Modelling of insulation characteristics of Solar Photovoltaic (SPV) modules

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ScienceDirect Solar Energy 120 (2015) 1–8 www.elsevier.com/locate/solener

Modelling of insulation characteristics of Solar Photovoltaic (SPV) modules J.N. Roy School of Energy Science & Engineering & Advance Technology Development Centre, Indian Institute of Technology, Kharagpur 721302, India Received 12 March 2015; received in revised form 17 June 2015; accepted 29 June 2015

Communicated by: Associate Editor Arturo Morales-Acevedo

Abstract Dry and wet insulation characteristics of a Solar Photovoltaic (SPV) module have been studied through theoretical modelling supported by experimental results. A new equivalent circuit model approach has been used to understand the effect of resistances of the individual material used in the SPV module and the overall impact on the insulation characteristics. The electrical resistances of all the individual material have also been measured separately prior to lamination. It has been seen that, post lamination, the characteristics change significantly. EVA (Ethyl Vinyl Acetate) and Back-sheet (Tedlar) looses their separate identity and exhibit properties of a different new combined material. The resistance of this material has been determined using the equivalent circuit modelling approach. It has been shown that the insulation behaviour is primarily governed by this material. The temperature characteristics of the insulation leakage currents follow Arrhenius behaviour with well defined activation energies. It has been determined from the activation energies that the primary leakage path is from cell to EVA–Tedlar to frame of the module. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Solar PV; Dry insulation; Wet insulation; Equivalent circuit

1. Introduction Solar Photovoltaic (SPV) modules have to undergo some stringent post fabrication tests (Rutschmann, 2009) to ensure safe and reliable performance for a long time after installation. One of the safety requirements is to ensure that there is no possibility of unwanted conduction between the current producing solar cells and the outside parts such as frame, back sheet and glass. As the solar modules produce large currents, any leaky current can be dangerous to person touching it during installation or maintenance. The current producing solar cells are embedded (sandwiched) between layers of glass and polymeric materials such as EVA (Ethyl Vinyl Acetate) and Tedlar. E-mail address: [email protected] http://dx.doi.org/10.1016/j.solener.2015.06.036 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

A typical configuration of a solar module is shown in Fig. 1. The polymeric materials such as EVA & Tedlar and insulating material such as glass as such have very high resistances. In normal condition therefore it is expected that the resistance between the cell and any external part of the module must be very high; in the order of tens of Mega Ohms. This is ensured by a test called “Dry insulation test” in which a high voltage (500–1000 V) is applied between the cell string and the frame. The current is then measured. The product of resistance thus determined and the area of the module must be greater than 40 MX m2 (10.3.5 of International Standard). These tests are to be performed in the specified temperature range (22 ± 3 °C). It is also important to ensure that the current producing cells and the external parts are properly insulated also during wet condition; causing due to rain, snow or cleaning.

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Fig. 1. Typical lay-up sequence of a Solar Photovoltaic (SPV) module during lamination.

This can be ensured by measuring the leakage current between the cells and external parts by dipping the module in water. This is known as “wet leakage test” during which the product of measured resistance and the area of the module must also be greater than 40 MX m2 (10.3.5 of International Standard). This test is to be performed in the specified conditions of temperature and water viscosity range (10.15 of International Standard). The installed SPV modules are expected to operate for a very long period, typically 25 years or more, in adverse conditions such as high temperature and high humidity. Long term reliability is ensured by extended life tests, such as TC200 (10.11 of International Standard) and DH1000 (10.13 of International Standard). The “Dry insulation test” and “wet insulation test” are also performed on sample modules after they have undergone the reliability test exposure (Rutschmann, 2009) and are found to meet the specifications (>40 MX m2). The failure of SPV modules during dry and/or wet leakage tests can be due to defect in the materials or process or a combination of both. In case there is a gross process or material defect, it may result in dry insulation as well as wet insulation failures. However, generally it is observed that the defective modules fail during wet leakage test. It is important to know which material is responsible for the failure. Resistance to degradation due outdoor environment and electrical insulation can be achieved by coating or encapsulation. Coating is widely used in SPV technology for enhancing Anti Reflection property to capture more light (Ali et al., 2014; Kaminski et al., 2014). Recent development of multifunction coating is also addressing outdoor stability. Their applications in Dye Sensitized Solar Cells (DSSC) (Bella et al., 2015), energy storage and conversion (Griffini et al., 2014, 2013) have been reported recently. Encapsulation technology is widely used for various SPV technologies and systems such as Solar Thermoelectric (Sundarraj et al., 2014), silicon carbide (SiC) based SPV cells (Cheng et al., 2015), flexible polymer SPV cells (Moaven et al., 2015). Polymeric materials are most commonly used for encapsulation of c-Si SPV modules. The polymeric materials used for construction of SPV modules are the back sheet and EVA. These materials are selected based on their aging behaviour. During the course of maturing of Solar PV technology several types of back sheet, e.g. Polyethylene Terephthalate (PET), Polyvinyl Fluoride (PVF), Polyvinylidene Fluoride (PVDF), etc. have tried (Oreski and Wallner, 2005). Tedlar or its variants, e.g. Tedlar Polyvinyl Fluoride (Gambogi et al., 2014) is now a most widely used back sheet material.

