Glossary · DC and AC electrics
MPPT and the inverter voltage window
The inverter’s maximum power point tracker; its voltage range sets the minimum and maximum number of panels in a string.
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Definition
MPPT (maximum power point tracking) is the inverter stage that keeps adjusting the voltage at which the panels deliver the most power. Each tracker works within its range, the "MPPT window", between the lower and upper voltages on the datasheet. The datasheet also gives the absolute maximum input voltage Vmax, above the top of the window.
These three numbers and the panel’s temperature-corrected voltages give the allowed string length. Solispec computes it as:
nmin = ⌈UMPPT,min / Ump(гаряча)⌉, nmax = min(⌊Vmax / Uoc(холодна)⌋, ⌊UMPPT,max / Ump(холодна)⌋ + 2)The first part of nmax is a hard safety limit: UOC MAX at Tmin must not exceed Vmax. The second is practical: in frost the operating voltage should not run far past the window, or the tracker works off its optimum. The lower bound nmin watches the heat: in summer the panel voltage drops, and a short string can fall out of the window.
Inverters with two or more MPPTs let you connect arrays with different orientations or panel counts to different inputs. Strings on one tracker should match in length and orientation.
What to read on an inverter datasheet. The MPPT voltage range is the tracker window; the maximum DC input voltage is the absolute limit Vmax; the input current and short-circuit current per tracker are given separately. The number of trackers sets how many independent string groups you can connect, and the input current rating sets how many strings can be paralleled on one tracker. With three or more strings on an input, the question of fuses comes up. So an inverter with several trackers is sometimes simpler to install than a single-input one of the same power.
How Solispec applies it
The Solispec engine takes the MPPT window, Vmax and input current rating from the inverter catalogue and the panel voltages from the panel catalogue (cold Uoc per DSTU HD 60364-7-712:2022, Annex B). For each input it shows the allowed panel range and checks the actual layout. If the inverter record has no window (lower bound not below the upper), only the Vmax limit applies.
Norms and clauses
- DSTU HD 60364-7-712:2022, Annex B
Related terms
- PV stringA chain of panels connected in series: voltages add up, the current is shared; frost and the inverter MPPT window limit its length.
- Maximum open-circuit voltage UOC MAXThe open-circuit voltage of a panel or string at the site minimum temperature; inverters, fuses and isolators are selected by it.
- Site minimum temperature TminThe lowest ambient air temperature at the PV site; it sets the maximum open-circuit voltage of a string, U_OC MAX.
- DC/AC ratio (inverter loading)The ratio of the panels’ peak power to the inverter’s rated power; no norm sets it, it is an engineering choice.
- Maximum short-circuit current ISC MAXThe design current of a PV array, I_SC MAX = K_I·I_SC STC with K_I of at least 1.25; cables and protection are sized by it.
- DSTU HD 60364-7-712:2022: the electrics of PV systemsNorms explained
Solispec’s calculation is preliminary: for a design it has to be checked by an engineer. The values and clauses follow the current editions of the norms on the date of the page.