ELECTRICAL RESISTANCE MEASUREMENTS OF BUILDING INTEGRATED PV MODULES AFTER EXPOSURE TO CONTROLLED CORROSIVE CONDITIONS

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1 ELECTRICAL RESISTANCE MEASUREMENTS OF BUILDING INTEGRATED PV MODULES AFTER EXPOSURE TO CONTROLLED CORROSIVE CONDITIONS Michele Pellegrino, Giovanni Flaminio, A. Grasso and Angelo Sarno ENEA Centro Ricerche, Località Granatello. P.O. Box 32,I Portici (NA), Italy. Tel: , Fax: ; Abstract Photovoltaic modules addressed to be fully integrated in buildings imply additional safety aspects as regards not particularly trained people, as for example to prevent electric shock due to an accidental touch. The aim of this paper is to investigate the effects of the main climatic parameters (humidity and temperature), of the operating voltage and of the ageing process on the insulation properties for the new building integrated PV laminated modules. The existing code Crystalline Silicon Terrestrial Photovoltaic Modules Design Qualification and Type Approval, IEC 625, states that, in order to pass the insulation test, the module electric resistance must be not less than 50 MΩ. The experimental results show that the insulation depends on the environmental conditions giving some suggestions to enhance that standard so to include the wet leakage current test. That kind of test was already introduced on the standard Thin Film Terrestrial Photovoltaic Modules Design Qualification and Type Approval, IEC From experimental data it resulted that even new PV modules can be subjected to a rapid decrease of the insulation properties in some specific climatic conditions. Therefore particular attention should be paid, for this kind of application, in designing these systems.. INTRODUCTION PV modules to be integrated in the building may involve some additional safety aspects towards not particularly specialized or even unaware people that could accidentally come in touch with some part of them. At present the applicable standards for these modules, both crystalline and amorphous, do not state any particular special requirements about the insulation for this kind of application. This paper intends to investigate about the effects on the modules insulation characteristics of the main climatic parameters, like the temperature and the relative humidity, taking into account the operating voltage and the ageing process. These modules, in fact, can be used as elements to delimit air conditioned rooms and can operate at different values of temperatures and humidity. This work has been accomplished by ENEA, the Italian Agency for New Technologies, Energy and the Environment, within the frame of the activities related to the PV modules characterization. 2. STANDARDS The existing regulation for crystalline silicon terrestrial photovoltaic modules design qualification and type approval, IEC 625, states that, in order to pass the insulation test, the resistance must not be less then 50 MΩ at dc or Vdc plus two times the maximum system voltage, if this voltage exceeds 50 V. Successive issued standards have included the wet leakage current test to verify if the insulation holds even when the module is working under wet condition; this kind of test was already introduced in the standard for thin film terrestrial photovoltaic modules design qualification and type approval, IEC 6646 and it will be probably included also on the standard Safety Testing Requirements for PV modules, now in progress status at the Technical Committee 82, intended to give more attention to the new PV modules applications. 3. PV MODULE DIELECTRIC RESISTANCE 3. General The electrical resistance is the capacity for a device to oppose to the flow of current under a difference of electrical potential. The resistance depends on the physical characteristics of the materials and on the geometry as well. In the case of the PV module is almost difficult to define a geometry. Anyway if we tried to determine the resistance, by applying a voltage drop and by measuring the current, we would find a variation with time as it is shown in figure. This kind of behaviour is typical of capacitance effect in an electric circuit.

2 Resistance (W ).E+ 9.E+9 8.E+9 7.E+9 6.E+9 5.E+9 4.E+9 3.E+9 2.E+9.E+9 0.E+0 vs. time; experimental and calculated curves. So, the simplest equivalent electric circuit we can imagine consists of a parallel of a capacitance and a resistance series connected to another resistance. Figure 2 depicts this simple circuit. Figure Fejl! Ukendt argument for parameter.. PV module insulation equivalent electric circuit. For this kind of circuit it is easy to show that the resistance vs. time, when a DC bias step is suddenly applied, follows a time dependency expressed by () () R= Rp C Time (sec) Rs(Rs+ Rp) experimental calculated Rs + Rpexp-t/τ where τ is the time constant given by RsRpC/(Rs+Rp). The time constant could be of some tens or hundreds of seconds or more, depending on the size and the type of the module. Figure reports also the theoretical curve for the model. It can be seen that the experimental charging curve presents some disagreements with the theoretical equation plot. This is probably due to the actual geometrical shape; the depicted model is too simple and it should be probably improved to take into account more than one Rs time constant. After all, PV modules are not so simple as a standard capacitor where the dielectric material is just a layer between two parallel plates. Anyway the modelling of the PV module dielectric is beyond the scope of the paper and the scheme has been introduced only in order to give a general comprehension. 3.2 The measurements But even if the resistance can not be geometrically well defined, it can be measured so allowing us to speak about insulation property. The measurements of very high insulating materials are not so easy and we followed some of the precautions described in the standard IEC 625, even if in some parts we adopted some changes. The measurement procedure can be so summarized: ) connect the shorted output terminals of the module to the positive terminal of a D.C. insulation tester and the exposed metal parts to the negative terminal of the tester, 2) increase the voltage applied by the tester at a rate not exceeding s- at 540 V and maintain the voltage at this level for some minutes, 3) reduce the applied voltage to 0 and short-circuit the terminals of the tester for some minutes, while still connected to the module, 4) remove the short circuit, 5) apply a D.C. voltage and determine the insulation resistance. To investigate about the insulation dependency on environmental parameters the measurements were performed at fixed temperature and humidity values inside a climatic chamber. The module was put inside the chamber and the instrumentation was electrically connected to the device through a hole on the wall. The values of the climatic parameters have been measured and controlled just near the module. If the module was provided without a frame or the frame was made of non conductive material, it was connected to a metallic holder during the measurement. 4. MODULE INSULATION RESISTANCE The modules under investigation were: polycrystalline Si as shingle, polycrystalline Si as laminated, polycrystalline Si as double glazed windows, triple junction α-si module, herein referred as type A,B,C and D, respectively. On the following the resistance dependency for the 4 type modules under investigation will be presented and discussed. 4. Type A

