Measurement of the I-V characteristics of PV panels using the Sonel PVM-1530 Max in accordance with IEC 61829
The measurement of current-voltage (I-V) characteristics of photovoltaic panels plays a key role in evaluating system efficiency and detecting potential issues within a PV installation. Learn how to perform accurate measurements in compliance with IEC 61829 using the advanced Sonel PVM-1530 Max meter. Discover the impact of environmental factors such as irradiance and temperature on the measurement results, as well as the importance of correcting these to standard test conditions (STC).
I-V characteristic – why is it so important?
One of the core features of the Sonel PVM-1530 meter is the measurement of I-V curves. The I-V characteristic is a graph that presents the panel’s power output as a function of current and voltage. According to the IEC 62446-1 standard, this is one of the two measurements defined in the category 2 tests. I-V curves are very useful in evaluating PV installations in terms of performance and potential issues. The meter records the relationship between current and voltage by adjusting the load from open-circuit voltage (Uoc) to short-circuit current (Isc).
To understand what a current-voltage characteristic is, a typical I-V curve is shown below.
Figure 1. Typical I-V characteristic
Although seemingly simple, this type of measurement can often yield results that differ significantly from expected values. This article describes how to perform these measurements using the Sonel PVM-1530 Max so that the results accurately reflect the actual condition of the tested PV panels, based on the IEC standard 61829:2015 Photovoltaic (PV) array – On-site measurement of current-voltage characteristics.
Photo 1. Sonel PVM-1530 Max meter set
Impact of irradiance and temperature on measurement results
The Sonel PVM-1530 is capable of measuring connected panels and generating an I-V curve within a few seconds. However, just connecting the panels to the meter is not enough, as such a curve is usually difficult to compare with the reference curve provided by the PV panel manufacturer under STC.
Figure 2. I-V and P-V curves displayed on the Sonel PVM-1530 meter screen
STC conditions assume:
- irradiance E = 1000 W/m²
- PV module temperature TPV = 25°
- air mass AM = 1.5
In practice, it is rare for field measurements to be conducted under STC conditions. That is why it is necessary to correct measured values to STC to assess whether the I-V curve is correct. This requires recording irradiance and temperature so the meter can recalculate the results accordingly. Only then can the results be reliably compared to the manufacturer’s data.
Why are irradiance and temperature measurements so important?
Current and voltage values generated by PV panels are highly dependent on these parameters, which in turn directly affects power output. Figure 3 shows how irradiance impacts generation. As irradiance increases, current and power increase linearly (assuming constant panel temperature), while voltage changes only slightly. For this reason, irradiance should be measured using the external Sonel IRM-1 sensor at the same tilt angle and orientation as the panel string being tested.
Figure 3. Impact of irradiance on the I-V characteristic
IEC 61829 – the PV measurement standard
IEC 61829 specifies that to perform I-V curve measurements with STC correction, irradiance should be at least E = 700 W/m². The Sonel PVM-1530 checks the irradiance value using the primary irradiance sensor (IRM-1) before starting the I-V curve measurement. If the value is too low, the device will display a warning (see figure 4).
Figure 4. Warning about low irradiance level
Because a string can contain dozens of panels installed over a wide area, it is important that all of the panels are mounted in the same direction, on a level surface, and exposed to a uniform level of sunlight. If a PV installation consists of strings with different tilt angles, orientations, technologies, or electrical configurations, the measurements should be performed separately for each string.
Two irradiance sensors – the key to compliance
The IEC 61829 standard requires irradiance to be measured in at least two points along the string, and the difference between them should not exceed 2%. This is why the PVM-1530 Max comes equipped with two Sonel IRM-1 irradiance sensors as standard—one main and one auxiliary.
Photo 2. Two irradiance sensors at both ends of the PV string
To comply with the standard, the irradiance should be checked at both ends of the string. If the PVM-1530 detects a difference greater than 2%, it will display a warning (see figure 5).
Figure 5. Warning about excessive irradiance difference between sensors
It is crucial that the Sonel PVM-1530 measures and records irradiance values from both sensors during the entire I-V curve scan (100 points). These values are displayed as graphs on the screen, allowing the user to verify the stability of lighting conditions during the measurement.
Figure 6. Irradiance and temperature graphs during I-V curve measurement using both irradiance sensors
Measurement stability – how to avoid errors
As explained earlier, controlling irradiance stability is essential—but temperature stability is also important. Large differences in panel temperature across the string can cause unreliable I-V results. While temperature does not impact power output as significantly as irradiance, it must still not be ignored. As the panel temperature increases, the current rises slightly while the voltage drops more significantly, resulting in reduced power. These relationships are illustrated in figure 7.
Figure 7. Impact of temperature on the I-V characteristic
According to IEC 61829, the difference in temperature between panels in the measured string should not exceed 1°C. Once again, the Sonel PVM-1530 uses both sensors to monitor this parameter. If a greater difference than 1°C is detected, the meter displays a warning (see figure 8). During the I-V measurement, temperature from both sensors is recorded and presented in graphical form, just like the irradiance data (see figure 6).
Figure 8. Warning about excessive temperature difference
How to ensure the reliability of I-V measurements
In summary, it must always be kept in mind that I-V measurements can produce results that differ from expectations. However, these results should indicate an actual issue within the PV system rather than a mistake in the measurement procedure.
All the functions of the meter described in this article are designed to ensure that the measurements taken with the Sonel PVM-1530 Max and two IRM-1 sensors are reliable and provide the highest possible accuracy, while minimising the risk of user error. The user can then focus on interpreting the results, without having to worry about correctly setting up the meter or the weather conditions affecting the I-V curve.
It is also worth mentioning that the Sonel PVM-1530 Max is ready to make measurements according to IEC 61829 immediately straight out the box once paired with the sensors. Other versions of the meter (Sonel PVM-1530 and Sonel PVM-1530 Pro) offer the same functionality once the necessary IRM-1 sensors are added.
References:
1. IEC 61829:2015 Photovoltaic (PV) array – On-site measurement of current-voltage characteristics.
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