10 An ex A informative Historical stu ies of the volume resistivity of en a s lation materials.. The test is p rformed on dry, h mid or wet precon itioned samples.. This test is desig ed
Preconditioning
Test materials must be preconditioned according to specific conditions before reporting results For dry resistance testing, samples should be placed in a desiccated atmosphere with relative humidity below 5% at a temperature of 23 ± 2 °C for at least 48 hours For wet resistance testing, samples need to be preconditioned in an enclosed container with liquid water, ensuring they do not come into direct contact with the water.
Before testing, the film samples (t, d3, d2, d1, d4, g) should be conditioned in water at a temperature of (23 ± 2) °C for at least 48 hours, which can be done using a desiccator jar filled with water instead of desiccant Additionally, the samples should be preconditioned in laboratory air at (23 ± 2) °C and (50 ± 5) % relative humidity for a minimum of 48 hours, or alternatively, in a humidity-controlled chamber.
For some classes of materials, such as desiccant filled edge seals, it may take significantly more than 48 h for them to equilibrate In this case verification of equilibrium is required
If the samples are laminated to a metal foil on one side, then they shall be equilibrated for
Samples should be conditioned for 96 hours instead of 48 hours, ensuring that they are not stacked and maintain a minimum air gap of 2 mm between each specimen It is essential to place the specimens in the test fixture immediately after removing them from the preconditioning chamber.
The moisture content of samples is unlikely to change significantly during the experiment due to their close contact with gas impermeable electrodes To minimize moisture in dry conditions, it is advisable to place the test fixture in a dry environment or incorporate desiccant Additionally, performing the experiment at 50% humidity can help maintain consistent moisture levels.
RH in a lab or chamber at 50 % RH will eliminate this concern for humid samples
The thickness of each test specimen must be measured after conditioning and before testing This measurement will be determined by averaging three readings taken from various locations on the specimen.
Test conditions
Room temperature
Measurements will be conducted at a temperature of T = (23 ± 2) °C, with both temperature and relative humidity (RH) included in the test report Due to the limited diffusion of water in most polymers, a sample placed between two impermeable electrodes does not require the atmosphere to be maintained at the preconditioned RH setpoint However, the annular region, which typically accounts for around 15% of the area, does have a slight impact on conductivity and may introduce a negligible error in the measurement.
Elevated temperatures
Resistivity in photovoltaic (PV) modules can fluctuate significantly across their operating range, making its temperature dependence crucial It is advisable to conduct elevated temperature measurements at specific points: (40 ± 2) ⁰C, (60 ± 2) ⁰C, and (85 ± 2) ⁰C.
To avoid complications from the sample drying out during the course of the test, pre- conditioning using the “dry” method, at the temperature of the test, is recommended,
To measure volume resistivity at elevated temperatures using preconditioned "humid" or "wet" samples, it is essential to precondition the samples at the target temperature The testing should then be conducted in an environmental chamber, maintaining the same relative humidity (RH) as during the preconditioning process.
At high temperatures, certain materials may undergo a melting transition, leading to adhesion or contamination of the test cell Additionally, elevated temperatures can cause materials to flow and deform, which may significantly affect the results.
Measurement voltage
Method A voltage
Measure the resistance (R) at (23 ± 2) °C, with applied voltage (V) of (1 000 ± 1 0) V DC If sample thicknesses greater than 0,75 mm or less than 0,1 5 mm are used, the applied voltage
The IEC 62788-1-2:2016 standard specifies that measurements must be conducted with an electric field of 2,000 V/mm For materials designed for higher voltage systems, testing should occur at the maximum rated voltage Any use of voltages other than 1,000 V must be documented in the test report.
For this method, the voltage will be cycled between a positive voltage and a zero voltage condition Here Vmax = V and Vmin = 0.
