PV Inverters - Some questions of understanding

Question

Good afternoon 
 For the first time I was called to do a PV design for a hotel we are designing so excuse my 'beginner level' questions I am going to ask. 
 We have established (through the energy efficiency report) that we need 35kWp of PV panels on the roof. 
 The inverter I am thinking to use has max DC power 37.5kW to cover for the 35kWp we need. 
 The inverter say in the specs 3 MPPTs , 2 strings per MPPT . 
 Some initial questions 
 1. When the inverter say max DC power 37.5kW should I assume that this is the total sum across all 6 strings that can be connected? 
 2. Should I assume that the 2 strings per MPPT are connected internally within the inverter in parallel? Or i dont know what I am talking about? Because if the two strings of each MPPT are internally connected in parallel then I have to balance the voltages of the 2 strings to be quite equal. If they are connected internally in series then the voltages add and I may exceed the max voltage allowed. Or maybe they are not connected at all inside the inverter and they stay independent. Sorry if the question is a bit stupid. I am trying to understand what does the 2 strings per MPPT mean 
 This is the terminals picture if it helps

3. The AC power of the inverter is going to be balanced (current per phase) or it somehow depends on how I connect the strings on the DC side? 
 
 Any help will be much appreciated

AJJewsbury · Accepted Answer

When the inverter say max DC power 37.5kW should I assume that this is the total sum across all 6 strings that can be connected? 
 
 I'd say that was the maximum total - there might even be an assumption that it's evenly divided (e.g. max 12.5kW per MPPT or max 6.25kW per string). It's not unknown for inverters to be rated somewhat lower than the panel outputs though - on the basis that much of the UK (especially more northern areas) don't ever get to 'standard' conditions, and the loss of output on the rare occasions output is maxed out doesn't justify the cost of a larger inverter (I've a 2.14kWp system with a 2kW inverter). 
 
 what does the 2 strings per MPPT mean 
 
 Good question about the series/parallel connection internally - I don't know, I'd suggest asking the manufacturer. But on the basis of the MPPT is meant to optimise the panel outputs I would have thought the two strings would have to be pretty much matched for it to work properly - i.e. same number of same type of panels, subject to the same aspect/shading - otherwise what'll be optimal for one string won't be optimal for the other. I could be wrong, but that's my (erring on the side of safety) guess. 
 
 The AC power of the inverter is going to be balanced (current per phase) or it somehow depends on how I connect the strings on the DC side? 
 
 I would hope that everything went onto the same DC bus and the three inverters ran from there - often you have two MPPTs on single phase inverters - and one might be used for East facing panels and the other for West facing. It would be odd if the 3-phase inverter was less flexible in that respect - but I've been wrong before. - Andy.

mapj1 · Answer

you clearly have a unit in mind as you show the connector drawings - if you tell us what model it is, it may be possible to be more specific with the answer - usually such things are in the documentation, if somewhat disguised or can be deduced from the performance data. 
 And given the way inverters work, I'd expect each of the panel strings to be boost converted up to a common DC bus, at some voltage higher then any of them and then that DC is then sliced and diced to generate the 3 phases. But as always, there maybe some manufacturer specific caveats. Mike. 
 PS missed the exam question. The two strings into any one MPPT boost converter are in effect wired in parallel, but probably (well, I hope) diode combined, so if one is very weak, the winner (higher voltage) takes all the load, or if the outputs are comparable, the current share out varies until the voltages are the same (i.e, the better lit one droops a bit more until it reaches the voltage the other, less well lit, string can sustain). As solar panels under a given level of solar flux are more or less constant current devices, this is not as poor practice as it sounds, but even so the 2 strings should made be as similar as possible, and if you have the choice it is more efficient to use one MPPT per string. 
 Realize that the mppt alters its 'transformation ratio' (I know its DC and it is a lot of switched mode cleverness, but this is a simplified explanation) to try to set the input impedance to try and get the highest voltage before the current falls away , so is always settles at about 80-90% of open circuit voltage and about 70-80% of short circuit current.

PV Inverters - Some questions of understanding

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