IV testing and its significance

Sep 7

With the increasing fame of solar energy worldwide and reducing rates of system installation, it becomes crucial to understand the quality of modules with other BoS components installed in the plant. Various on-site tests are performed periodically or on a requirement basis to assess the quality. One such testing is IV testing which assesses strings and modules degradation rates.

IV curve of a PV string (or module) shows the relationship between the output voltage and current at the operating temperature and irradiance conditions.

Two types of system voltage are common in solar power plants - 1000 V and 1500 V. This system voltage limits the number of modules that can be connected in series by limiting the maximum operating voltage for the system. For a 72 module configuration, typically 20 or 30 modules are connected in series for 1000 and 1500 volts system respectively but can vary with design.

Technical understanding

Current-voltage (I-V) curve shows the relationship between output voltage and current of a pv module/string. Maximum voltage is obtained when a module/string is in open-circuit and no current is flowing. This is called the open-circuit voltage (Voc). Ideally, infinite resistance is considered during this condition, therefore, no current flows through the circuit.

Conversely, the pv device (module/ string) produces maximum current when there is no resistance in the circuit, i.e., there is a short-circuit between the positive and negative terminals. The maximum current is known as short-circuit current (Isc). When the module is shorted, the circuit voltage is ideally zero. These extreme conditions in load resistance along with the range of conditions in between them represents an IV curve. Current, in amps (A) and voltage, in volts (V) is depicted on the y-axis and x-axis respectively.

Figure 1: A typical example of an IV curve

Power available from a photovoltaic device at any point in the curve is the product of current and voltage at that point and is expressed in Watts. From the above figure, we can conclude that the power will be zero at two point:

At the short-circuit current point, since the voltage is zero
At the open-circuit voltage point, since the current is zero

The highest power corresponds to the maximum power point of the IV curve.

Significance

As the PV arrays are exposed to environment and other on-site factors, they become susceptible to various degradation mechanisms like module cracks, snail trails, thermal failure, PID, soiling losses, etc., leading to their under performance. Since IV curve tracers captures all the operating points of current and voltage of PV source, they quantify and indicate if there is any degradation in the modules and/or strings.

But how? When a measured IV curve differs significantly in height, width or shape from the predicted IV curve (based on the module datasheet) after adjusting it to STC conditions of irradiance and temperature, the nature of deviations provides clues about the possible performance setbacks. Additionally, EL Imaging helps in the detection of internal defects in the module. IV measurement can be conducted in the following ways:

In a stationary lab
In a mobile lab
On site with a portable IV tracer tool

In case of a stationary or a mobile lab, modules are unmounted from the structures and are sent for testing in a controlled setting. In the case of on-site IV testing, testing can be conducted while modules still mounted on the structures. It is a relatively cheaper and faster alternative to lab testing with satisfactory qualitative results. On-site testing is less accurate as the outside conditions cannot be controlled but it is a very good indicator of module performance when coupled with EL and/or thermography.

IV measurement on the site

Conducting IV measurements on-site has its own set of challenges. Environmental parameters like temperature and irradiation directly impacts the power output of the modules. Fluctuations in these parameters during IV measurements can cause transient errors, therefore, IV measurements on the site follow a systematic procedure to reduce/ nullify the environmental variations as much as possible.

Once the measurements are taken on the site, the data is translated to STC conditions and are then analyzed. The STC translation is required to translate the IV data to STC condition from the operating conditions so that the current power of the module can be compared with datasheet values measured at STC condition. This helps in obtaining the degradation value.

For string IV measurements: It is critical to identify the system voltage to conduct the IV measurement on the strings. Since the maximum design string voltage can change with system voltage, the number of modules connected in series will also change with the system voltage. Therefore, tool specifications also change accordingly. These measurements can be taken either near the table at the string connectors or at the SCB level itself. For both the cases, measured strings should be isolated from the remaining strings of the SCB before taking the measurement. This is necessary from the safety point of view.