3 0. Module type A Temperature 25 C vs. relative humidity in semilogarithmic scale for the shingle module at 25 C and for different Module type A Temperature 50 C V Figures 3-5 show the variation of the logarithm of the resistance vs. the relative humidity for the type A module. The logarithm scale has been chosen since the resistance variation is of some orders of magnitude. The curves show that the module is highly insulated at low relative humidity, where the measurement is out of the instrument full scale, about 2 TΩ, 2 2 Ω, then the resistance decreases as the humidity increases, almost linearly. The slope of the straight line depends on the value of the temperature and decreases as the temperature increases. At 50 C and 70 C and for humidity values exceeding 90 % rain drops begun to fall over the module and a liquid film was formed. The change of the slopes for the straight lines can be attributed to this phenomenon. Finally, for this kind of module it resulted that the resistance drops from very high values down to hundreds of MΩ at 25 C or even to some MΩ at higher temperatures and relative humidity. 4.2 Type B V Module type B Temperature 25 C vs. relative humidity in semilogarithmic scale for the shingle module at 50 C and for different 0. V Module type A Temperature 70 C Figure 6. Resistance vs. relative humidity in semilogarithmic scale for the laminated module at 25 C and for different V vs. relative humidity in semilogarithmic scale for the shingle module at 70 C and for different

4 4.3 Type C Module type B Temperature 50 C V Module type C Temperature 25 C V Figure 7. Resistance vs. relative humidity in semilogarithmic scale for the laminated module at 50 C and for different Module type B Temperature 70 C Figure 9. Resistance vs. relative humidity in semilogarithmic scale for the double glazed module at 25 C and for different Figure 8. Resistance vs. relative humidity in semilogarithmic scale for the laminated module at 70 C and for different V Module type C Temperature 50 C V Figures 6-8 confirm that type B module behaves more or less as the type A, being based on the same technology. It is worth noting that at dry conditions it exhibits very high values of the resistance, to rapidly decrease at higher humidity, falling down to some tens of MΩ at higher temperatures. Figure. Resistance vs. relative humidity in semilogarithmic scale for the double glazed at 50 C and for different Module type C Temperature 70 C 0. V Figure. Resistance vs. relative humidity in semilogarithmic scale for the double glazed at 70 C and for different Figures 9- indicate that the type C module behaviour is not quite different also. At dry conditions the resistance is

5 Module type D very high but rapidly decreases for higher values of humidity; if we plot the logarithm of the resistance the variation is a straight-line. Again the resistance can be as low as some tens of MΩ at high temperature and humidity values. Temperature 70 C V 4.4 Type D Module type D Temperature 25 C Figure 4. Resistance vs. relative humidity in semilogarithmic scale for the triple junction at 70 C and for different Figure 2. Resistance vs. relative humidity in semilogarithmic scale for the triple junction module at 25 C and for different 0. Module type D Temperature 50 C Figure 3. Resistance vs. relative humidity in semilogarithmic scale for the triple junction module at 50 C and for different V V Figures 2-4 show the type D module presents a quite different behaviour. The resistance is measurable even at very low humidity and it can be seen that at 25 C and 50 C there is a slight linear decrease of the resistance on a large range of humidity, from dry conditions up to 80 % and 60 % of humidity respectively; then the resistance drops more steeply towards lower values. The logarithm appears to be a piecewise linear graph, consisting of two straight-line curves. In figure 4, related at 70 C, the resistance value is almost constant within all the humidity range. The reason for this behaviour, in comparison with the other modules, could be ascribed to the presence of the metallic frame that is tightly sealed to the encapsulant material and acts as a cold point for the condensation phenomenon. 4.5 Voltage effects The figures from 3 to 4 show also the effect of the voltage on the resistance. In all the pictures we can see that at high voltages the resistance values decrease. This is due to the greater level of polarization produced by the voltage on the dielectric material, e. g. the local electric field, so increasing the parasitic resistance in the dielectric. 4.6 Temperature effects The temperature effect on the dry conditions resistance has been investigated only for the module type D because of the above mentioned instrumentation limits. The variation of the resistance vs. the reciprocal of the absolute temperature, reported in figure 5, is linear; that means that the correlation can be considered as an Arrhenius like energy activated phenomenon.

