Frequently asked questions about curve tracing by means of BENNING PV 2

Installation tester and curve tracer

The BENNING PV 2 provides a highly efficient and effective test and diagnostic solution for PV systems, carrying out all commissioning tests required by IEC 62446 and performing fast and accurate measurement of I-V curves in accordance with IEC 61829.
When used in conjunction with the BENNING SUN 2 irradiance meter, the BENNING PV 2 measurement data can be converted to STC, using either the BENNING PV Link app or BENNING SOLAR Manager PC software, allowing direct comparison with the PV module manufacturer’s published data.

1. What is solar PV I-V curve tracing?

The electrical power produced by a solar PV cell or module is a function of the current (I) and voltage (V) characteristics.
Measuring the relationship between current and voltage whilst varying the electrical load (resistance R) connected to the PV cell or module from short circuit (R → 0) to open circuit (R → ∞) produces a characteristic current vs voltage (I-V) curve as shown below in figure 1.
The intersection point of the characteristic with the voltage axis (x-axis) corresponds to the open-circuit voltage Uoc and the intersection point of the characteristic with the current axis (y-axis) corresponds to the short-circuit current Isc.

Power is the product of voltage and current and so the power vs voltage curve shown in figure 2 can be generated from the measured voltage and current data. The power vs voltage curve shows the point at which the power is a maximum (Pmax). The corresponding maximum power point Mpp is shown on the I-V curve. Loading the PV module such that the current is Impp and voltage is Vmpp will operate the PV module at the maximum power point (Mpp) and result in the maximum power generation.

2. What is STC and why is it significant?

All PV modules have information shown on the rating plate which relates to the module performance, including values for open circuit voltage (Vo/c), short circuit current (Is/c), voltage and current at the maximum power point (Vmpp and Impp) and the maximum power (Pmax).
When PV modules are produced, manufacturers check the performance and determine the rating plate values by carrying out an I-V curve measurement. The voltage generated by a PV module or the current generated by this module depend on the solar irradiance, the spectral composition of the incident solar radiation and the module temperature.
In order to ensure that the performance values are meaningful and comparable, well-known and homogeneously defined framework conditions for measurement are required. Manufacturers measure the rating plate values under standard test conditions (STC) with an irradiance of 1000 W/m2, a module temperature of 25 °C and using an irradiance source equivalent to an air mass AM = 1.5.
If we want to compare performance data measured in the field directly with the rating plate values, the measurements must be converted to STC. The conversion requires measured values for the irradiance level and PV module temperature at the time of measurement.

3. Where should the PV module temperature sensor be positioned?

The standard IEC 61829 Photovoltaic (PV) array - On-site measurement of current-voltage characteristics recommends that a flat thermal sensor with fine leads is mechanically attached directly to the backsheet in the middle of a module. The thermal sensor should be at least 10 cm from any junction box but opposite an active part of the module. The attachment method should not change the temperature of the PV module.

4. Can I use an infrared thermometer to measure the PV module temperature?

The accuracy of any infrared temperature measuring device is influenced by the emissivity (ability to emit infrared energy) of the surface being measured. An infrared thermometer should only be used if it has been calibrated for the PV module backsheet emissivity such that the temperature measurement accuracy meets the ± 1 °C required by IEC 61829.

The electrical output will vary significantly with changes in the level of in-plane irradiance.
Solar PV panel manufacturers quote the electrical output at standard test conditions (STC) with an irradiance level of 1000W/m2. Therefore, when commissioning a PV system, it is necessary to measure the level of irradiance at the same time as testing its electrical output, to know whether it is working to its potential under the existing irradiance levels. If the electrical output is different from the manufacturers’ quoted values, it must be determined whether this is due to a fault in the PV installation, or simply because irradiance was different from STC.
Simultaneous measurement and recording of irradiance, open circuit voltage (Voc) and short circuit current (Isc) is required for the PV Array Test Report for IEC 62446.

6. How is solar irradiance measured?

Solar irradiance metering equipment used during the assessment of PV modules must have a spectral response that matches that of the PV module or system under test.
There are two irradiance measurement methods defined and accepted by international standards covering the performance measurement of PV systems:

Pyranometer
An instrument for measuring the intensity of solar irradiance, normally used to measure global irradiance on a horizontal plane. Pyranometers are generally high precision, high cost instruments using thermal sensors in a glass dome.

PV- Referenzzelle
A small PV cell with a known current vs irradiance characteristic, constructed using the same cell technology as the PV system under test. If they are not constructed using the same cell technology, an estimate of any uncertainty can be made. Reference cells commonly have temperature compensation to ensure that the measurement accuracy is not affected changes in temperature.
Instruments such as light meters, lux meters or devices using photo diode sensors may appear to offer a low cost solution for measuring irradiance however they do not have the same spectral response as a PV module; they do not compensate for temperature and are likely to introduce significant measurement errors if used for solar PV applications. They are not suitable for PV system performance testing and assessment.