The standards used for IV measurements on-site are IEC 60904-1 (Measurement of IV characteristics of PV devices in natural or simulated sunlight), IEC 60904-3 (Basic measurement principles to determine electrical output of PV devices), IEC 60891 (Temperature and irradiance corrections to the measured IV characteristics of PV devices), IEC 61829-2015 (On-site measurement of flat-plate PV array characteristics, the accompanying meteorological conditions, and use of this data for STC translation)

Pros

Cost-effective to identify the possible areas of issues
Faster
Less manpower support is required since the modules need not be unmounted and mounted back
Since no mounting is needed, no chance of damage being introduced into the modules during the measurements.

Cons

Radiation and temperature dependent, therefore, highly environment sensitive
Gives a relative understanding of possible issues in the modules
Measurement tolerance is higher than the mobile or stationary lab measurements
Due to higher measurement tolerances, on-site testing might not be sufficient during warranty claims.
Module cleaning is a challenge since modules are mounted.

Inspite of all the above challenges, IV measurements are equipped to provide insightful understanding of the plant’s current degradation. This degradation rate needs to be perceived carefully since IV measurements on the site are weather dependent. Some of the major factors colluding the IV results are as follows:

Cloudy weather
Low irradiation during testing
Smoke or other particles in the air due to nearby industry
Soiling levels on the modules

PV Diagnostics has analyzed more than 4,00,000 IV curves till date which gives us an in-depth understanding on the interpretation of IV curve results. As a part of a project with one of the developers, IV testing was done for modules and strings. Other tests included thermal imaging and EL testing. The same study was replicated to their other plant as well. Our client was able to get a warranty claim of approx. 1 Crore with the help of our exhaustive reports. For this study, a sample diagnostics study was conducted which was followed by a detailed diagnostics study. After conducting IV measurements, we observed that 80% modules had degradation of over 7.5% (threshold expected degradation) while ~60% modules had degradation of over 10% (threshold value of stepwise performance warranty clause). EL and thermal testing further identified the reasons for degradation rates which were severe solder contact failures causing hotspots and corrosion. As these defects are directly related to manufacturing and material quality, our client was successfully able to claim the module warranty.

Image 1: Hotspot due to solder contact failure

Image 3: Hotspot due to solder contact corrosion at bus-bar and the corresponding failure

IV and EL Correlation

While IV gives quantitative understanding of modules performance, EL gives an internal structure level image of silicon material of the suspected defects in the module. This makes them a perfect duo for module quality inspection. Few module defects and their correlations are shared below.

1. Potential Induced Degradation (PID): Higher PID means high number of cells affected due to PID. In the EL image 5 shown below, the affected cells look dark since they are shunted. Due to the shunt path, less current is available for external load, therefore, less conversion to the useful energy. Shunt resistance impacts Vmp and Imp of the module, thereby, reducing the maximum power. More severe PID implies greater reduction in maximum power of the module.

Image 6: IV curve of the affected module

2. Multiple cracks: Bypass diode gets activated during IV measurement if any string has a current value different from other strings by a certain minimum value (generally, there are three diodes inside the PV module). These diodes can be activated during any internal cell issue that affects Isc or Im of the cell or obstacles from the outside like shadow or uneven dust deposition on a pv module glass. This bypass diode activation is evident in the image 8 shown below in the form of steps called kinks, identifying inherent defects affecting Isc or Im.

Image 8: IV curve of the defective module

3. Diode failure: In the case of diode failure, the related module’s sub-string gets shunted and no current passes from that substring. As a result, Voc also shifts in the IV curve. The maximum power of the module decreases by at least 1/3rd of the total Pmax of the module.

Image 10: IV curve of the defective module

4. Solder failure: It is a manufacturing defect which leads to the increment in series resistance of the module. This increment in series resistance impacts the Vmp of the PV module and hence reduces the maximum power. Reasons for increment in series resistance are corrosion, loose connections, bad connector quality, moisture ingression, etc.

Image 12: IV curve of the defective module

To conclude, IV measurements are a good indicator of a plant's condition but club it with EL testing and thermal Imaging for modules and you can get a more comprehensive picture of condition of modules in the plant.

Ayush Mahajan