Solar cell system (PV) — installation and DC safety

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.

• 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.

Anyone may mount and connect a solar PV system together if it was bought as a complete product with installation instructions. But the connection in the distribution board always requires authorisation (section 2 of the Authorisation Act). The connection is typically made at the inverter's output terminals. Once the system is permanently connected, it may only be repaired by authorised electrical contractors and by companies that service solar PV systems commercially.

The Danish Business Authority (formerly the Danish Safety Technology Authority) has collected the general requirements for solar PV installations. The installation follows HD 60364, including HD 60364-7-712:2016. Here are the most important ones for the DC side:

• 01The DC side must have double or reinforced insulation, and the equipment must be class II. Use single-core cables.
• 02The cables must not be placed directly on the roof surface. EN 50618 is the standard for DC cables.
• 03SELV or PELV may be used, but at most at 120 V DC.
• 04There must be a switch on the DC side of the inverter.
• 05DC connectors must comply with EN 50521 and EN 62852. Connectors that laypersons can reach must only be separable with a key or a tool.
• 06If there are more than two parallel strings, the string cables must have overcurrent protection (HD 60364-7-712, section 712.53).
• 07If there is more than 10 m to the inverter, there must be surge protection (SPD) at the inverter. If an SPD is required on the AC side, it is also required on the DC side.

The AC cable is connected as a generating set on the supply side of the protective devices, unless the requirements of HD 60364-5-551:2010, item 551.7.2, are met. A plug and socket-outlet is not an option. 1-phase inverters must be spread evenly across the phases so that the current is not unevenly distributed.

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.

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.

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.

Module frames and support structure (frame) are exposed conductive parts that are assembled in an equipotential bonding and connected to the installation's grounding — among other things so that isolation monitoring in a transformer-less inverter functions correctly. With increased lightning risk this grounding is coordinated with the building's lightning protection system (DS/EN 62305) and overvoltage protection (SPD) is used on both DC and AC sides. The module string itself plus/minus is normally NOT grounded in a transformer-less inverter — the DC system is floating (without ground connection) and the inverter monitors isolation resistance to ground.