Adding dissimilar panel string to existing install

[Island mon]

I want to add a similar, but different series string to a existing array.
do I separate it with a separate controller?
if not, do I bog the entire array down adding this 12% lower amp string?
I

[bob-n]

Please give us more information on what you have and are adding. Can you find the brand and model of the inverters? How many panels are there now? How many are you adding?

If you have an inverter with two string inputs and only one string input is used, then you can put something different on the other string input with no penalty.

There are limits on the number of panels that you can put on one string input. Also, it is different if you are off-grid vs grid-tied. That's where it gets more complex.

[MichaelK!]

It's ok to have two different strings of panels as long as their combined voltages are within 5% of each other. Remember, in series, volts add, and amperage stays the same. In parallel, voltage stays the same but amperage adds.

Let's say for example you have two strings of panels, one producing 5.5 amps at 72V, and a second string producing 8.0amps at 74V. Combine the two strings in parallel and you might get about 13.5amps at ~ 73V.

[Island mon]

I have 7 strings of 3 panels in series (4 strings on one controller, 3 on the other)
those 21 panels are 255w 8.39a open 37.50
New string to be added is 230w 3 panels in series 7.82a open36.30
its a off grid system panels have no inverters inverter is a 4.4kw split phase Can I add this new string to one of the existing controllers, or do I need a sep controller (moot)

[MichaelK!]

I assume what you are trying to say is that the first 7 strings are made with panels having an Voc of 37.5? We would say 37.5 open circuit volts. The new string panels have an Voc of 36.3? It would be better to compare the Vmp, the maximal power point Volts, which for each I would guess is about 30.0Vmp? All these numbers should be pasted on the back of your panels.

Working with the numbers you've provided, the first 7 strings would have a Voc of 112.5V and 108.9Voc for the second, which is only a 3.3% difference, so it should be fine. Is the Vmp even less?

So, you'd have 4 + 4 strings to the two controllers. Does each individual string have a breaker, or fuse before going to the controller? With three or more parallel strings, it is considered unsafe to have unprotected strings converging.

[Island mon]

Michael, yes, I have understand.
i ordered breakers, but they never showed up (thanks for the reminder).
Is it common for a panel to short?
I'm glad I don't need another controller, thanks!

[MichaelK!]

Is it common for a panel to short?

I've never had one, but I've never had a house fire either. I still have fire insurance though. The breaker is the insurance that IF something goes wrong, you are protected.

BTW, do NOT try to substitute an AC breaker for a DC breaker. AC current arcs far less than DC current, so the breaker contacts aren't designed to open as far. If there is a DC overload flowing through an AC-only breaker, the contacts can weld together, creating an even greater danger.

If you can't get the proper DC breakers right now, it would be a good idea to substitute an appropriate DC fuse, about 10-12 amps per string.

[Mike90250]

Be aware that I know of no solar rated (DC) fuse holders, that are rated to to be opened or disconnected under load. That means if the sun is up, you can't open to check a fuse, if it's working, you will draw a large arc and fry the fuse holder

[headsaver]

Here is what to consider in solar installation. There are basically 2 types of Inverters used in Grid Tied Installation. One is the String Inverter and the other one is the Microinverter. A panel when exposed to light will produce a DC voltage and develop a DC current ( Photo Voltaic Effect ). A 250 watt panel will produce 30 volts DC and develop 8.3 Amperes DC current ( 30 X 8.3 = 249 watts which is Power ) when exposed to good sunlight. The String Inverter is used when the panels are connected in series. When panels are connected in series, it is important that all panels receive the same light exposure so that the developed current of each panel will be the same and the individual panel voltage will Add up. If they develop different currents, the least current will prevail ( that is why same light exposure is important ). This is where the String Inverter is used and the AC side of the String Inverter is connected to the Utility Grid ( 240 volts ) via an AC disconnect switch.

The Microinverter is used when the panels cannot be installed to receive the same amount of light exposure, therefore will all develop different amount of current. Each panel will have a microinverter installed in the back of the individual panels and will be independent of each other. The output of the microinverter is already AC, 240 volts and the current will depend on the amount of light exposure the panel receives. In this case the 240 volt AC output will be connected in parallel and the currents will ADD up. This will be tied to the Utility Grid ( 240 volts ) via an AC disconnect switch.

I hope this theory helps in understanding the principle of the difference between Series connected panels and Parallel connected panels. You are welcome to ask questions for clarity.

[Island mon]

Interesting.
I don't know of, but is there a slow blow fuse of the automotive blade type? Would the slow blow handle the anticipated surge?

The fuse idea is gaining approval of me as I think my outdoor junction box would be stuffed if I had breakers that are much larger than fuses.
My original ideas was to put 2 60;amp breakers inside next to the controller (that would protect it from the weather, but not trip on a individual string short).
I am off the grid, no no grid tie or individual panel inverters.
I have a 4.4kw split phase inverter with a 16 battery golf cart bank with 2 60amp moot controllers

[Island mon]

MPPT not moot

[foo1bar]

Interesting.
I don't know of, but is there a slow blow fuse of the automotive blade type? Would the slow blow handle the anticipated surge?

He didn't say surge - it's arcing.
The current flowing through a fuse will try to keep flowing, even if it has to arc through a little bit of air.
And that arc means heat.
And that heat can mean damage to the contacts that hold the fuse. (and the plastic surrounding it)

[MichaelK!]

The fuse idea is gaining approval of me as I think my outdoor junction box would be stuffed if I had breakers that are much larger than fuses.
My original ideas was to put 2 60;amp breakers inside next to the controller (that would protect it from the weather, but not trip on a individual string short).

You really want to keep them separate. Companies make solar combiner boxes exactly for this purpose. This is the one I use.

https://www.amazon.com/Midnite-Solar-Photovoltaic-Combiner-6-Position/dp/B0041OJ2J6/ref=sr_1_1?dchild=1&keywords=Midnight+combiner+box&qid=1607789194&sr=8-1

It houses all the breakers that protect the solar arrays from short-circuits. The main cables from the combiner then are routed to the terminals for the charge controller

[headsaver]

This is Romeo Lampa, my post on What to consider in Solar Installation in not meant to directly reply to a post but rather a sort of a lesson in general about how solar panels behave in the 2 different type of Grid Tied Inverters namely: String Inverter and a Microinverter. I teach Home Solar Installation and this is one of the lessons that I teach. If anybody has questions, I will try and answer them.

[headsaver]

What to consider when using a String Inverter in a Grid Tied Installation: The bigger size inverter can take 3-strings of panels ( solar arrays ), Each string must be composed of the same model panels and they must be oriented the same way ( azimuth and tilt ). This way, the current generated by every panel in the string is about the same. Connecting panels in series is like joining together different diameter pipes in Plumbing, the smallest pipe will dictate the flow of the water.

What to consider when using Microinverters in a Grid Tied Installation: Since the panels are basically connected in Parallel via the microinverter, they are independent of one another, but each panel must match the wattage rating of the microinverter that connects to it.