A silicon PV module makes more voltage when it is cold and less when it is hot. The data sheet gives the voltages at standard test conditions, with the cell at 25 °C. A string is many modules in series, so a small change per module adds up to tens of volts across the string.
Cold: the maximum voltage
On a clear, frosty morning the cells sit near the air temperature, and the sun can arrive before they warm. The open-circuit voltage is then at its highest. AS/NZS 5033 Cl 4.2.1.2 corrects Voc to the lowest expected operating temperature, three ways:
| Method | When to use it |
|---|---|
| The maker's temperature coefficient | The data sheet gives it, usually as % per K |
| The maker's own method | The maker gives one |
| Table 4.1 factors | Crystalline silicon with no maker's figure, down to −40 °C |
The string's maximum voltage is the corrected Voc times the modules in series. It has to stay at or under the inverter's maximum d.c. voltage, and under the Cl 3.1 limit: 1000 V for a domestic installation, 1500 V for any other.
Hot: the MPPT minimum
On a hot afternoon the cells run far above the air: 25 K to 40 K on a roof is common. The voltage at the maximum power point falls. If the string's Vmp drops below the inverter's MPPT minimum, the inverter loses the string's output. AS/NZS 5033 sets no cell rise, so the PV string tool asks for one, with 35 K as AmpSize's default.
The inverter's limits
AS/NZS 4777.2 Cl 7.2.2 has every inverter marked with its ratings, among them the absolute maximum PV voltage and Isc for each PV input. Read them off the plate or the data sheet, not a brochure.
Current
A module's current rises with the sunlight, and can pass its rated Isc in bright, cool weather or with light off the ground. Cl 3.3.3.1 takes the maximum string current as 1.25 × KI × Isc. KI is 1 for a common module, and higher for a bifacial one.
Strings in parallel can feed a fault in one string. With SA strings, the others can push (SA − 1) × the maximum string current back into it. Where that, plus any backfeed from the inverter, passes the module's maximum series fuse, Cl 3.3.4.1 calls for string fuses. Table 3.1 sets their rating above 1.2 × the maximum string current and at or under the module's fuse rating.
Cables and the isolator
String and array cables carry the circuit currents of Table 4.2, and a cable near the modules runs hot: Cl 4.4.2.2 points to the record high plus 40 K. The conductor is at least 4 mm². The d.c. isolator at the inverter is rated for the array current at the string's maximum voltage.
Worked example
Solar string sizing for a 600 V inverter works one array: 5 to 11 modules per string, 10 chosen, 535.1 V cold and 349.6 V hot.