Degradation of peak power can be caused by an increase of the internal series resistance. For the determination of the internal series resistance out of one dark IV-curve several methods are known, e. g. [7]. The dark IV-curve can be easily measured for single cells or singele modules. As for the measurement of the dark IV-curve an external DC-current source is necessary, such strong external DC-current sources for large PV-generators (several kW) are very expensive and so hardly available.

For the measurement of the internal series resistance two IV-curves of different irradiance but of the same spectrum and at the same temperature are necessary according to IEC 60891 [6]. As the actual spectrum during the measurement is not relevant for the calculation of Rs, the measurement of the first characteristic can also take place under open air conditions with natural sunlight.

The second charcteristic can be obtained by the following simulation, so a second measurement is unnecessary.

Characteristic 1: Measurement

Isci Voci [17]p maxi Vp maxi (12)

Characteristic 2: Simulation

I ■V

FF Pmaxi pmaxi

f _ i ——— > no change in Voltage (14)

FF is the same for both characteristcs, so:

(15)

I _ f ■ I

p max2 i p maxi

V _ V

p max2 p maxi

The determination of the series resistance Rs of only one measured IV-characteristic now is possible.

The following example shows the accuracy of this method.

In order to demonstrate the effect of a higher Rs, the Rs of a BP585F-module first was measured without any manipulation and then a second measurement with an additional external resistor Rext=0.9 O was made.

Measurement A without manipulation:

(16)

Measurement B with manipulation +Rext=0.9O

IscB = 5 A VocB = 223V 1

} RB = 1.3O

Ip max B = 4.51 AVp max в = 14.56 V J SB

The manipulation can be detected here.

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