Method B voltage
Method B uses ± (1 000 ± 5) V DC for the measurement of all films The voltage is cycled alternatively from a positive to negative polarity Here Vmax = +V = + (1 000 ± 5) V and
Measurement cycle
Method A cycle
For Method A, a 1-hour cycle time is utilized for datasheet reporting This involves a 1-hour "on" cycle at a test voltage (Vmax) of 1,000 V, followed by a 1-hour "off" cycle at 0 V, with the process repeated For thicker samples, the voltage is adjusted to achieve an electric field of 2,000 V/mm.
Method B cycle
For method B use a 1 min cycle time The 1 min cycle is intended to be used for quality control or process control Here the voltage is cycled between + (1 000 ± 5) V and – (1 000 ± 5) V.
Results
At the conclusion of the cycles, current measurements will be documented The current from the first cycle (I1) will be excluded, while the subsequent four measurements (I2, I3, I4, I5) will be retained To reduce the impact of background currents, a weighted average (IAve) of the current will be computed.
PV modules experience prolonged exposure to high voltages, alternating with extended periods of no voltage during the night To accurately reflect their application use, an on/off measurement method is employed to generate relevant values.
Here I2 , I3 , I4 and I5 , are sequential current measurements from voltage-on, and voltage-off conditions in Method A, and alternating polarity voltages in Method B The volume resistivity (ρ) is determined from the formula:
In Method A, Vmax denotes the applied "on" voltage, while Vmin indicates the applied "off" voltage, which is 0 V Conversely, in Method B, Vmax is a positive voltage and Vmin is a negative voltage Additionally, t represents the specimen thickness, and A signifies the effective area of the smaller electrode, defined by a specific formula.
A=π d + (3) where g is the gap for the guard electrode and d1 is the diameter of the test electrode, Figure 1
Because the first cycle is discarded, this measurement should take 5 h to perform for method
A or 5 min for method B If in doubt about the operation of an instrument, use method B, noting the total test time, to verify operation of the instrument
The measurement averaging technique effectively reduces the impact of fluctuating background currents and is commonly utilized in commercial electrometers Method B is generally recognized as the standard protocol for these devices Alternatively, Method A can be employed by configuring the electrometer to operate in alternating polarity mode, applying a 500 V alternating voltage combined with a 500 V DC offset, resulting in a 1,000 V/0 V "on/off" measurement pattern Additionally, many commercial instruments automatically discard the initial measurement for improved accuracy.
Electrometers commonly utilize formula (1) to determine average current, particularly when employing the alternating polarity method In this approach, the voltage from the alternating polarity method is integral to calculating volume resistivity Notably, typical electrometers do not account for offset voltage in their volume resistivity calculations, and their formula lacks the factor of ẵ present in formula (2).
V max is double the alternating polarity voltage set in the electrometer to generate the "on/off" voltage As a result, these factors negate each other, allowing standard electrometers to calculate values that align with formula (2).
In the case where the measured current is less than 2 nA, it may be necessary to disengage the ‘autorange’ feature on the electrometer in order to obtain valid measurements
If a negative current value is recorded during measurements, except when the voltage is zero, the test specimen cannot be accurately measured with the current instrument setup In such cases, it is essential to assess the instrument's limitations and report the resistivity as exceeding the instrument's measurement capability The resolution of a well-shielded instrument and test fixture is based on the lowest current range and sample geometry, as outlined in formulas (1), (2), and (3) Additionally, when determining the lowest current range, it is crucial to ensure that environmental noise does not restrict the instrument to higher current ranges.
All measurements will be reported to two significant figures as the average resistivity (\( \rho \)) in \( \Omega \cdot \text{cm} \), based on five samples, along with the standard deviation Any results equal to or exceeding \( 1.1 \times 10^{17} \, \Omega \cdot \text{cm} \) will be reported as greater than \( 1.1 \times 10^{17} \, \Omega \cdot \text{cm} \).
