Glossary · DC and AC electrics
PV string
A chain of panels connected in series: voltages add up, the current is shared; frost and the inverter MPPT window limit its length.
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Definition
A string is a group of panels connected in series, plus to minus. The panel voltages add up, while the current through the string is the same. Several strings can be paralleled on one inverter input, and then their currents add up.
Two conditions bound a string’s length. From above, frost: the string’s UOC MAX at the site minimum temperature must not exceed the inverter’s maximum input voltage. From below, heat: the operating voltage of a hot string must not drop below the lower end of the MPPT window, or the inverter loses power or fails to start.
Example for Kyiv (Tmin = −40 °C): an illustrative panel with Uoc 49.5 V and α −0.25 %/K gives 57.5 V in frost. An inverter with an 800 V limit takes at most ⌊800 / 57.5⌋ = 13 panels per string. At −25 °C the same arithmetic would allow 14 panels, which is 805 V in a real Kyiv frost.
The string current matters too: ISC MAX = 1.25·ISC STC sets the DC cable section, and with three or more parallel strings on one input it decides whether fuses are needed. Panels in one string should be identical and face the same way: the weakest or shaded panel drags the whole chain down.
A few layout tips. Start from the top limit: divide the inverter's Vmax by the panel's UOC MAX in frost and round down. Then check the lower end of the MPPT window in heat. If there is room between the limits, a longer string has higher voltage and lower current, and so lower losses in the cable. Equal panel counts on every string of one tracker are a must, and different orientations (east and west) are a reason to put strings on different inputs. On a ground frame it helps when one table row is one or two strings: the cable runs are then shorter and alike.
How Solispec applies it
The Solispec engine assigns panels to MPPT inputs automatically: for each input it computes the allowed panel count at the site Tmin and checks voltage, input current, fuses and cable per DSTU HD 60364-7-712:2022. You can change the layout by hand in the constructor and the check reruns. In the free string calculator you only pick a panel, an inverter and a temperature.
Norms and clauses
- DSTU HD 60364-7-712:2022, Annex B
Related terms
- MPPT and the inverter voltage windowThe inverter’s maximum power point tracker; its voltage range sets the minimum and maximum number of panels in a string.
- 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.
- 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.
- 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.
- 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.