7. Where should the solar irradiance meter be positioned?

It is important that the solar irradiance meter is correctly positioned in relation to the PV system under test to ensure that meaningful data is collected. Varying the angle between the reference cell and the sun can cause significant changes in measured irradiance. The irradiance meter must be positioned so that it is in the same plane as the PV module within ± 2°.
The irradiance meter must be positioned so that it does not shade any part of the PV system under test. Accurate and repeatable measurements are best achieved by mechanically securing the irradiance meter to the PV module framework.

8. Can I perform I-V curve measurements at low irradiance levels?

The shape of a I-V characteristic varies depending on the insolation. If the insolation is below a critical value, the change of the curve shape is so drastic that it is impossible to assess the performance of the PV system due to incorrect measurements. In case of low insolation values, poor measuring results are to be expected.
The standard DIN EN 62446 (VDE 0126-23) “Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance” defines that a power characteristic must be measured at stable radiation conditions of at least 400 W/m².
The standard DIN EN 61829 (VDE 0126-24) “Photovoltaic (PV) array – On-site measurement of current-voltage characteristics” recommends stable radiation conditions of at least 700 W/m² for on-site measurements that are converted to STC conditions.

9. What happens if the irradiance changes during an I-V curve measurement?

Ideally, I-V curve measurements should be made when there is a clear sky and little wind. Changes in irradiance will produce variations in PV module temperature that can affect the accuracy of I-V curve measurements. If the irradiance has increased significantly immediately before measurements are made, the PV module temperature may not have stabilised. Changes in irradiance during an I-V curve measurement can influence the shape of the I-V curve.
However, the reality is that time and contractual constraints limit the periods in which it is possible to perform a test. I-V curve measurements should be made under the most possible stable conditions. Variations in irradiance and module temperature should be recorded. If the changes in irradiance alter the shape of the I-V curve the measurement should be repeated.

10. What are the benefits of I-V curve tracing?

Power is a function of current and voltage. Anything that reduces the current or voltage generated by a PV system will reduce the power that is produced. The shape or profile of the I-V curve therefore provides a highly effective visual indication of the performance of a PV module or string.
Real-time power monitoring systems provide an indication of the actual power generated however, if the actual yield is less that the design figure, they provide no information. Other measurement tools are required to identify the root cause of an under-performing PV system.
If, during the test, the I-V characteristic of the installed PV string shows a significant change of the angle of inclination, bumps or jumps in the curve shape, this indicates errors within one or more individual modules.
Thus, the documentation and evaluation of the I-V characteristic serves as an indispensable evidence during installation, commissioning or as part of a periodic inspection to ensure perfect condition of the modules and compliance with the manufacturer’s specifications.

11. Why is I-V curve tracing important?

The I-V curve provides a quick and effective means of accessing the true performance of solar PV modules or strings. In a correctly performing PV system the shape of the curve should follow the normal profile and the measured values of Is/c, Impp, Vo/c, Vmpp and Pmax should be as expected for the environmental conditions at the time of measurement.
As part of the manufacturing process, modules are tested under standard conditions (STC) at an irradiance of 1000 W/m2, a temperature of 25 °C and air mass of 1.5. Measurements of irradiance and temperature captured at the same time as the I-V curve data can be used to convert field I-V curve measurements to STC. Corrected measurements can then be used for a direct comparison with the rating plate figures.
At the factory stage, the testing is used to identify any manufacturing problems and also to verify the power rating of a particular module for inclusion in product datasheets and specifications.
Once a module or string has been installed on site, I-V curve tracing can be carried out to create an operational I-V curve to confirm that the actual power output is close to the predicted value of the new system.
If there is a discrepancy, analysis of the I-V curve shape can be used to help identify the root cause for the under-performance and remedial measures can be implemented.

12. Is I-V curve tracing only useful during installation?

No, over time, periodic I-V curve tracing is a highly effective tool to check for deterioration in performance of the system. It can be used to identify and locate module or wiring issues and compare power generation performance against previous performance data or product warranty data. I-V curve measurements can also highlight the effect of partial or uniform shading and demonstrate the improvement in performance after module cleaning.

13. What is the PV Fill Factor?

Fill factor (FF) is the ratio of the actual maximum obtainable power, represented by the dark blue box, to the product of short circuit current Is/c and open circuit voltage Vo/c, represented by the light blue box.

The Fill Factor is essentially a measure of the efficiency of a PV module, the theoretical maximum value depending on factors such as the type of silicon used to construct the module. However, deviation from the expected value or changes in Fill Factor can provide an indication that a fault is present.

14. How does temperature and irradiance affect I-V curves?

There are various factors that can influence the performance of solar PV modules, including temperature and irradiance.
The open circuit voltage of a PV module varies with cell temperature. As the temperature increases, due to environmental changes or heat generated by internal power dissipation during energy production, the open circuit voltage (Voc) decreases. This in turn reduces the power output. The design of a solar PV system must take into account the PV module temperature coefficient, comparing the expected average cell temperature in its operational environment, against the STC data used to calculate the module output.
In the same way, irradiance will also affect module performance, with a reduction of sunlight resulting primarily in a reduction in current and consequentially a reduced power output.