A certified report of tests, prepared by the test agency in accordance with ISO/IEC 17025, must detail the performance characteristics of the material Each certificate or test report should include essential information such as the title, the name and address of the test laboratory, and the location of the tests It must also feature a unique identification for the certification or report, along with the client's name and address when applicable Additionally, the report should describe the tested item, including specimen thickness, characterization, and condition, as well as the method of specimen preparation, measurement temperature, and preconditioning conditions Important dates, such as the receipt of the test item and testing dates, should be documented, along with the identification of the test method used and any relevant sampling procedures Finally, the report must note any deviations from the test method and provide additional information pertinent to the tests, including environmental conditions or voltage.
IEC 62788-1 -2:201 6 IEC 201 6 – 1 1 – k) measurements, examinations and derived results supported by tables, graphs, sketches and photographs as appropriate including the resistivity measured for both the wet, 50 %
The report must include the average and standard deviation of resistivity measurements for specimens at room temperature and other temperatures, indicating that for multi-layer specimens, the measured resistivity reflects effective bulk resistivity It should also state the estimated uncertainty of the test results when applicable, include a signature and title of the responsible individual along with the date of issue, and clarify that the results pertain only to the tested items Additionally, it must specify that the certificate or report cannot be reproduced in part without written approval from the laboratory.
Annex A (informative) Historical studies of the volume resistivity of encapsulation materials
The degradation of photovoltaic (PV) modules is influenced by electrochemical corrosion, which is affected by the resistivity of the encapsulation materials Effective encapsulation must restrict ion flow, especially when wet, to mitigate degradation caused by electrical potential or polarization Historical data on the volume resistivity of photovoltaic encapsulation materials can be found in references [1] and [2] In PV applications, increasing the encapsulant resistivity from \$10^{13} \, \Omega \cdot \text{cm}\$ to approximately \$10^{16} \, \Omega \cdot \text{cm}\$ can significantly alleviate potential induced degradation concerns, while further increases in resistivity yield minimal improvements [4].
Figure B.1 – Example data showing current and voltage as a function of time for Method A measurement
Table B.1 – End of cycle current measurements and values used for calculation of volume resistivity according to Method A
Measured currents and calculated data
Ap pl ie d vo lta ge ( V)
[1 ] G R Mon and R G Ross, "Electrochemical degradation of amorphous-silicon photovoltaic modules," in Proceedings of the 18th IEEE PV Specialists Conference, Las Vegas, Nevada, USA, 1 985, pp 1 1 42-1 1 49
[2] G Mon, L Wen, J Meyer, R Ross, Jr., and A Nelson, "Electrochemical and galvanic corrosion effects in thin-film photovoltaic modules", in Photovoltaic Specialists Conference, 1988, Conference Record of the Twentieth IEEE, 1 988, pp 1 08-1 1 3, vol.1
[3] Adam Daire, “Improving the Repeatability of Ultra-High Resistance and Resistivity
Measurements”, White paper by Keithley Instruments, 2001
[4] C Reid, S Ferrigan, I Fidalgo, and J T Woods, "Contribution of PV Encapsulant
Composition to Reduction of Potential Induced Degradation of Crystalline Silicon PV Cells," 28 th European Photovoltaic Solar Energy Conference and Exhibition, 201 3
[5] Keithley, Low Level Measurements Handbook: Precision DC Current, Voltage, and
Annexe A (informative) Études menées sur la résistivité transversale des matériaux d'encapsulation 27
Annexe B (informative) Exemple de données 28
Figure 1 – Schéma de l'appareillage d'électrodes pour les mesurages de la résistivité 21
Figure B.1 – Exemple de données indiquant le courant et la tension en fonction du temps pour la méthode A de mesure 28
Tableau B.1 – Mesurages et valeurs du courant de fin de cycle utilisés pour le calcul de la résistivité transversale conformément à la méthode A 28
PROCÉDURES DE MESURE DES MATÉRIAUX UTILISÉS
Partie 1 -2: Encapsulants – Mesurage de la résistivité transversale des encapsulants photovoltạques et autres matériaux polymères
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The official decisions or agreements of the IEC on technical matters aim to establish an international consensus on the studied topics, as the relevant national committees of the IEC are represented in each study committee.
