Comparing I-V Curve Tracing to Open Circuit Voltage and Short Circuit Current Testing of Solar Modules

By Will White, Fluke Solar Product Application Specialist

In the field of photovoltaic (PV) module testing, two common methods are used to assess the performance and health of solar panels: I-V curve tracing and open circuit voltage (Voc)/short circuit current (Isc) testing. While both methods provide valuable insights, they differ significantly in their diagnostic capabilities and the depth of information they offer.

Technician using the Fluke PVA-1500 Series PV Analyzer to perform detailed I-V curve tracing in a solar plant, enabling precise diagnostics and optimization of PV module performance.

Sometimes operating current (Iop) testing is used during PV analysis as well. (Iop is the string's operating current when the PV system is producing power normally.) For the purposes of this article, we’ll discuss the two main types of PV testing and their advantages and disadvantages.

Open Circuit Voltage and Short Circuit Current Testing

Open circuit voltage (Voc) and short circuit current testing (Isc) are straightforward methods that measure the voltage and current of a solar module at just the endpoints of the I-V curve. Voc is measured when the module is not connected to any load, while Isc is measured when the module's terminals are shorted.

These tests provide basic information about the module's maximum voltage and current capabilities. However, they do not offer insights into performance under actual operating conditions.

I-V Curve Tracing

I-V curve tracing involves measuring a solar module's current (I) and voltage (V) characteristics under varying loads. This method generates a detailed I-V curve, which is a graphical representation of the module's performance across its entire operating range.

By analyzing this curve, technicians can identify various deviations that indicate specific classes of issues within the module or the PV circuit. Each of the deviations reduces the maximum power that the string can produce.

Six Deviations to the I-V Curve

When conducting an I-V curve test, it’s important to pay attention to deviations. They can indicate current mismatch and potential issues impacting the string – information Voc and Isc testing may not provide. For example, damaged cells and shading can cause deviations in current.

Here are six common deviation types:

1. Steps or notches: These deviations are often caused by partial shading, debris, or non-uniform soiling on the module but may also be caused by problems in individual solar cells. They appear as distinct steps or notches in the I-V curve, indicating that certain cell groups are underperforming. I-V curve tracing can detect these steps, which inform troubleshooting, whereas Voc and Isc testing does not reveal the steps.
2. Low current: This deviation can result from uniform soiling, inter-row shading, or reduced conversion efficiency of the cells. It manifests as a lower-than-expected current across the I-V curve. I-V curve tracing and Isc testing can identify low current issues.
3. Low voltage: Causes of low voltage include shorted bypass diodes, missing modules, or potential induced degradation (PID). This deviation appears as a reduction in the width of the I-V curve (reduced Voc). I-V curve tracing provides valuable diagnostic insight into possible causes of low Voc and can positively identify cases of PID. Voc testing can also identify low-voltage issues.
4. Soft knee: A soft knee in the I-V curve indicates a gradual transition from the maximum power point to the open circuit voltage, often caused by increased series resistance or degraded cell connections. This deviation can be detected through I-V curve tracing but not through Voc and Isc testing.
5. Reduced slope in the vertical leg: This deviation is typically caused by increased series resistance within the module, such as broken bonds between cells, corroded connections, or increased resistance in the external wiring. This results in a less steep vertical leg of the I-V curve. Excess series resistance does not affect Isc or Voc, so while I-V curve tracing can identify this issue, Voc and Isc testing cannot.
6. Increased (steeper) slope in the horizontal leg: In newer PV arrays, this deviation may be caused by specific patterns of shading or soiling, and in older arrays, the deviation may be caused by reduced shunt resistance in the cells, a degradation effect, and it is one of the deviations caused by PID. It appears as a steeper slope in the horizontal leg in the I-V curve. I-V curve tracing can detect this problem, whereas Voc and Isc testing cannot.

I-V Curve Testing Offers the Most Comprehensive Results

While open circuit voltage and short circuit current testing provide basic information about a solar module's maximum voltage and current, I-V curve tracing offers a comprehensive analysis of the module's performance under actual operating conditions. It also provides valuable diagnostic information for troubleshooting performance issues.

I-V curve tracing can detect issues such as steps or notches, low current, low voltage, soft knee, reduced slope in the vertical leg, and increased slope in the horizontal leg. When testing voltage and current, only the endpoints of the I-V curve are measured so only reduced current or voltage can be measured with these tests. Therefore, I-V curve tracing is the preferred method for thoroughly assessing PV module health and performance.

With I-V curve tracing, technicians can commission new PV arrays in greater detail and ensure more accurate diagnostics when maintaining solar modules, improving PV system efficiency and longevity.

About the Author

Will White has been a Solar Product Application Specialist at Fluke since 2022, supporting renewable energy testing equipment like I-V curve tracers, electrical meters, and thermal imaging cameras. With nearly two decades of experience in the solar industry, Will’s expertise spans residential and small commercial PV systems, energy storage, and wind power. He is a NABCEP Certified PV Installation Professional and a seasoned educator in solar energy, having developed and taught courses for Solar Energy International (SEI). Connect with Will on LinkedIn.