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.
Figure 1: temperature dependency of the open circuit voltage Figure 2: irradiation dependency of the open circuit voltage
@ constant irradiation (1000W/m²) @ constant cell temperature (0°C)
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)
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Calculating the maximally arising DC Voltage (Open Circuit Voltage = Uoc,max)
The most established and easiest way to calculate the maximum open circuit voltage is to use the STC value
from the datasheet with a certain estimated lowest occurring cell temperature.
Uoc,max = Uoc,stc [V] + Uoc,stc [V] * (Tcell,min [°C] – Tstc [°C]) * TC,Uoc [%/°C] / 100
As a matter of fact, STC (1000W/m²) doesn’t take into account the influence from the irradiation.
(lower voltages at lower irradiations)
For finding the real (correct) Maximum DC Voltage (Open Circuit Voltage),
a complete set of module characteristic curves with
different irradiation levels and the resulting cell temperatures at
the lowest occurring ambient temperature (Tamb,min)
would have to be made. (graphs in figure 3)
As this would be quite a big effort (software, module detail data, …), the upper formula can be used with a
modified Minimum Cell Temperature (Tcell,min).
Table 1: resulting cell temperatures Figure 3: realistic open circuit voltages versus calculation with simple formula
at an constant
ambient temperature (0°C)
at different irradiation levels.
For most modules the highest open circuit voltages would occur at an irradiation of 400 – 500 W/m²
(see figure 3).
At normal operation, high open circuit voltages won’t appear because the PV system (inverter) operates in its
MPP (dots in figures 1 – 3).
As a matter of fact the PV system (inverter) would have to shut down exactly at a moment @ lowest ambient
temperature and @ high irradiation, only then the highest open circuit voltage can appear!
(= quite unlikely worst worst case)
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