The IEC publications are issued as international recommendations and are approved by the national committees of the IEC The IEC makes every reasonable effort to ensure the technical accuracy of its publications; however, it cannot be held responsible for any misuse or misinterpretation by end users.
To promote international uniformity, IEC National Committees strive to transparently implement IEC Publications in their national and regional documents Any discrepancies between IEC Publications and corresponding national or regional publications must be clearly stated in the latter.
Préconditionnement
Testing materials must be preconditioned under specific conditions, which should be detailed in the test report For dry environment resistance testing, five samples should be conditioned in a dehydrated atmosphere (HR < 5%) at a temperature of (23 ± 2) °C for at least 48 hours prior to testing For wet environment resistance testing, the samples should be placed in a sealed container filled with liquid water (without direct contact with the water) at the same temperature for a minimum of 48 hours This can be achieved using a desiccator by adding water to the bottom of the container typically containing a desiccant For laboratory ambient conditions, samples should be conditioned in ambient air at (23 ± 2) °C and (50 ± 5)% HR for at least 48 hours, with the option of using a humidity-controlled chamber Certain material classes, such as peripheral sealing joints filled with desiccant, may require a longer equilibrium time exceeding 48 hours, necessitating an equilibrium check.
If the stratification of samples is performed on one side of a metal sheet, these samples must be equilibrated for 96 hours instead of 48 hours During conditioning, the samples should not be stacked on top of each other and must include air gaps of at least 2 mm The specimens should be placed in the test setup immediately after being removed from the preconditioning chamber.
When samples are in direct contact with gas-impermeable electrodes, significant variations in water content are not expected during the experiment To minimize this for dry testing conditions, the setup can be placed in a dry environment or a desiccant can be integrated into the assembly Additionally, conducting the experiment in a laboratory or chamber with a relative humidity of 50% effectively addresses issues related to wet samples.
L'épaisseur de chaque éprouvette doit être mesurée après conditionnement, mais avant l'essai L'épaisseur doit être prise comme la moyenne de trois mesurages effectués en différents emplacements sur l'éprouvette.
Conditions d’essai
Température ambiante
Measurements should be conducted at a temperature of T = (23 ± 2) °C The test report must include both the temperature (T) and the relative humidity (RH) For most polymers, water diffusion is insufficient to significantly dry a sample placed between two impermeable electrodes, making it unnecessary to control the atmosphere at this point.
IEC 62788-1-2:2016 specifies the initial relative humidity conditions The annular zone, indicated as zone g in Figure 1, typically accounts for about 15% of the area and has a minor impact on conductivity, potentially leading to a negligible measurement error.
Températures élevées
The variation in resistivity across multiple orders of magnitude within the operating range of a photovoltaic module highlights the importance of its temperature dependence It is advisable to conduct measurements at elevated temperatures of (40 ± 2) °C, (60 ± 2) °C, and (85 ± 2) °C.
Afin d’éviter les complications dues à l’assèchement de l’échantillon au cours de l’essai, il est recommandé d’appliquer, lors du préconditionnement, la méthode “à l’état sec” à la température de l’essai
For measuring transverse resistivity at high temperatures on preconditioned "wet" samples, the samples must be conditioned at the target temperature The testing should be conducted by placing the test setup in a climate chamber, maintaining the same relative humidity as during the preconditioning process.
At high temperatures, certain materials may melt, leading to their adhesion to the test cell and potential contamination Additionally, elevated temperatures can cause flow and unacceptable deformation of materials, thereby affecting the results.