6 R(G W ) Figure 5. Arrhenius plot for the type D module. 5. EXPERIMENTAL The above plots refer to the unexposed new modules; even in this case some unfavourable conditions can cause an insulation decrease, less then 50 MΩ. But the situation could get even worse when the modules have been in service for long time. For this reason we have started to expose the modules to artificial harsh conditions in the corrosion chamber to provide an ageing process. This chamber, 3.8 m. in length.8 m. in depth and.9 m. in height, is provided with 4 equally spaced nozzles just above the floor level; from here a mixture of water and salt is sprayed upright so that the following fall of the drops can impinge the modules. The water solution has been prepared in advance and a tank of 25 l was filled. NaCl was added at the concentration of 5% in weight and the ph was adjusted at 3.7 by using acetic acid. This value of ph is representative of polluted urban area. The test consists in alternating dry, spray off, and wet, spray on; the duty cycles, i.e. the intermittence between the dry and the wet phases has been put at 5%. All the modules have been held on a special rack at a fixed slope, 5 respect the vertical line, and let in open circuit conditions. One of them, belonging to the type D, has been instead continuously biased during the test at dc between the frame and the short-circuited terminals. The insulation measurements were performed in accordance to the same procedure as described in RESULTS Type D module /T k- 45V V 250V 500V The modules have been exposed for 2 months in the salt spray chamber at the above described conditions. The results are shown in tables -5 for all the modules. We can see that 2 months of exposure have represented quite a short period to detect some heavy effects for all the modules; anyway while the unbiased modules, tables -4, exhibit only slight evidence of significative variations of the resistance, in the case of the biased module in table 5 we observe a clear decrease. One possible explanation for that is the presence of a permanent electric field applied inside the module that can be responsible of metallic electromigration or permeation of some small quantity of water. type A after >2 3 >2 3 V >2 3 >2 3 > > Tab.. Variations of the resistance after 2 months of testing for the type A module. type B after >2 3 >2 3 V >2 3 >2 3 > > Tab. 2. Variations of the resistance after 2 months of testing for the type B module. type C after >2 3 >2 3 V >2 3 >2 3 > > Tab. 3. Variations of the resistance after 2 months of testing for the type C module. type D after V Tab. 4. Variations of the resistance after 2 months of testing for the unbiased type D module. type D after

7 V Tab. 5. Variations of the resistance after 2 months of testing for the biased type D module. These results agree with the findings of a previous work (Pellegrino, Parretta and Sarno, 998); in that case the insulation of some standard silicon-crystalline PV modules was investigated and we showed that: ) the rate of resistance could decrease, at the established experimental conditions, about order of magnitude every three months, 2) the bias application of between electrical leads and the frame increased that rate at about order of magnitude per month, 3) the heat treatment leaded to a partial recovery of the resistance value. 7. CONCLUSIONS PV building integrated modules must be grounded as any other metallic structure for safety reasons; but if this remedy is good for lightning protection it could be not sufficient for accidental indirect touch. In fact since the active circuit is in floating condition, if there were a single failure this would not constitute a danger for an accidental touch; but in the case of another failure a loop could create and an accidental touch would allow the dispersion current to enter through it and put the person at risk. This case would be even worse if the operating voltage were higher, usually more than 20 V. The solutions are:. set the operating voltage as low as possible; 2. install an automatic ground fault detection instrument; 3. use PV module with protection class II rating; 4. put the cells not too near to the frame; 5. include the wet leakage current test in the applicable standard; Solution is not always applicable and then we will have the problem to face higher current values. Solution 2 is alternative to solution and it is compulsory for application with operating voltage higher then 20 V. Solution 3 is expensive and there are only a few solar modules of this type. Another possibility is to improve the quality of materials used as encapsulant to electrically separate the frame from the active circuit. It is important to characterize them and to test for long time against corrosion, water permeation and electromigraton phenomena. In this work we demonstrated that even new PV module can be subjected to a rapid decrease of the insulation properties not only for the dew condensation or for the wetting conditions after a rain shower, but even when the conditions approach the humidity saturation. This performance can be accelerated by high temperature and high voltage effects. From the presented experimental data it results that the module insulation model, depicted in figure 2, is not enough to understand and to predict the degradation phenomenon. Experimental work is in progress to improve the theoretical model and to take into account the influence of different materials. REFERENCES IEC 625 Crystalline Silicon Terrestrial Photovoltaic Modules Design Qualification and Type Approval IEC 6646 Thin-film Terrestrial Photovoltaic (PV) Modules - Design Qualification and Type Approval IEC 730-working draft Safety Testing Requirements for PV Modules Pellegrino M., Parretta. A. and Sarno A. (998) A Survey on the Electrical Insulation Behaviour of the PV Module Encapsulant Materials. In proceedings of PVSEC, 6- July, Vienna Austria. F. Sick and T. Erge (996) PV in Buildings. James and James (Science Publisher) Ltd., London.