6 deviations to the I-V curve

In photovoltaic (PV) systems, understanding module performance characteristics is crucial for maintenance and optimization. I-V curve tracing is an essential diagnostic tool that provides a detailed snapshot of a PV array’s health. By identifying deviations in the I-V curve, technicians can diagnose and address performance issues effectively. This article delves into the common types of I-V curve deviations, their causes, and how to interpret them.

What is an I-V curve deviation?

An I-V curve deviation occurs when the measured current-voltage relationship of a PV module or array differs from the expected curve. These deviations can indicate various performance issues, from shading and soiling to hardware failures. Understanding these deviations is key to diagnosing and resolving problems in PV systems.

Common types of I-V curve deviations:

1. Stepped I-V curves
2. Low short-circuit current (Isc)
3. Low open-circuit voltage (Voc)
4. Rounder knees
5. Low voltage ratio
6. Low current ratio

1. Stepped I-V curve

A current mismatch in the PV system can cause notches or steps in the I-V curve. When bypass diodes activate and pass current around weaker cells (or cells receiving less light), a stepped curve is a common finding. The number and width of the steps vary according to the density and extent of the shade. A stepped I-V curve can result from debris, partial shading, or non-uniform soiling on the module blocking sunlight. Additionally, hardware concerns such as damaged cells or cell strings and shorted bypass diodes can also produce this deviation.

Interpretation:

• Distinct steps in the I-V curve typically indicate small areas of shading or debris.
• Multiple steps in the curve may suggest widespread unevendirtaccumulation.

Troubleshooting steps:

• Inspect for visibleshading from tree branches or debris. Clean any soiled surfaces of dust, bird droppings, or other matter.
• With a multimeter, test the bypass diodes. If necessary, replace any faulty modules or diodes.
• Inspect modules for physicaldamage that could cause shading effects.

I-V curve testing before and after cleaning can quantify the impact of soiling on system performance. (Follow the same testing procedure before and after addressing shading concerns.)

2. Low short-circuit current (Isc)

Low short-circuit current indicates that the PV system isn’t producing as much current as it should. A lower-than-expected value of Isc in an otherwise ordinary I-V curve can have one of several causes. This deviation may result from operator error, poor irradiance measurement, uniform shading or soiling across the array, module performance issues, or module degradation.

Interpretation:

• Check for environmental factors such as shading or soiling; they’re the most common causes.
• Verify the placement and calibration of irradiance sensors.
• Evaluate previous measurements and consider long-term trends to identify gradual performance degradation (aging equipment).

Troubleshooting steps:

• Ensure the irradiance sensor is correctly calibrated and positioned.
• Remove any dirt or debris from the PV modules.
• Look for signs of module degradation, including aging or damage. Compare current performance with historical data.

3. Low open-circuit voltage (Voc)

Low Voc deviations are characterized by a lower-than-expected voltage when the circuit is open. Inaccurate temperature readings and increased circuit resistance are common causes. Other possible causes of low Voc are hardware issues such as shorted bypass diodes, long conductors, inadequate wire size, or pinched conductors. However, due to low Voc having one of the lowest aging rates of all PV module parameters, initially consider other causes besides cell degradation.

Interpretation:

• Double-check temperature sensor placement and accuracy.
• Look for wires that are smaller or longer than expected.
• Inspect hardware for physical damage or faults in the system.

Troubleshooting steps:

• Verify that the temperature sensor is correctlyplaced and producing accurate readings.
• Identify and remove any wiring issues, or account for their impact on the IV curve.
• Inspect for shorted bypass diodes or pinched conductors and repair or replace as necessary.

4. Rounder knees

A rounder-than-expected knee in the I-V curve indicates a gradual slope change at the maximum power point, which can signify age-related module degradation. In addition, increased resistance due to poor connections or damaged interconnections lead to a rounder knee.

Interpretation:

• Monitor the curve over time to track gradual degradation related to equipment age.
• Inspect interconnections and junction boxes for signs of increased resistance.

Troubleshooting steps:

• Monitor I-V curves overtime, track gradual changes in performance, and analyze data trends.
• Look for sources of increased resistance like poorconnections or corrosion.

5. Low voltage ratio

Low voltage ratio presents as a lower-than-expected slope in the vertical leg of the I-V curve. Detection involves visually comparing the measured and predicted curves or comparing voltage ratio values across the population of string measurements, with the prerequisite that the curves be free of steps from mismatch effects. The voltage ratio is great for identifying a string with an atypical slope in the vertical leg of the I-V curve. Causes include damaged solder joints, corroded connectors, or poorly made interconnections that increase series resistance. Additionally, using conductors that are too small for the current they carry is another possible culprit.

Interpretation:

• Check all connections and junctions for signs of corrosion or damage.
• Verify that conductors are appropriatelysized for the current they carry.

Troubleshooting steps:

• Examine all connections for signs of corrosion or damage.
• Check conductor sizing.

6. Low current ratio

Description: A low current ratio is identified by a higher-than-expected slope in the horizontal leg of the I-V curve. This metric is excellent for identifying a string with atypical slopes in the horizontal leg of its I-V curve. Over time, decreasing shunt resistance can lead to this deviation. Additionally, severe, localized shunts can heavily impact current output. I-V curve tracing can identify low current issues. Before searching for hardware issues, inspect for shade, soiling, and irradiance measurement errors.

Interpretation:

• Identify gradual performance degradation by tracking changes in shuntresistance over time.
• Infrared (IR) imaging can help detect localized issues, like shunts that need urgentattention.

Troubleshooting steps:

• Eliminate shading or soiling issues first.
• Use an IR camera to detect hotspots or localizedshunts that point to performance problems.