Once the installation stage of the photovoltaic systems is complete, the project commissioning phase begins. At this stage, the system's compliance with regulatory standards, especially ABNT NBR 16274:2014, is inspected and verified, in addition to good engineering practices. The aforementioned standard has three test branches:
- The first, which governs a detailed visual inspection of compliance with the project and good practices;
- The second, which employs electrical tests to ensure that the system operates safely and;
- The third, which presents performance tests.
Due to the fact that performance testing equipment is relatively expensive, it is not unanimous among professionals, which makes it impossible for data collected in the field to be properly processed and analyzed as required by the manual. Without a temperature and irradiance sensor, data measurement and analysis becomes more empirical and less accurate.
The minimum instrument required for measurements during the commissioning stage of a photovoltaic system is a multimeter to measure the open-circuit voltage and short-circuit current of the photovoltaic modules or strings. It is possible to obtain valuable information even with such simple equipment.
However, the data provided by photovoltaic module manufacturers are based on pre-established climatic conditions (STC or NOCT, as we will explain below). Therefore, the measured voltage and current variables need to be adjusted or normalized to the same conditions specified in the data sheets. The standard test condition (STC), in which the electrical parameters of the modules are presented in the data sheets, corresponds to a cell temperature of 25 °C and irradiance of 1000 W/m2.
Since the STC condition is not achieved in practice, manufacturers occasionally provide another option to provide a different perspective on the electrical parameters of the modules. The NOCT (nominal operating cell temperature) condition corresponds to an ambient temperature of 20 °C, with irradiance of 800 W/m2 and wind speed of 1 m/s.
Since the main parameters of photovoltaic modules depend on the irradiance and operating temperature of the cells, there is an undoubted need for sensors capable of measuring these two variables. This article will demonstrate a practical example of normalizing the open-circuit voltage and short-circuit current of a photovoltaic module, enabling an effective comparison with the data in the datasheet provided by the manufacturer.
Experiment carried out in the field
We will be based on a practical experiment to demonstrate the procedures for adjusting the open circuit voltage (VOC) and short circuit current (ISC) of a photovoltaic module, based on the adjustment of the cell temperature and irradiance measured in the local. The following instruments were used in this experiment:
- Irradiance and temperature sensors coupled to a display device;
- 320 Wp polycrystalline silicon module, positioned on a roof with a 5º slope;
- A conventional multimeter.
In order to measure the climatic conditions in a situation closer to the NOCT condition, it was decided to perform the tests at a time around noon, when the irradiance and ambient temperature are higher. The ambient temperature at the time of the tests was 35 °C. The fact that the experiment was performed at a time of high irradiance is also useful to improve the analysis of the open circuit voltage.
The open-circuit voltage of the photovoltaic module is directly dependent on temperature and is practically unaffected by irradiance, provided that the irradiance level is high. In other words, the adjustment of the open-circuit voltage to one of the standard conditions (STC or NOCT) can be done only as a function of the temperature variable.
In the assembly of this experiment, the temperature sensor was installed in with the central region of the back of the module (backsheet). The irradiance sensor was installed coplanarly to the surface of the photovoltaic module, as shown in Figure 1. Finally, the sensors are connected to the display terminals, enabling the visualization of the parameters during the tests.
Once the sensors were installed, the multimeter test leads were connected to the open circuit terminals of the photovoltaic module. The photographic record of the results collected in the field is shown in Figure 2 below.
Figure 3 shows the module short-circuit current being measured simultaneously with temperature and solar irradiance. This measure must be carried out cautiously, as closing and opening the circuit can cause a dangerous electrical arc. The connection of the photovoltaic module in series with the multimeter and the closing and subsequent opening of the circuit must be done with the aid of a disconnector switch. Direct current photovoltaic circuits must never be sectioned under load (i.e., with the presence of electric current) without a sectioning device capable of extinguishing the electric arc.
STC and NOCT characteristics from the datasheet
To compare the measured results with the characteristics specified by the manufacturer, we need to look for the electrical parameters in the tables presented in the data sheet. In this example, the 320P6K-36 module was evaluated. The parameters we look for are the open circuit voltage (VOC) in STC and NOCT and the thermal coefficients of current and voltage in relation to temperature, normally expressed in %/oC. The %/oC unit indicates that the thermal coefficient determines the percentage variation in voltage or current (in relation to the nominal value in STC) as a function of temperature variation. As can be seen in the data sheet, the current coefficient is positive and the voltage coefficient is negative. A negative coefficient indicates that an increase in temperature causes a reduction in the open-circuit voltage of the photovoltaic module.
From the tables we can extract the following parameters of the 320P6K-36 module:
- Open circuit voltage at STC: VOC,STC = 46,39 V;
- Open circuit voltage in NOCT: VOC,NOCT = 42,8 V;
- Short-circuit current in STC: ISC,STC = 9,15 A;
- Short-circuit current in NOCT: ISC,NOCT = 7,42 A;
- Current thermal coefficient ISC: Alpha = 0,07 %/oC;
- VOC voltage thermal coefficient: Beta = -0,31 %/oC.
Open circuit voltage adjustment
After measuring the open circuit voltage, cell temperature and solar irradiance variables, our goal will be to analyze the voltage value and compare it with the value shown in the module data sheet. In order for this comparison to be possible, it is necessary to adjust the value in relation to the temperature and irradiance, to obtain a normalized voltage value in the STC condition (25 oC and 1000 W/m2).
Alternatively, we can also use the module parameters in NOCT as a basis for our analysis. In other words, we can choose to evaluate the module using its NOCT condition as reported in the data sheet. In this case, we will adjust the VOC value measured under the experimental conditions to the NOCT condition, and then compare the result of this adjustment with the VOC,NOCT value (open circuit voltage in the NOCT condition) presented in the data sheet.
