IEC 60904-2:2015 pdf free download – Photovoltaic devices
4.3Additional requirements for reference modulesAdditional requirements apply to reference modules:a)Bypass diodes:
– general reference modules,that are used to measure a range of module types andgeometries,should not contain bypass diodes. The presence or absence of bypassdiodes shall be noted and considered in conjunction with the measurement conditions,in particular spatial non-uniformity of the irradiance on the module during measurement;for reference modules，that are intended to be matched to the module under test,thenumber,type and connection of bypass diodes (if present) shall match those in themodule under test.
b) lf they are made from discrete cells, these shall be matched as follows depending on the
intended use of the reference module:
– if only the short circuit current of the reference module will be used the short circuit current of the individual cells shall be matched to within ±1 %;
– if other parameters (such as maximum power) are used additionally or exclusively,both the short circuit current and the fill factor of the individual cells shall be matchedto within 1 %.
The matching of the individual cells is the responsibility of the manufacturer of thereference module, bearing in mind that matching may also be influenced by encapsulationor lamination. The cell matching needs not be checked by the calibration laboratory.However,if l-V curves of the reference module indicate inconsistent response (i.e. stepsare noted in the l-V curve), the l-V curve should be measured under light that is known tobe uniform (e.g. natural sunlight) to determine whether there is evidence that the cellswithin the module are matched within 1 %. lf the module exhibits evidence of > 1 %mismatch between cells, the module shall not be used as reference module.
4.4Requirements for built-in shunt resistors
The resistor shall be chosen such as to ensure that the reference device operates sufficientlynear to short-circuit condition, meeting the requirement:
The long-term stability of such resistors shall also meet the stability requirements of thereference device.Calibration values of such reference devices shall be measured as thevoltage drop across the shunt resistor and stated with the dimension [V] at standard testconditions (see Clause 7). The temperature coefficient of the built-in shunt resistor is part ofthe temperature coefficient of the calibration value of the reference device.As the uncertaintyin the calibration may be strongly dependent on the shunt resistor stability and temperaturecoefficient, respective values should be provided with the reference cell data sheet.
lf a shunted reference cell is to be used for low irradiance measurements, either a dedicatedcell can be constructed with the restriction of formula (1), where the short circuit current isconsidered at the desired low irradiance rather than at STC.Alternatively a shunted cell canhave a larger shunt resistor，but requires a separate calibration for each irradiance andtemperature it is to be used at.
lt is recommended that the shunt resistor be a removable 4-wire resistor, to allow for periodicchecking of the reference device stability by taking an l-V curve per lEC 60904-1.
Formula (1) means that the measured output voltage of a shunted reference cell shall be lessthan 3 % of its open circuit voltage.For typical crystalline Silicon this equates to about 20 mvoutput.
5 Temperature measurement
Means shall be provided for determining the reference cell temperature or, for referencemodules,the equivalent cell temperature (ECT),according to lEC 60904-5. The requireduncertainty for temperature measurements shall be less than ± 2,0 °C for all referencedevices.A minimum accuracy of ±1,0°C for the temperature sensor is suggested to achievethis uncertainty in the temperature measurement.