Tension de mesure
Tension pour la méthode A
Mesurer la résistance ( R ) à une température de (23 ± 2) °C, en appliquant une tension ( V ) de
For measurements in direct current, a voltage of 1,000 ± 10 V should be applied If sample thicknesses exceed 0.75 mm or are less than 0.15 mm, the applied voltage must be adjusted to achieve an electric field of 2,000 V/mm during measurement For materials intended for use in higher voltage systems, testing should be conducted at the highest rated voltage It is important to document in the test report any use of voltages other than 1,000 V.
Pour cette méthode, les conditions de tension varient entre une tension positive et une tension nulle Dans le cas présent, Vmax = V et Vmin = 0.
Tension pour la méthode B
Method B employs a voltage of ± (1,000 ± 5) V in direct current for measuring all films The voltage alternates between a positive and a negative polarity Specifically, the maximum voltage is Vmax = +V = + (1,000 ± 5) V, while the minimum voltage is Vmin = –V = – (1,000 ± 5) V.
Cycle de mesure
Cycle pour la méthode A
For Method A, a cycle duration of 1 hour is recommended This 1-hour cycle is intended for the completion of technical data sheets The measurement includes a 'powered' cycle lasting 1 hour at the test voltage (Vmax) of 1,000 V, followed by a 'powered off' cycle of 1 hour at 0 V, and this process is repeated For thicker samples, the voltage is adjusted to ensure an electric field of 2,000 V/mm.
Cycle pour la méthode B
For Method B, a cycle duration of 1 minute should be used, which is intended for quality control or process monitoring In this case, the voltage fluctuates between + (1,000 ± 5) V and – (1,000 ± 5) V.
Résultats
Current measurements are recorded at the end of each cycle The current measured at the end of the first cycle (I1) is discarded, while the subsequent four measurements (I2, I3, I4, I5) are retained To minimize the impact of background currents, a weighted average (IMoy) of the current is calculated.
Given that photovoltaic modules are subjected to prolonged periods of high voltage followed by extended periods of no voltage (such as at night), a method of measuring under voltage and no voltage is employed to generate values that accurately reflect their application usage.
In this case, I2, I3, I4, and I5 represent the sequential current measurements under both energized and de-energized conditions for method A, as well as under energized conditions with alternating polarity for method B The transverse resistivity (ρ) is calculated using the following formula:
Pour la méthode A, Vmax représente l’application de la condition sous tension et Vmin représente l’application de la condition hors tension (0 V dans ce cas) Pour la méthode B,
Vmax represents a positive voltage, while Vmin indicates a negative voltage In this context, t refers to the thickness of the specimen, and A denotes the effective surface area of the electrode, which is the smallest value provided by the measurements.
A=π d + (3) ó g est l’espace pour l’électrode de garde et d1 est le diamètre de l’électrode d’essai, Figure 1
Due to the rejection of the first cycle, the measurement duration should be 5 hours for method A or 5 minutes for method B If there is any uncertainty about the operation of an instrument, it is advisable to use method B while recording the total test time to verify its functionality.
This calculation technique for averaging measurements effectively eliminates a constant background current and is commonly applied automatically in commercial electrometers Method B typically serves as a standardized protocol for these devices In contrast, Method A can be implemented by adjusting the electrometer to measure in alternating polarity mode with an alternating voltage of 500 V and a direct current offset of 500 V, creating a measurement model of 1,000 V/0 V Many commercial instruments automatically discard the initial measurement.
Electrometers commonly use formula (1) to calculate average current When employing the alternating polarity method, the calculation of transverse resistivity relies on the voltage obtained through this method Notably, standard electrometers do not account for the offset voltage in their transverse resistivity calculations, and their formula does not include the factor of ẵ as seen in formula (2) Since Vmax corresponds to double the programmed alternating polarity voltage in the electrometer, which produces values under voltage and no voltage, these factors cancel out, allowing standard electrometers to compute values equivalent to those derived from formula (2).