In the module used in this experiment, we found the open circuit voltage VOC,NOCT = 42,8 V and the thermal coefficient Beta equal to -0,31%/ºC, which expresses the variation of the open circuit voltage as a function of temperature. We will show how the adjustment of the measured open circuit voltage is done. With this procedure, we will be able to compare the measured voltage with the voltage presented in the manufacturer's data sheet.
Open circuit voltage adjustment for NOCT condition
According to the manufacturer's datasheet, the cell's operating temperature under NOCT conditions is 45 ± 2 ºC. Therefore, we must take this value as a reference to adjust our data. According to the temperature sensor, we have a temperature of 61,3 ºC in the experiment, a value 16,3 ºC above the NOCT standard. The temperature difference must be multiplied by the Beta coefficient, resulting in a correction factor that expresses the percentage variation of the open circuit voltage in relation to the NOCT value. The normalized open circuit voltage is obtained as follows:
DT = TEXPERIMENTAL – TNOCT
fc = Beta x DT
VOC,ADJUSTED = VOC,EXPERIMENTAL – fc * VOC,NOCT
Where:
- DT = temperature interval between the experimental value and the reference value (NOCT) [ºC];
- fc = correction factor calculated over the temperature range;
- Beta = thermal stress coefficient [%/ºC];
- VOC,EXPERIMENTAL = VOC measured at experimental temperature [V];
- VOC,NOCT = VOC measured at reference temperature [V];
- VOC,ADJUSTED = VOC adjusted for the NOCT [V] condition.
Numerical example, based on the experiment carried out:
DT = 61,3 ºC – 45 ºC = 16,3 ºC
fc = -0,31 x 16,3 = -5,053 %/ºC
fc * VOC,NOCT = -5,053/100 x 42,8 V = -2,16 V
VOC,SET = 40,19 V – (-2,16 V) = 42,35 V
By numerically comparing the values of the VOC,ADJUSTED voltage (42,35 V) with the VOC,NOCT voltage from the catalog (42,8 V), we can see that the values are very close. This allows us to confirm that the photovoltaic module is intact from the point of view of the open circuit voltage. This procedure of measuring and adjusting by temperature becomes more important the greater the number of modules connected in series.
The professional, when measuring the voltage of a string with numerous modules, is not sure of measuring the correct value except by adjusting the measured variable in relation to a reference condition (STC or NOCT), so that the measured value can be compared with the values found in the product data sheet.
Setting the short-circuit current for STC condition
The short-circuit current (ISC) treatment process follows a different methodology. In this case, the adjustment of the measured variable is based on two variables: temperature and irradiance. In the previous case, we disregard the effect of irradiance, since for a high irradiance value its effect on the open-circuit voltage is negligible, and this variable depends directly on the temperature.
In the case of short-circuit current, on the other hand, the main influencing variable is irradiance, together with a minor effect of temperature. For our calculations, we must employ the thermal coefficient Alpha of the short-circuit current, which can also be found in the datasheet. We have the following equation to fit the data collected under experimental conditions to the STC condition [1]:
ISC,ADJUSTED = ISC,EXPERIMENTAL * [ 1 – Alpha x (TSTC – TEXPERIMENTAL) ] * IrradSTC/IrradEXPERIMENTAL
Where:
- ISC,SET = current adjusted for the STC condition [A];
- ISC,EXPERIMENTAL = experimentally measured current [A];
- Alpha = thermal coefficient of the short-circuit current [%/oC];
- TSTC = STC temperature (25 oC);
- TEXPERIMENTAL = experimental temperature [oC];
- IrradSTC = irradiance in STC (1000 W/m2);
- IrradEXPERIMENTAL = experimental irradiance [W/m2].
Numerically, we have:
ISC,ADJUSTED = 8,089 * [ 1 – 0,07/100 x (61,7 – 25) ] * 1000/903 = 9,19 A
Numerically comparing the ISC,ADJUSTED values (9,19 A) with the STC value obtained from the catalog (9,15 A), the test presented an error of 0,04% in relation to the data provided by the manufacturer. The values are very close, showing that the tested module presents a short-circuit current compatible with the manufacturer's specification.
Conclusion
Photovoltaic commissioning tests are essential to validate the correct functioning of a system. The NBR 116274:2014 standard describes several categories of tests. Open-circuit voltage and short-circuit current tests are part of category 1, which are the minimum requirements of the standard. To correctly evaluate a photovoltaic module – or a string of modules – it is necessary to adjust the measured voltage and current values according to the temperature and solar irradiance, which must be measured simultaneously.
In this article we show how to adjust the open circuit voltage for the NOCT temperature and the short circuit current for the STC condition. In both cases, the results obtained experimentally were very close to the values obtained in the module manufacturer's datasheet.
References
- [1] Roy, J., Bliss, M., Betts, T. R., & Gottschalg, R. (2010). Effect of IR translations of irradiance-temperature on the energy yield prediction of PV module and spectral changes over irradiance and temperature. Proceedings of the 6th Photovoltaic Science Applications and Technology Conference (PVSAT-6), 149–152. https://dspace.lboro.ac.uk/
- [2] ABNT NBR 16274:2014 – Grid-connected photovoltaic systems – Minimum requirements for documentation, commissioning tests, inspection and performance evaluation
An answer
Dear Elson, thank you very much for sharing this information. The method you propose to be applied in commissioning is quite practical to be operationalized. To make it perfect, I suggest you review the last equation: ADJUSTED ISC. The temperatures T_EXPERIMENTAL and T_STC are in the wrong positions in the equation. The result is correct, but in the equation – probably an error when editing – they are in the wrong positions. Thank you.