Solar cells are one of the fastest growing task types — and the DC side behaves completely differently from the AC current the electrician knows. Here are the system's structure, string voltage and its temperature dependency, DC breakers, potential equalisation of modules and the correct RCD choice according to DS/HD 60364-7-712.
Introduction
A solar cell system (PV — photovoltaic) converts sunlight to direct current (DC) in the modules, and an inverter converts it to alternating current (AC), which is connected to the installation and the grid. It sounds simple, but the DC side breaks with several habits an electrician has from AC installations — and that is where the danger and errors lie. The normative requirements are in DS/HD 60364-7-712.
Facility structure
- 01Modules — the individual panels that each supply a DC voltage and current depending on light input.
- 02String — modules connected in series, so their voltages add up to the inverter's working range.
- 03DC cabling and DC breaker — between strings and inverter, with the ability to disconnect the DC side.
- 04Inverter — converts DC to 230/400 V AC and synchronises with the grid.
- 05AC connection — through its own circuit breaker with overcurrent and residual current protection to the panel, and on to the grid according to the Common Regulations.
The DC side is different — and dangerous
On AC the voltage crosses zero 100 times per second; this helps extinguish an arc when a switch opens. Direct current has no zero crossing — a DC arc will burn on and is hard to extinguish. Meanwhile, a solar string is live as soon as there is light — it cannot be 'switched off' at the source. This means two things: DC equipment (switches, plugs, cables) must be specifically DC-marked for the task, and you cannot make the modules de-energised by disconnecting the inverter.
String tension — and why the cold is determining
A string's open-circuit voltage (Voc) must be kept below the inverter's maximum input voltage AND below the system's maximum voltage. The critical point is that the module's Voc RISES when it gets cold — opposite to what you'd expect. Therefore the number of modules per string is sized according to the lowest expected module temperature, not a summer day.
Eksempel: Et modul har Voc,STC = 41 V og β = −0,29 %/°C. Ved T_min = −10 °C bliver stigningen 0,0029 · (25 − (−10)) = 0,0029 · 35 = 0,102, altså ca. 10 %. Voc(kold) = 41 · 1,102 = 45,2 V pr. modul. Med en maksimal systemspænding på 1000 V bliver største strenglængde 1000 / 45,2 = 22,1 → 22 moduler pr. streng. Regner du kun med de 41 V ved 25 °C, ville du fejlagtigt tro, at 24 moduler var i orden — og overskride grænsen på en kold vinterdag.
DC breakers and arc protection
There must be the possibility of interrupting the DC side so the inverter can be serviced safely — typically a DC load switch close to the inverter rated for the full string voltage and current. Some systems are supplemented with module-level electronics (optimizers/microinverters) with a safety function that lowers the string voltage to a safe level on disconnection — an advantage for fire safety because the roof otherwise stands under full DC voltage as long as the sun shines. Arc detection (AFDD-like function in the inverter) is found on newer systems and disconnects at an incipient DC arc.
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