BBQ your controller.

Hi, Thanks for the info. Just for learning purposes, why would a fuse not be needed in series connected panels? Is it because we use less current than parallel connected? And a circuit breaker basically does the same thing as a fuse correct? But you suggest it for control (to shut down on the spot) and just to be safe right?

Thanks

[TheIntern]

While you will not need a combiner box if you wire those 4 panels in series you will need on later one if you start to utilize even half of that 30kw inverter system.

So plan ahead if you are thinking about putting in a much bigger solar array in the future.

Thanks, I'm sure we will get there soon. Thanks for the advice!

[Naptown]

I hope you plan on a LOT more panels and a huge battery bank to run that inverter.

[TheIntern]

I hope you plan on a LOT more panels and a huge battery bank to run that inverter.

Oh yes, there will be more panels as it will be a 6kW deployment. And for the battery, I don't have the details but I've been told it will be a large battery bank. We just need this small setup for testing purposes

[sensij]

Hi, Thanks for the info. Just for learning purposes, why would a fuse not be needed in series connected panels? Is it because we use less current than parallel connected? And a circuit breaker basically does the same thing as a fuse correct? But you suggest it for control (to shut down on the spot) and just to be safe right?

Thanks

Here is how I think of it:

Start with the idea that an over-current protective device (OCPD), IE, a fuse or a circuit breaker, has a primary function to protect the circuit's conductors from overheating. The PV panel comes with leads that are typically 12 AWG (or 4.0 mm2). 12 AWG has an ampacity of 20 A. The panel has an Isc of something like 8-9 A, which is the maximum current it can produce in a short circuit. Putting panels in series only increases voltage, not current. There is no fault condition for a single series string of panels that could cause current exceeding the conductor rating to flow (NEC has more specific calculations for this, leading to 1.56 * Isc as the PV circuit current rating). In fact, you could put two strings in parallel, and still have a maximum fault in each conductor equal to just the Isc (rated as 1.56 * Isc). This is because when the circuit earth faults, it turns into a single string in series on each side of the fault. Once you get to three strings in parallel, the available current is too high because one side of the fault could have the power of two parallel strings, and the OCPD is required on each string.

It turns out that my understanding is a bit simplistic, and the series fuse rating for each panel is also governed by how much reverse current the panel itself can tolerate (not just the leads), which is why some panels might have a 12 A, 15 A, or 20 A series fuse rating even with the same size leads. Here is a nice paper about it:

http://solar.org.au/papers/08papers/408.pdf

As sort of an aside, the installation manual for LG panels states:

Maximum parallel strings without proper measures,
e. g. fuse 20A : 1 string

I think that must mean that they are counting 1 additional string in parallel to the first series string (for two total).

[inetdog]

The simple explanation is that in the case of a fault in a string a second parallel string could not drive more current back through the damaged string than the damaged string itself would produce.
But once you have two or more strings in parallel you have potentially exceeded the fault current limit into a damaged string.

If there is a fault in the CC itself in a battery based installation, then, particularly with a PWM CC, you could have current back into a damaged string (single panel string?) which is limited only by the CC to battery fuse. In that particular case (two simultaneous faults) it might be useful to have a lower current fuse even on a single string.

[TheIntern]

Here is how I think of it:

Start with the idea that an over-current protective device (OCPD), IE, a fuse or a circuit breaker, has a primary function to protect the circuit's conductors from overheating. The PV panel comes with leads that are typically 12 AWG (or 4.0 mm2). 12 AWG has an ampacity of 20 A. The panel has an Isc of something like 8-9 A, which is the maximum current it can produce in a short circuit. Putting panels in series only increases voltage, not current. There is no fault condition for a single series string of panels that could cause current exceeding the conductor rating to flow (NEC has more specific calculations for this, leading to 1.56 * Isc as the PV circuit current rating). In fact, you could put two strings in parallel, and still have a maximum fault in each conductor equal to just the Isc (rated as 1.56 * Isc). This is because when the circuit earth faults, it turns into a single string in series on each side of the fault. Once you get to three strings in parallel, the available current is too high because one side of the fault could have the power of two parallel strings, and the OCPD is required on each string.

It turns out that my understanding is a bit simplistic, and the series fuse rating for each panel is also governed by how much reverse current the panel itself can tolerate (not just the leads), which is why some panels might have a 12 A, 15 A, or 20 A series fuse rating even with the same size leads. Here is a nice paper about it:

http://solar.org.au/papers/08papers/408.pdf

As sort of an aside, the installation manual for LG panels states:

Maximum parallel strings without proper measures,
e. g. fuse 20A : 1 string

I think that must mean that they are counting 1 additional string in parallel to the first series string (for two total).

Fantastic response. Thank you for the understanding and the link is very helpful as well!

[Naptown]

Oh yes, there will be more panels as it will be a 6kW deployment. And for the battery, I don't have the details but I've been told it will be a large battery bank. We just need this small setup for testing purposes

In a balanced system using 48 v battery bank that 6kw array could charge a 1250A battery bank at a comfortable C/10 rate.
Now to get that power out of the batteries you need to discharge at no more than a C/20 rate which means a draw of 62.5 amps or 3000 watts
Orr peukert is going to bite you hard if you get any where near the capacity of that 30000 watt inverter.
Or was the 30k inverter a typo and it is actually a 3k