Although some other materials were tried by researchers, EVA has emerged as most acceptable material for encapsulation of SPV modules (Oreski and Wallner, 2009; Addeo et al., 1983; Poulek et al., 2012). High voltage stress test on SPV modules are conducted to ascertain the partial discharge behaviour (Dechthummarong et al., 2011) occurring due to unwanted and unprotected lightening strike and Potential Induced Degradation (PID) (Hacke et al., 2011; Del Cueto and Rummel, 2010; Raykov et al., 2012). Dhere et al. (2014) has a detail test and analysis for modules made by four different manufacturers primarily to address System-Voltage-induced Degradation (SVID), which is similar to PID, under different environmental conditions. Biasing effect on SPV module made by depositing polycrystalline silicon directly on tempered glass, i.e. without using polymeric material for front side encapsulation, is also studied by Park et al. (2015). In this paper a method has been described to understand and model the dry and wet leakage phenomenon applicable to SPV module under high voltage stress. It has been developed based on the temperature dependency of the dry and wet leakage resistances using a new equivalent circuit model approach. The model relates the properties of the individual materials used for construction of the SPV modules. 2. Measurement set up The measurements are carried out as per IEC standard (10.3 of International Standard). The schematic of the test set up is shown in Fig. 2. For dry insulation test, the modules are not dipped in water. Industry standard equipment, Megger High Pot Tester (10.3.2 of International Standard), has been used for this measurement. Both positive and negative ends of the SPV module are shorted and connected to one end of the power supply of Megger. The other end of the power supply is connected to the frame of the module. A high voltage (1000 V) is applied and the current is measured. The calculated resistances are also recorded. The corresponding module temperatures are recorded by a Pyrometer. The measurements are done outdoor and are conducted at various times of the day and also at different days to collect dry insulation resistance data at various

Terminal1

Megger High Pot Tester Ground Terminal

SPV Module

Terminal2

Fig. 2. Typical test set-up for insulation test. Terminal 1 is connected to both positive and negative ends (shorted) of the SPV module. Terminal 2 is connected to frame of the SPV module for dry insulation test and water for wet insulation test.

J.N. Roy / Solar Energy 120 (2015) 1–8 140

Table 1 Dry insulation test results.

120

Leakage Current (nA)

3

100 80 60 40

S. no.

T (°C)

T (K)

IR (GX)

LC (nA)

1 2 3 4 5

24 36 41 49 55

297 309 314 322 328

86.9 35.4 21.9 12.1 7.5

11.5 28.2 45.6 82.2 132

20 0 295

300

305

310

315

320

325

330

335

Temperature (K) Fig. 3. Leakage current as function of temperature during dry insulation test.