Lorsque le courant mesuré est inférieur à 2 nA, il peut être nécessaire de désactiver la fonction de commutation automatique de calibre de l'électromètre afin d'obtenir des mesurages valides
If a negative current value is recorded during measurements, except when V = 0, the sample cannot be measured with the instrument's configuration, regardless of the cycle or the instrument's most sensitive current range In such cases, it is essential to separately determine the instrument's limits and report the resistivity as exceeding the instrument's measurement capacity The instrument's resolution for a well-protected setup is established using the lowest current range value and the sample's geometry in formulas (1), (2), and (3) To identify the lowest current range, it is crucial to ensure that environmental noise does not practically limit the instrument setup to higher current ranges.
All measurements (expressed with 2 significant figures) are taken as the average resistivity ρ, in Ω ∙ cm, of 5 samples, along with the standard deviation of the measurements Results equal to or greater than 1.1 × 10¹⁷ Ω ∙ cm must be reported as exceeding 1.1 × 10¹⁷ Ω ∙ cm.
A certified test report must be established by the testing laboratory in accordance with ISO/IEC 17025, including specific material specifications Each certificate or test report should contain essential information such as: a title; the name and address of the testing laboratory and the testing site; a unique identification for the certificate or report; the client's name and address, if applicable; a description and identification of the tested item, including specimen thickness; characterization and condition of the test item, detailing specimen preparation methods, measurement temperature, and relative humidity; the date of receipt and testing; identification of the testing method used; references to sampling procedures, if applicable; any deviations from the testing method; measurements, examinations, and results supported by tables, graphs, sketches, and photographs; for multi-layer specimens, an indication that the measured resistivity is effective volumetric resistivity; an estimated uncertainty statement for test results, if applicable; signatures or equivalent identification of responsible individuals, along with the issuance date; a statement that results pertain only to the tested items; and a statement that the certificate or report must not be reproduced without written consent from the laboratory.
Annexe A (informative) Études menées sur la résistivité transversale des matériaux d'encapsulation
The degradation of photovoltaic modules is partly attributed to electrochemical corrosion, which is influenced by the resistivity of the encapsulation used It is essential for the encapsulation to limit ionic flow, especially in humid conditions, to prevent degradation caused by electrical potential or polarization Historical data on the transverse resistivity of photovoltaic encapsulation materials can be found in references [1] and [2] In photovoltaic applications, increasing the resistivity of encapsulants from \$10^{13} \, \Omega \cdot \text{cm}\$ to approximately \$10^{16} \, \Omega \cdot \text{cm}\$ can significantly mitigate potential-induced degradation issues However, higher resistivities yield only minor or negligible improvements [4].
Annexe B (informative) Exemple de données
Figure B.1 – Exemple de données indiquant le courant et la tension en fonction du temps pour la méthode A de mesure
Tableau B.1 – Mesurages et valeurs du courant de fin de cycle utilisés pour le calcul de la résistivité transversale conformément à la méthode A
Courants mesurés et données calculées
Te ns io n ap pl iq ué e (V )
[1 ] G R Mon and R G Ross, "Electrochemical degradation of amorphous-silicon photovoltaic modules," in Proceedings of the 18th IEEE PV Specialists Conference, Las Vegas, Nevada, USA, 1 985, pp 1 1 42-1 1 49
[2] G Mon, L Wen, J Meyer, R Ross, Jr., and A Nelson, "Electrochemical and galvanic corrosion effects in thin-film photovoltaic modules", in Photovoltaic Specialists Conference, 1988, Conference Record of the Twentieth IEEE, 1 988, pp 1 08-1 1 3 vol.1
[3] Adam Daire, “Improving the Repeatability of Ultra-High Resistance and Resistivity
Measurements”, White paper by Keithley Instruments ,2001
[4] C Reid, S Ferrigan, I Fidalgo, and J T Woods, "Contribution of PV Encapsulant
Composition to Reduction of Potential Induced Degradation of Crystalline Silicon PV Cells," 28 th European Photovoltaic Solar Energy Conference and Exhibition, 201 3
[5] Keithley, Low Level Measurements Handbook: Precision DC Current, Voltage, and