SIZING THE MAXIMUM DC VOLTAGE OF PV SYSTEMS

The maximum DC voltage commonly is a safety relevant limit for sizing a PV system.

All components (modules, inverters, cables, connections, fuses, surge arrestors, ….) have a certain maximum voltage they can withstand or handle safely.

If this voltage gets exceeded, damage or even worse harm can result.

New technologies established a new standard, to build PV systems with voltages up to 1000V (for special purposes in big PV power plants with central inverter topology even 1500V are used).

This makes sense by causing lower losses (power / energy, voltage-drop) and gaining higher efficiencies (inverter). This is also reducing the string number and so far reducing cabling, connectors, fuses and so on, which leads to lower space requirements and higher reliability (less parts).

So the challenge is to size a PV system with the highest possible and safe DC voltage.

Open Circuit Voltage of a PV module

On the datasheet of a PV module the open circuit voltage normally is specified at STC (= Standard Test Conditions; defining the irradiation at 1000W/m² and a cell temperature at 25°C).

As the voltage correlates nearly linear with the cell temperature, a temperature coefficient (TC,Uoc) is specified, either in V/°C or %/°C.

This is an inverse correlation, meaning the highest voltage occurs with the lowest cell temperature.

Naturally also irradiation is necessary to produce voltage (and power).

So the voltage shows also a non-linear dependency from the irradiation, meaning at low irradiations also the
voltages will be low.

Modern PV Modules have an efficiency of 15% to 20% (often also given on the datasheet).

While this percentage is converted into electricity the bigger “rest” (80% - 85%) of the irradiation is mostly converted into heat, meaning the PV cell (module) gets heated-up quite quickly.

As a matter of fact, with irradiation (= at daytime!) the cell temperature almost always will be higher than the
ambient temperature. (see also Table 1 below)