temperatures. The modules used here have a size of about 1.4 m2. For the wet leakage test, in addition to Megger, a large water tub is also used. One end of the Megger power supply is dipped into water. The other end is connected to the shorted positive and negative ends of the SPV module as done for dry insulation test. The SPV module has a junction box which is fitted after the lamination process. The connection to the cells are taken out of the lamination and connected appropriately to the junction box diodes. Sometime the wet leakage failure occurs due to leaky junction box. The water may enter to the junction box and touch the connecting leads creating a short. A separate study is being undertaken to understand the junction box leakage. In this study, this has not been included. It has been ensured during the test that the junction box is not immersed in the water and therefore not contributing to the leakage current value. One set of measurements is taken by dipping only the front side; i.e. glass side in water. The next set of measurements is taken such a way that the front side as well as back side is in water. The measurements are taken at different temperatures. In this case temperature variation is achieved by varying the temperature of the water by preheating the water before pouring it in the tub. A standard thermometer is used to measure the temperature of the water. TC200 and DH1000 tests have been conducted to see if there are any differences between pre and post reliability test results. Environmental chambers from Weiss (Model WK-3100) and ESPEC (Model EW5270WS) have been used for these accelerated tests. The differences between pre and post reliability test results have been found to be not significant and within permissible limits (Oreski and Wallner, 2009). Therefore these values are not reported or used in this paper. Nevertheless, this confirms that there are no gross defects in the materials and process used to manufacture the SPV modules used for this study. 3. Experimental results The dry insulation test results as a function of module temperature is given in Table 1. It can be seen that it

followed an exponential behaviour. The wet insulation test results as a function of temperature are given in Tables 2 and 3. Table 2 and Fig. 4 represent the values from the experiment in which the back side of the module is not in water. The results with backside in water are represented by Table 3 and Fig. 5. These also follow exponential behaviours. In these tables the notations used are T denoting Temperature, IR denoting Insulation Resistance and LC denoting Leakage Current. 4. Modelling The typical configuration of a solar module is shown in Fig. 1. The electrical equivalent circuits during three different test conditions; dry insulation and wet insulation with and without backside in water, are shown in Figs. 6–8. In these figures, the various resistances are denoted as shown below: R1, R9: Resistances offered by the Aluminium (Al) frame R2, R8: Resistances offered by the foam tape, which is used in between Al frame and laminated module. R3: Resistance offered by the Back sheet. R4, R6: Resistances offered by the EVA. R5: Resistances offered by the Cell string. R7: Resistance offered by the Toughened glass. RWFS: Resistance offered by the water from front (glass) side of the module. RWBS: Resistance offered by the water from the back side (back-sheet) of the module. The resistances of the various components appearing in equivalent circuits have also been measured independently Table 2 Wet insulation test results when the backside of the module is not in water. S. no.

T (°C)

T (K)

IR (MX)

LC (lA)

1 2 3 4 5 6 7 8 9 10 11

35.0 36.8 37.6 39.2 40.8 42.5 44.1 46.1 47.5 50.1 54.2

308.0 309.8 310.6 312.2 313.8 315.5 317.1 319.1 320.5 323.1 327.2

52.6 50.8 45.4 39.1 31.0 27.7 25.1 21.9 18.0 14.1 10.5

19.0 19.7 22.0 25.6 32.3 36.1 39.7 45.6 55.4 70.9 94.8

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Table 3 Wet insulation test results when the backside of the module is in water. S. no.

T (°C)

T (K)

IR (MX)

LC (lA)

1 2 3 4 5 6 7 8 9 10

35.4 37.4 38.5 40.5 44.5 45.6 46.5 47.7 50.7 54.1

308.4 310.4 311.5 313.5 317.5 318.6 319.5 320.7 323.7 327.1

87.7 75.2 59.9 49.0 37.6 33.8 29.9 26.2 22.1 18.6

11.4 13.3 16.7 20.4 26.6 29.6 33.4 38.1 45.3 53.8 Fig. 6. Electrical equivalent circuit of dry insulation measurement.

100

Leakage Current (nA)

90 80 70 60 50 40 30 20 10 0 305

310

315

320

325

330

Temperature (K) Fig. 4. Leakage current as function of temperature during wet insulation test when the backside of the module is not in water. Fig. 7. Electrical equivalent circuit of wet insulation measurement when the backside of the module is not in water.

