Why Does Current Discrepancy Rate Reduce the Power Generation of Your PV Modules?

Introduction

In the operation and maintenance (O&M) of photovoltaic (PV) power plants, we often focus on macro indicators such as power generation, efficiency, and degradation rate. However, there is a more detailed parameter—current discrepancy rate—that helps us diagnose the health status of PV strings with greater precision. 

When there are significant differences in current between modules or branches within a PV string, it may indicate issues such as shading, hot spot effect, module degradation, potential-induced degradation (PID) effect, or wiring faults. The current discrepancy rate is the key indicator to quantify such differences. It not only helps O&M personnel quickly locate faults but also optimizes system design and improves overall power generation efficiency.

I.What is Current Discrepancy Rate?

The term “current discrepancy rate” may sound like an obscure technical jargon, but it is actually quite simple: it is an indicator that measures whether the output current of each module in a PV string is consistent. It is analogous to a group of people pulling a cart—they should all pull in the same direction with equal force; if some slack off while others overexert, the cart will definitely not move fast. 

Here is a straightforward example: A household’s PV string consists of 10 modules. Nine of them can output 8A of current, while one—blocked by tree branches—can only output 5A. In this case, the current of the entire string will be “dragged down” and limited to the lowest current (5A). A system that should theoretically generate 80 kWh of electricity will only produce 50 kWh in reality. The 30 kWh of electricity that “disappears” out of thin air is essentially “stolen” by the current discrepancy rate. 

Its calculation formula is equally simple: 

Current Discrepancy Rate = (Maximum Current in the String – Minimum Current in the String) ÷ Average Current in the String × 100%** 

There is a widely recognized industry standard: if the discrepancy rate exceeds 5%, power generation will decrease significantly; if it exceeds 10%, the annual loss in electricity costs may be enough to buy an additional air conditioner. However, in reality, the current discrepancy rate of many household PV strings has already exceeded 15%, yet homeowners remain unaware—mistaking the power loss for natural degradation of PV panels over time.

II.Factors Affecting Current Discrepancy Rate

Ideally, if all PV strings have identical performance and operate under the same environmental conditions (e.g., sunlight, temperature), the current of each string will be the same, and the discrepancy rate will be 0%. In practice, however, this ideal state rarely exists in operational PV power plants. The operation of a PV power plant is affected by numerous factors—from inherent quality differences in PV modules and minor deviations during installation to varying degrees of shading, temperature changes, and other environmental influences during operation. All these factors can cause current differences between strings, resulting in a non-zero current discrepancy rate. Specifically, three key factors influence the current discrepancy rate:

1. Shading

Shading is a common and critical factor that causes changes in the current discrepancy rate. When part of a PV module is shaded, the shaded area receives less sunlight, leading to a corresponding decrease in its current output. In a series-connected PV string, current follows the “barrel effect”—the current of the entire string is determined by the module with the lowest current output. Therefore, if even one module is shaded, the current of the entire string will be limited, creating a difference compared to other unshaded strings and ultimately increasing the current discrepancy rate. 

For instance, in a PV power plant, the growth of surrounding trees caused some PV modules to be shaded in the afternoon. The current of the originally normally operating strings dropped significantly after being shaded, and the current difference between these shaded strings and unaffected ones gradually widened. Monitoring data showed that the current discrepancy rate in this area rose rapidly from 5% to over 15%, seriously impairing the overall power generation efficiency of the plant. Such shading can come from natural objects (e.g., trees, buildings, mountains) or from the power plant’s own structures (e.g., brackets, frames).

2. Module Aging

PV modules gradually age over time, which is another key factor affecting the current discrepancy rate. Long-term exposure to outdoor environments subjects modules to erosion from ultraviolet (UV) radiation, high temperatures, humidity, sand, and other natural factors, leading to a gradual decline in the performance of their internal semiconductor materials. For example, aging of encapsulation materials may reduce their ability to protect the module, making it more vulnerable to external environmental damage. Aging of solar cells, on the other hand, lowers their photoelectric conversion efficiency, resulting in reduced current output under the same sunlight conditions. 

Different modules may age at different rates, which increases current inconsistency between strings. In a PV power plant that has been in operation for many years, some early-installed modules have aged severely, and their current output is significantly lower than that of newly replaced modules. The current of strings with these aged modules differs greatly from that of other strings, leading to a rise in the current discrepancy rate. Studies have shown that after 10–15 years of use, the photoelectric conversion efficiency of PV modules may decrease by 10%–20%, and the string current will also change significantly—thereby affecting the current discrepancy rate.

3. Equipment Faults

In addition to shading and module aging, equipment faults are another non-negligible factor. A PV power plant consists of numerous pieces of equipment, and a fault in any link can cause changes in the current discrepancy rate. Common equipment faults include inverter failures, combiner box failures, and wiring connection faults. 