When comparing I-V curves measured in the field with predicted profiles, full consideration of these factors needs to be taken into account if the comparison is to produce meaningful results.

15. What are the typical problems that can be identified by I-V curve tracing?

Differences in the curve shape and the Fill Factors associated with a particular module or sting can indicate a problem in the quality, power performance or correct installation of the solar PV system.
Typical problems encountered with an installation that will have an effect on the expected I-V curve could include soiling, shading of the modules, high resistance wiring or connection problems between modules, a mismatch of modules caused by manufacturing or specification differences or the PV cells are damaged.

The I-V curve tracer can be used to as a diagnostic tool to identify, locate and rectify any of these problems.

16. How do I-V curve tracers work?

I-V curve tracers are specialist types of test equipment that sweep an electrical load connected to the solar PV module or string and measure both the current and voltage at multiple points during the sweep. The pairs of current and voltage values are then used to plot an I-V curve directly or to calculate and plot a P-V curve.
To overcome the influence of external factors such as temperature and irradiation, I-V curve tracers typically measure and record irradiance and temperature to allow measurements to be converted to STC to enable an accurate comparison to be made with the PV module specification.
Some I-V tracers also incorporate databases of PV module data to enable measured values in the field to be immediately compared with the values declared by the manufacturer as a means of verifying the fitness for purpose and performance of installed systems.

17. How does I-V curve tracing rank alongside other solar PV commissioning and periodic tests?

The installation of solar PV systems involves not only performance and efficiency tests at the time of installation, but also checks to ensure that the system has been wired correctly and safely. These should be repeated periodically to determine ongoing safety and performance.
IEC 62446 Grid connected PV systems' defines the minimum requirements for solar PV system documentation, commissioning tests and inspection.
In summary, the standard sets out the testing, information and documentation that should be provided to the customer following the installation of a solar module system and also the initial (and periodic) electrical inspection and testing required.
The absolute minimum testing that needs to be undertaken during commissioning involves continuity measurements, open circuit voltage, short circuit current, insulation and irradiance. I-V curve tracing can also be added to this list to assess the performance characteristics of the installed system.

18. Why is it important to test PV installations?

The infographic below illustrates 10 reasons why you should carry out regular tests on PV installations.

Damage or corrosion to cabling and connectors caused by moisture can decrease performance or increase the risk of fire.

2. Damage to wiring
Wires hanging below the panels or touching the roof/ vegetation can become damaged, presenting a shock hazard.

3. Surface contamination and damage
PV modules can become dirty over time, and can be damaged by the elements or stones dropped by birds, resulting in decreased output.

4. Verification of system performance
It is important to identify electrical faults or wiring failures as early as possible. Periodic electrical testing verifies performance over extended periods.

5. Avoid fire risks
Fires started by electrical faults in rooftop PV systems have been reported worldwide. Regular testing of PV system cabling and components reduces the potential risk of fire.

6. Warranty fulfilment
Testing to identify and confirm continued safe operation and optimum energy output can be required by product warranties and component guarantees.

7. Ground faults
Poor insulation in underground cabling can cause electricity to leak to earth. This can significantly reduce the efficiency of the system.

8. Customer documentation
Copies of all test and commissioning data should be provided to the customer as part of the system documentation when a PV system is installed.

9. Effective grounding
If the grounding system degrades over time there is a chance of electric shock.

10. Compliance with IEC 62446
The international IEC 62446 standard recommends that periodic verification of an existing PV installation should be performed.

19. Is periodic inspection and testing of PV installations necessary? If so, why?

The verification of system performance and energy output from the panels is particularly important and a major reason why periodic verification and testing of the system can also be very important – as well as being essential to comply with warranty and PV system guarantees.
Undetected faults may also develop into a fire hazard over time. Without fuse protection against such faults, elimination of a fire risk can only be achieved by both good system design‚ and careful installation with appropriate inspection and testing.

20. What equipment is required for testing a solar PV installation?

The absolute minimum testing that needs to be undertaken involves continuity measurements, open circuit voltage, short circuit current, insulation and irradiance.
Other tests involving the use of I-V curve tracers, power analysers and thermal imaging cameras are not mandatory but may be regarded as useful to carry out certain diagnostic testing or to assess different performance parameters of the solar PV system.
It is therefore largely up to the installer to decide whether he wishes to purchase individual items of equipment or select one or possibly two instruments which provide a combination of tests to enable measurements to be taken in a fast, safe and efficient fashion.

21. What are the key considerations that should be addressed prior to selecting a piece of test equipment?

There are many instruments available on the market that are sold under the title of Solar Testing so it is vital to ensure that the instruments selected are capable of performing all of the tests required by the IEC 62446 standard.
The nature of PV testing is such that it can expose the installer to high voltages, so the selection of an instrument which is capable of automatically and safely performing tests greatly improves efficiency and safeguards the installer.
The availability of new multi-purpose solar PV test instrumentation also means that the functions of a number of individual test instruments can be combined in a single tool – with consequent savings in cost and improved practical considerations.

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