Internal Series Resistance determined of two IV-curves

For the measurement of the internal series resistance (which desribes internal losses and losses due to bad contacts as well) two IV-curves of different irradiance but of the same spectrum and at the same temperature are necessary according to IEC 60891 [6].

From the two characteristics two working points V1 and V2 have to be obtained of which the series resistance can be calculated.

The two working points are determined as follows: Definition of a current interval AI. Here:

(8)

(9)

(10)

(11)

AI = 0.5 — Isc2

Determination of the working points V1 and V2 with (2)

Vi=V(Isci-AI, Rpvi, Vti, I01, Iphi) V2=V(Isc2-AI, Rpv2, Vt2, I02, Iph2) Calculation of the series resistance

Isc1 Isc2

Fig.4. Rs-Measurement of single PV-Modules

For single cells and single modules the second IV-curve can be obtained by covering the cell or module with an insect-screen.

For large PV-generators (several kW) this method could not be applied. A new method is here necessary which allows to determine the internal series resistance out of only one IV — curve under illumination.