BS/EN 62446-1-2016 pdf download.Photovoltaic (PV) systems Requirements for testing, documentation and maintenance Part 1: Grid connected systems — Documentation, commissioning tests and inspection.
7.2.3 I-V curve measurement — Array performance
Given suitable irradiance conditions, an I-V curve test provides a means to measure that the
performance of a PV array Is meeting the rated (nameplate) performance.
PV string and array performance measurements shall be performed at stable irradiance conditions of at least 400 W/m2 as measured in the plane of the array.
NOTE I Poor results may be expected where measurements are taken In low irradiance or where the angle of Incidence Is too oblique.
NOTE 2 The maximum power current and voltage of a PV string are directly affected by irfadiance and temperature, and are Indirectly affected by any changes In the shape of the I-V curve. In general. I-V curve shape varies slightly with irradiance, and below a critical level of irradiance the curve shape changes dramatically. The details of the variation depend on the PV technology and the extent to which module performance has been degraded over time. changes In the shape of the curve can cause errors In evaluating array performance. regardless of the method used to characterize siring performance (I-V curve tracing or separate current and voltage measurements).
The procedure for undertaking the I-V curve test Is as follows:
• Ensure system is shutdown and that no current Is flowing.
• The string under test should be isolated and connected to the I-V curve test device.
• The test instrument should be programmed with the characteristics, type and quantity of modules under test.
• The irradiance meter associated with the I-V curve tester should be mounted such that it matches the plane of the array and checked to ensure it is not subject to any localized shade or reflected light (albedo), Where a reference cell device Is used. this shall be checked to ensure it is of the same cell technology as the array under test, or suitably corrected for the difference in technologies.
• Where the I-V curve tester uses a cell temperature probe, this shall be in firm contact with the rear of the module and in the centre of a cell towards the centre of a module. Where temperature corrections are calculated by the I-V curve test device, a check shall be undertaken to ensure that the correct module characteristics are inputted onto the device and that the string V value is within the range expected.
NOTE 3 A check of is performed to ensure that the string is not missing a module and, to the extent possible, that It has no shorted bypass diodes. Either condition would cause an error in the calculation of temperature from the measured
• Prior to commencing the test, the irradiance levels shall be checked to ensure they are greater than 400 W/m2 in the plane of the array.
On completion of the test, the measured maximum power value should be compared to the rated (nameplate) value of the array under test. The measured value should lie within the stated power tolerance for the modules under test (together with an allowance for the accuracy of the I-V curve test equipment).
7.2.4 I-V curve measurement — Identification of module I array defects or shading issues
The shape of an I-V curve can provide valuable information on the array under test. Defects including the following may be identified:
• Damaged cells I modules.
• Short circuited bypass diodes.
• Local shading.
• Module mismatch.
• The presence of shunt resistance in cells I modules I arrays.
• Excessive series resistance.
If the goal of the I-V curve measurement is to verify that there are no steps or notches of the type caused by mismatch effects, the measurement may be conducted at lower irradiance and greater incident angles than required for performance testing.
For most shape tests, irradiance values should be greater than 100 W1m2. However, useful data may also be obtained at lower irradiance levels. Where shape defects are spotted at irradiance levels of less than 100 W/m2, while it may merit investigation of the potential fault, the test should also be repeated at a time when values over 100 W/m2 are present.
On recording an I-V curve, the shape shall be studied for any deviation from the predicted curve. Deviations to I-V curves demand particular attention as they can signal otherwise undetected and significant faults within the PV array. Information on interpreting deviations to an I-V curve is contained in Annex D.BS/EN 62446-1-2016 pdf download.
BS/EN 62446-1-2016 pdf download
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