strings with different number of modules going into an inverter

[francie]

How do you connect 2 strings (with different number of pv modules on each string) into a SMA 3800 inverter?

 

[JulianC]

The SMA Sunny Boys only have one MPP tracker, so the short answer is 'you can't'.

 

[francie]

Is there anyway to connect to 2 different size strings - 10 and 8 - before the inverter. Modules are 220w. Shading is an issue on lower 8 panels

 

[JulianC]

The only options that I am aware of:

1. An inverter with dual MPP trackers
2. Two separate, smaller inverters
3. Connected as 2 x 9 module strings (depending on the extent of shading, this may be possible)

Unless anyone knows of some sort of external dual MPP box of tricks that can be fitted between inverter and modules?

 

[francie]

Thank you so much for your time! We will have to go with the 2 x 9 option. The shading only occurs early in the morning which is why we are going for 2 strings and the 10 and 8 option would have helped reduced the shading issue. Space restricts the layout of the panels. Very grateful for your help.

 

[JulianC]

No probs. You may find that if shading is for short periods only, particularly at times of low overall insolation like early morning, the overall reduction in generation won't be much to worry about. The important thing is that any strings connected to a single MPP input inverter are of the same length and use the same modules. Did find a good article about multi-strings with different shading issues per string and even mounted on different roof aspects but I'm buggered if I can find it. It was surprising what you could get away with though!

 

[Solarmark]

Sma do a 4000Tl inveterate that has two Mpp trackers on which will enable you to connect a string of 10 and a string of 8 modules. This would usually give you better efficiency even without issues of shading, rather than an even 9 plus 9 split.

 

[goodwilld]

I have used the Aurora 3600 for 10 and 11 moduels per string, not sure if it would do your config.

 

[JulianC]

Both these are transformerless inverters. Are people fitting class B RCD's with these, as per BS7671, and earthing the array frame? Or, as I suspect, are people just not bothering?

 

[francie]

A type B RCD will be used but was advised that earthing the array frame was not necessary unless the array is at particular risk of lighting strike.

 

[JulianC]

No need for a class B RCD (the hard to find DC blocking type, not your common or garden bog standard type) with your Sunny Boy 3800 - it's got a built in isolation transformer. Therefore generally no need to earth the array frame. These things are only an issue with transformerless inverters. It seems a lot of people are fitting these, and a lot of wholesalers are peddling kits with transformerless inverters included but I see no mention of a suitable RCD being supplied in the kit and they're not easy to find (the only one's I've seen are the ABB ones from RS Components). Just wondering really if people aren't bothering through either laziness or ignorance.

 

[Markc]

No need for a class B RCD (the hard to find DC blocking type, not your common or garden bog standard type) with your Sunny Boy 3800 - it's got a built in isolation transformer. Therefore generally no need to earth the array frame. These things are only an issue with transformerless inverters. It seems a lot of people are fitting these, and a lot of wholesalers are peddling kits with transformerless inverters included but I see no mention of a suitable RCD being supplied in the kit and they're not easy to find (the only one's I've seen are the ABB ones from RS Components). Just wondering really if people aren't bothering through either laziness or ignorance.

Pays to keep an eye on RS for the RCD's. Our wholesalers aren't interested in getting them in and I don't know of anyone who keeps them on the shelf. We try and stick to transformered inverters.

 

[Solarmark]

You are ok with the SB 3800 as I believe it has a transformer. You only need to earth the array if like you say there is an increased risk of lightning, or if you are using an inverter without an isolating transformer like the Sma 3000 and 4000 TL inveters.

 

[SolarCity]

I heard something earlier which surprised me and I'm not sure it's correct. Someone has designed a system on house we have looked at where he has 12 panels on the main area of roof and then 4 panels on another section of roof in which the installer intends to install the 4 panels on a seperate string. Can this be possible? Surely we have two uneven strings which will require a dual MPP tracker but the voltage of the second string will be far below what is needed to fire the inverter and therefore effectively useless? Or will the inverter be fired by the 1st string and therefore allow the smaller string to operate?

I'm confused, as I'm sure you can see.

 

[dansk]

I dont really know, but i susupect as you say it will need duel MPP and even then the 4 panels may not have enough voltage - odd, or perhaps they using two inverters to suit?

Question - the bonding wire to the array, do you take it back to the MET? or A/C isolater?