IEC 61829-2015 pdf Photovoltaic (PV) array – On-site measurement of current-voltage characteristics

5 Measurement procedure 5.1 Choose and record appropriate conditions for measurement The ideal conditions for an outdoor I-V curve test are clear skies (no clouds and no fog) and little wind. Variable irradiance and wind both introduce temperature transients in the array that confound the accuracy of the measurements. In practice, time and contractual constraints limit the periods in which it is possible to perform a test. Therefore, it is the responsibility of the person(s) conducting the test to ensure that all tests are performed under the most stable conditions possible, and that special attention is given to noting variable irradiance, wind, and array temperatures. For example, even though irradiance during the course of the I-V measurement does not vary more than 2 %, it may be that the irradiance increased by 30 % in the 5 min leading up to the test, and that the array temperature may not have equilibrated before the test was run. Record the weather conditions, qualitatively, and periodically note when and how conditions change over the period during which I-V curves are being taken. It is recommended to take a picture of the sky and record the time periodically. NOTE This information is for identifying potentially erroneous data, and is not directly used in the analysis. 5.2 Clean the modules The cleanliness of the module surfaces shall be consistent with the intent of the test. The state of cleanliness, whether or not cleaning has been attempted, shall be reported. If the intent of the test is to detect any possible differences between fielded modules and laboratory or factory measurements, either a) the array shall be cleaned thoroughly immediately prior to the measurement, or b) a representative string shall be tested immediately prior to and immediately after a thorough cleaning. The level of soil on the array is determined by comparing the results of the string I-V test before and after the cleaning.