60

Leakage (nA)

50 40 30 20 10 0 305

310

315

320

325

330

Temperature (K) Fig. 5. Leakage current as function of temperature during wet insulation test when the backside of the module is also in water.

prior to lamination. These values are given in Table 4. The resistance measurements of the polymer materials and glass have been done using the same set-up as shown in Fig. 2. In this case, instead of the SPV module, the respective polymer material or glass has been connected to Megger. The contact was taken with the help of aluminium foil. Resistance of aluminium frame has been measured using standard multi-meter and the cell sting resistance has been measured by dark forward I–V characteristics. It may be noted that these resistances have strong dependence on temperature and therefore must be taken as indicative values. Further analysis of the equivalent circuit of Fig. 6,

Fig. 8. Electrical equivalent circuit of wet insulation measurement when the backside of the module is also in water.

Table 4 Resistance values of various components at ambient temperature of the equivalent circuits of Figs. 6–8. S. no.

Name of the material

Resistance

1 2 3 4 5 6 7

Toughened glass EVA-1 Cell string EVA-2 Back sheet Foam tape Al frame

3 TX 3 TX 0.6 X 3 TX 1.68 TX 48.1 MX 164 mX

J.N. Roy / Solar Energy 120 (2015) 1–8

5

results the resistance network circuit shown in Fig. 12. It may be noted that the voltage drop across the cell string is neglected for simplicity. The drop is anyway not significant as the resistance of the cell string is small (Table 4). The equivalent resistance value can be obtained from Eq. (1). Req ðDry InsulationÞ ¼ ðR1 þ R2 þ R3 þ R4 ÞðR6 þ R7 þ R8 þ R9 Þ=ðR1 þ R2 þ R3 þ R4 þ R6 þ R7 þ R8 þ R9 Þ

ð1Þ

Similar analysis of Fig. 7 results the equivalent resistance of wet insulation test when the backside of the module is not in water (wet insulation 1). This is given in Eq. (2). Equivalent resistance of wet insulation test when the backside of the module also in water (wet insulation 2) is given in Eq. (3).

Fig. 9. Modified electrical equivalent circuit of Fig. 6 assuming EVA and back sheet as a single material post lamination.

Req ðwet insulation 1Þ ¼ ðR1 þ R2 þ R3 þ R4 ÞðR6 þ R7 þ R8 þ R9 þ RWFS Þ=ðR1 þ R2 þ R3 þ R4 þ R6 þ R7 þ R8 þ R9 þ RWFS Þ

ð2Þ

Req ðwet insulation 2Þ ¼ ðRWBS þ R1 þ R2 þ R3 þ R4 Þ  ðR6 þ R7 þ R8 þ R9 þ RWFS Þ =ðRWBS þ R1 þ R2 þ R3 þ R4 þ R6 þ R7 þ R8 þ R9 þ RWFS Þ ð3Þ Calculated resistances (Eqs. (1)–(3)) and the experimentally measured (Fig. 2) resistances values are shown in Table 5. The differences in the theoretical and experimental values, during dry insulation test, can be attributed to the fact that the EVA resistance changes drastically after lamination. In fact, it indicates that, post lamination, for all practical purpose this should not be considered as separate materials. The laminated combination of back sheet and EVA can be considered as a single material and the resistance of this can be estimated by suitably modified the equivalent circuit as shown in Figs. 9–11. The corresponding equivalent resistances are given in Eqs. (4)–(6). The measured values are indicated in Table 5 and at the bottom of Table 6. In these the resistance of EVA–Backsheet combination after the lamination is denoted as R34, which can be determined from Eq. (4) assuming other resistances are not changing due to lamination process. It is a reasonable assumption as the resistance of the tempered glass, which is very high and primarily (other than EVA–Backsheet) determining the equivalent resistance of Fig. 9, is not expected to change at lamination temperature of about

Fig. 10. Modified electrical equivalent circuit of Fig. 7 assuming EVA and back sheet as a single material post lamination.

Fig. 11. Modified electrical equivalent circuit of Fig. 8 assuming EVA and back sheet as single material post lamination.