Inverter Faults: As the key equipment that converts DC power generated by PV modules into AC power, the inverter’s performance directly affects current output. If an inverter malfunctions (e.g., damaged power modules, faulty control circuits), it may process string current abnormally, resulting in unstable output current and creating a difference compared to the output of normally operating inverters. In a large-scale PV power plant, for example, a faulty power module in one inverter caused its output current to drop sharply, creating a significant difference from other normal inverters and leading to a sudden increase in the current discrepancy rate in that area. 

Combiner Box Faults: Combiner boxes collect and transmit current from multiple PV strings. If internal components of a combiner box fail (e.g., blown fuses, loose terminals), it can disrupt current transmission and affect the consistency of string currents. 

Wiring Connection Faults: Issues such as aging, damage, or poor contact in cables increase line resistance, causing current loss during transmission. The degree of current loss varies across different strings, leading to current differences between strings—which is ultimately reflected in changes to the current discrepancy rate.

III. How to Address the Challenge of High Discrepancy Rate?

(1) Intelligent Monitoring Systems

In the era of rapid technological development, intelligent monitoring systems have become a key tool to address high current discrepancy rates. Real-time monitoring systems built using smart sensors, big data, and artificial intelligence (AI) algorithms are playing an increasingly important role. 

Smart sensors—acting as the “sensory organs” of the system—are widely deployed at key locations in PV power plants, such as PV modules, combiner boxes, and inverters. These sensors accurately collect multi-dimensional data (e.g., current, voltage, temperature, sunlight intensity) and transmit it to a data processing center in real time. 

When the current discrepancy rate exceeds the normal range, the system quickly issues an early warning to notify O&M personnel to take timely action. Using machine learning algorithms, the system can also analyze historical data to continuously optimize its monitoring and diagnostic capabilities, improving the efficiency of identifying and addressing high discrepancy rate issues. 

Through intelligent monitoring systems, O&M personnel can remotely monitor the operating status of the power plant at any time, enabling real-time monitoring and precise management of the current discrepancy rate—providing strong support for the stable operation of the power plant.

 (2) Fault Diagnosis and Localization

After the monitoring system detects an abnormal current discrepancy rate, rapid and accurate fault diagnosis and localization are crucial. By analyzing the abnormal discrepancy rate alongside other indicators and technical tools, fault points can be quickly identified. 

For example, thermal imaging technology plays an important role in detecting hot spots on modules. Hot spots—one of the most common PV module faults—are usually caused by local defects in modules or shading. Since the temperature at hot spots is significantly higher than that of normal areas, thermal imaging technology can clearly identify the location and scope of hot spots by detecting the temperature distribution on the module surface. 

In practice, O&M personnel use handheld thermal imagers or drone-mounted thermal imaging equipment to scan PV modules. These devices quickly capture thermal images of modules, where areas with abnormal temperatures are displayed in different colors—intuitively showing the location of hot spots. 

In addition to thermal imaging, other technical tools can be used for fault diagnosis: 

Electroluminescence (EL) Testing: This technology detects subtle internal defects in PV modules, such as hidden cracks or broken grids. 

Data-Driven Fault Diagnosis: By analyzing correlations in monitoring data, this method infers the possible location and cause of faults.

 (3) Optimized O&M Strategies

Developing reasonable optimized O&M strategies based on different fault causes is an important step in resolving high current discrepancy rates. 

Adjusting Module Layou: If shading (caused by unreasonable module layout) leads to a high discrepancy rate, adjusting the module layout can solve the problem. This requires a detailed analysis of the power plant’s terrain, surrounding environment, and the sun’s movement trajectory to re-plan the installation position and angle of modules—ensuring each module receives sufficient sunlight and minimizing shading. In some mountainous PV power plants, for example, adaptive terrain mounts are used to automatically adjust module angles based on terrain undulations, effectively avoiding shading and reducing the current discrepancy rate. 

Regular Module Cleaning: Dust accumulation on PV module surfaces blocks sunlight, reducing module performance and increasing the current discrepancy rate. Regular cleaning is therefore an important measure to minimize dust impact and improve power generation efficiency. 

O&M teams should develop a cleaning schedule based on local environmental conditions and dust accumulation rates: in dusty areas, modules may need cleaning once a month; in relatively clean areas, quarterly cleaning may suffice. Professional cleaning equipment and tools (e.g., high-pressure water guns, soft brushes) should be used to ensure cleaning effectiveness while avoiding damage to modules. Intelligent cleaning robots can also be deployed to automate module cleaning, improving efficiency and quality.  

By implementing these optimized O&M strategies, the current discrepancy rate can be effectively reduced, and the power generation efficiency and economic benefits of the PV power plant can be enhanced.