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# Optimum setup of 2 strings on Solis RHI hybrid inverter

Discuss Optimum setup of 2 strings on Solis RHI hybrid inverter in the Solar PV Forum | Solar Panels Forum area at ElectriciansForums.net

#### Tom S

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Hi,
We have a setup of 2 strings in series of Viridian panels, at present these go to the inverter which has dual mppt inputs so the strings are independent.
The strings are slightly different, 7 x 250w clearline mono and 7 x 270w clearline poly.

I have been looking at the settings in the Solis inverter and and it seems it has a parallel mode so the 2 mppt's can be combined in the inverter to make one parallel string.

What I am wondering if anybody has the knowledge is which would produce the best system performance?
Would it be just leave it at 2 independent series strings or parallel both strings in the inverter settings?

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#### marconi

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I am new to solar pv technology so please do not take what follows as gospel - I recommend you contact the manufacturer and read what others contribute. But my understanding of parallel operation of dual/multiple MPPT trackers refers to them operating in the same battery charging stage at the same time (see diagram above) and not to the electrical paralleling of their outputs nor to only one MPPT algorithm being applied to the combined array of paralleled strings. So, in your case - each MPPT will track the pv voltage and pv current and battery voltage to determine the pv voltage and pv current from each string to produce the maximum power output from each string; this is then converted into the optimum current at the most appropriate voltage to suit the state of charge of the battery and the stage of the battery charging sequence viz bulk, absorption, float, equalization. I think the parallel mode (sometimes called synchronisation) refers then to each MPPT controller being in the same battery charging stage as required by the state of charge of the battery at any instant and charge already delivered to the battery (ie: Coulombs Q = integral of i(t)dt).

Other more experienced folk can correct, clarify or amplify further.

So, in answer to your question I reckon you should select parallel mode.

Dual MPPT allows separate arrays installed in different azimuths and angles and irradiation profiles to be combined eg:

#### TJ Anderson

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Basically the Solis Inverter has Dual MPPT which means it can operate two strings completely independently (2 MPPT as Solis say). As yours are unbalanced it would be detrimental to parallel them together so therefore keep them independent.

#### Tom S

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Great, thanks for the information. I'll leave them independent and see how it preforms.

#### marconi

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Could you send me a link to the Solis RHI hybrid inverter you have please so I can read up on it and the parallel mode setting?

#### Tom S

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Here's the datasheet,
and here is the manual
There is not much info on the parallel mode but from my own observations in parallel mode the current from both strings is exactly the same with a +5 to 10 volts on the bigger series of panels (270w) compared to the smaller series of panels(250w).

In independent mode there is a difference in current and voltage between both series strings of panels as to be expected with the 270w panels producing slightly more.

#### marconi

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Are you able to flip between parallel and independent mode quickly, and on a number of time separated occasions, to see whether at each time you take both measurements, the sum of the measured pv array currents is always higher for the parallel mode than the independent mode? (I suspect it will be.)

#### marconi

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For Tom S: For my own education and clarification I asked Ginlong Technologies about the parallel mode setting on the solis hybrid inverter and have just received this response:

No you shouldn’t use parallel mode, because they are different panels and will have different maximum power points, so the trackers must be independent of each other.

The parallel mode is only for installs that want to save a little on cabling and want to run a single thicker cable down from the roof. This requires that the panels are facing the same direction.

Kind regards,

Paul Carpenter
Electrical Design Engineer
MSc Electrical Engineering (UCT)
Ginlong UK Office (Solis Inverters)

#### Tom S

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This makes sense now to me.
Thank you for updating this for me , many thanks

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