145C and time of about 15 min. RWFS and RWBS can now be calculated from Eqs. (5) and (6) respectively. The resistance values of all the components can now be determined. These are listed in Table 6. It can be seen that the resistance of the glass is much larger than the other components. Therefore effect of the resistance of the EVA after

Table 5 Calculated and experimental resistance values of the equivalent circuits of Figs. 6–8. S. no.

Description of the test

Calculated resistance values

Experimental resistance values

1 2 3

Dry insulation test Wet insulation test without water on the back sheet (Wet Insulation 1) Wet insulation test with water on the back sheet (Wet Insulation 2)

3.15 TX 146 MX 219 MX

78.9 GX 146 MX 219.16 MX

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Table 6 Experimentally determined resistance values of material used for SPV modules. It may be noted that post lamination EVA and back sheet are considered to be a single material. S. no.

Material name

1 2 3 4 5 6 7 8

Resistance values

Toughened glass EVA-1 Cell string EVA-2 Back sheet Foam tape Al frame Water on the front side of the module Water on the back side of the module

9

RExperimental (Total)

Dry insulation test

Wet insulation test without water on its back side

Wet insulation test with water on its back side

3 TX 3 TX 0.6 X 4.68 TX

3 TX 3 TX 0.6 X 97.86 MX

3 TX 3 TX 0.6 X 97.86 MX

48.1 MX 164 mX NA

48.1 MX 164 mX 34.7 KX

48.1 MX 164 mX 34.7 KX

NA

NA

73.6 MX

78.9 GX

146 MX

219.16 MX

1000 V DC

-3.9 -4

35

36

37

38

39

40

41

-4.1

R2

R3

R4

-4.2

Log I

R1

R6

R7

R8

-4.3 -4.4 -4.5

R9

-4.6

Fig. 12. Equivalent resistance network circuit of dry insulation measurement.

-6.6 -6.8

35

35.5

36

36.5

37

37.5

38

38.5

39

39.5

-4.7 -4.8

1/KT (eV -1)

Fig. 14. Log (I) Vs (1/KT) plot for wet insulation test results without backside in water.

Log I

-7 -4.1

-7.2

35.5

-4.2

-7.4

36

36.5

37

37.5

38

38.5

39

39.5

40

40.5

-4.3

-7.6

-4.4

-7.8

Log I

-4.5

-8 -8.2

-4.6 -4.7 -4.8

1/KT (eV -1)

-4.9

Fig. 13. Log (I) Vs (1/KT) plot for dry insulation test results.

-5 -5.1

lamination on the glass side is not considered. It is also shown later that the leakage currents during insulation test are mainly due to EVA–Backsheet combination. Therefore during insulation test majority of the current flows through backside and from sides (cell to EVA–Backsheet to Frame/Water). The current component through front side (cell to EVA to Glass) is negligible. Req ðDry Insulation : ModifiedÞ ¼ ðR1 þ R2 þ R34 ÞðR6 þ R7 þ R8 þ R9 Þ =ðR1 þ R2 þ R34 þ R6 þ R7 þ R8 þ R9 Þ

1/KT (eV-1)

Fig. 15. Log (I) Vs (1/KT) plot for wet insulation test results with backside in water.

Req ðwet insulation 1 : ModifiedÞ ¼ ðR1 þ R2 þ R34 ÞðR6 þ R7 þ R8 þ R9 þ RWFS Þ =ðR1 þ R2 þ R34 þ R6 þ R7 þ R8 þ R9 þ RWFS Þ

ð5Þ

Req ðwet insulation 2 : ModifiedÞ ¼ ðRWBS þ R1 þ R2 þ R34 ÞðR6 þ R7 þ R8 þ R9 þ RWFS Þ ð4Þ

=ðRWBS þ R1 þ R2 þ R34 þ R6 þ R7 þ R8 þ R9 þ RWFS Þ

ð6Þ

J.N. Roy / Solar Energy 120 (2015) 1–8

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Table 7 Values of I0 and Ea determined from Figs. 13–15. S. no.

Description of the test

I0 (A)

Ea (eV)

R0 (X)

1 2 3

Dry insulation test Wet insulation test without dipping the back sheet and junction box in water Wet insulation test with dipping the back sheet and junction box in water

2.01E+03 5.15E+07 1.58E+07

0.66 0.76 0.74

0.498 1.94E05 6.33E05

The electrical characteristics of the EVA–Backsheet material can be studied from the temperature dependence of the leakage current (Figs. 3–5). These are exponential curves which follow Arrhenius relationship as given in Eq. (7). Y ¼ aExpðb=X Þ

ð7Þ

In this case Y is Leakage current and X is absolute temperature in K. This is in the form of Arrhenius behaviours, having well defined activation energy (Ea), applicable to materials such as semiconductor and polymer. Eq. (7) can be re-written in the form of as given in Eq. (8). I ¼ I 0 ExpðEa =KT Þ

ð8Þ

I0 indicates Leakage current value at very high temperature. Ea is the activation energy characteristics of the materials. The value of I0 and Ea can be determined by plotting Log Y Vs (1/KX) i.e. Log I Vs (1/KT) as shown in Figs. 13–15. The slope of Log I Vs (1/KT) plot gives activation energy (Konovalov et al., 1998). These show excellent linearity, with a linearity co-efficient of almost 1.0 (>0.99). The values of I0 (A) and Ea (eV) determined from these plots (Figs. 13–15) are given in Table 7. The activation energy (Ea) is similar, within the measurement accuracy, for both the wet insulation test results. Lower activation energy indicates that primary component taking part during the current flow is a polymer; EVA– Backsheet combination in this case. Pure glass typically has activation energy of 9 eV. Even if there are impurities in glass, the activation energy cannot be as low as 0.7 eV. This clearly indicates that the current flow through glass is not significant. The R0 value increases if the backside is also in water. This is expected as the resistance of the extra water comes in series. However the room temperature insulation resistance would still be governed primarily by the activation energy as R0 value is very small. This also indicates another important point. It shows that the majority of the current conducts through the side; i.e. cells to EVA–Tedlar and then to frame. The conduction through the back side is negligible. Otherwise it should have shown a smaller resistance when the backside is also dipped in water. The R0 value determined from the dry insulation test are found to be higher than that obtained by wet insulation tests. This is in expected line as back sheet (Tedlar) absorbs some water, may be through micro pores, and the characteristics undergoes changes. Water resembles metal from the electrical conductivity perspective. The resistance value is therefore expected to decrease if the Backsheet is soaked

in water. The activation energy obtained from the dry insulation tests is a bit lower than that of wet insulation results. This is a surprise. As R0, this was also expected to be higher. It is not known to the author that how the water molecules would alter the bond structure of back sheet, which is a polymer. Therefore determination of exactly how and why this material properties change due to water absorption is not attempted here. 5. Conclusion In this paper a method has been described to understand and model the dry and wet insulation characteristics of Solar Photovoltaic (SPV) module. Two different experiments have been conducted for wet insulation measurement. One with only the glass side dipped in the water. In the other one, both glass side as well as back side is dipped in water. The resistance values of the material, used in the modules, have been determined from the measurements on individual materials as well as from the electrical equivalent circuits. These measurements confirm that post lamination EVA and Backsheet combine to form another new material with different electrical characteristics. The resistance of this material has been determined. From the study of activation energy, it has been shown that the insulation characteristics are primarily decided by the EVA–Tedlar combination. This activation energy has been determined by temperature measurements of the insulation leakage current. The characteristics are Arrhenius in nature and the semi log plot follows a nice linear behaviour. The activation energy and the extrapolated resistances at very high temperature have also been determined. It has also been concluded that the conduction from side; i.e. cell to EVA–Tedlar to frame is primarily responsible for leakage current. Acknowledgement The experimental data used here was from the experimental set up and modules (Roy et al., 2010) manufactured at Solar Semiconductor Pvt Ltd, Hyderabad, India. References 10.11 of International Standard for Crystalline Silicon Terrestrial Photovoltaic (PV) Modules – IEC 61215-2. 10.13 of International Standard for Crystalline Silicon Terrestrial Photovoltaic (PV) Modules – IEC 61215-2. 10.15 of International Standard for Crystalline Silicon Terrestrial Photovoltaic (PV) Modules – IEC 61215-2.

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