What abnormalities can diode aging cause in photovoltaic systems?

1, The technical causes and physical mechanisms of diode aging
The aging of diodes is the result of the combined action of material degradation and electrical thermal stress, and its core causes include:

Thermal stress accumulation: The operating temperature range of photovoltaic modules is usually -40 ℃ to+85 ℃, but the junction temperature of bypass diodes may exceed 125 ℃ when they are in a conducting state (such as when shaded). Long term high temperature environment will accelerate the diffusion of silicon lattice defects, resulting in an increasing forward voltage drop (Vf) year by year. Experimental data shows that the Vf of Schottky diodes running for 5 years may increase from the initial 0.3V to 0.5V, with a 67% increase in conduction loss.
Electrical stress shock: Transient overvoltage generated by lightning strikes and switch operations (such as peak voltage exceeding 100V in EL detectors) may cause breakdown of the PN terminal of the diode, resulting in hidden damage. In a certain photovoltaic power plant case, 30% of the bypass diodes experienced a surge in reverse leakage current (Ir) from μ A to mA after lightning strikes, leading to a significant increase in the risk of component thermal runaway.
Material oxidation and pollution: When the junction box is poorly sealed, water vapor intrusion can accelerate the oxidation of diode pins, causing the contact resistance (Rc) to rise from milliohms to ohms. A laboratory test showed that the contact resistance of oxidized diodes can increase the series resistance (Rs) of components by 15% and decrease the fill factor (FF) by 8%.
2, Component level anomaly: from efficiency decay to thermal runaway
The impact of diode aging on photovoltaic modules is directly reflected in the deterioration of electrical performance parameters and thermal management failure:

Decreased power generation efficiency: An increase in forward voltage drop will directly increase conduction loss. Taking a current of 20A as an example, when Vf increases from 0.3V to 0.5V, the power consumption of a single tube increases from 6W to 10W, resulting in a 4% loss in the output power of the component. If multiple diodes in the string age, the cumulative loss may exceed 10%.
The hot spot effect intensifies: an increase in reverse leakage current (Ir>10 μ A) will cause the obstructed battery cells to continue consuming electrical energy, resulting in a local temperature rise. A field test showed that a diode with Ir=50 μ A caused the temperature of the blocked battery cell to be 25 ℃ higher than normal, accelerating the cracking of the battery cell and aging of the packaging material.
Risk of junction box burnout: The double increase in contact resistance (Rc) and conduction voltage drop (Vf) can lead to a vicious cycle: Rc increases causing local heating → diode junction temperature rises → Vf further rises → heating becomes more severe. In a power station case, a diode with Rc=0.5 Ω generated 20W of heat loss at 20A current, ultimately igniting the insulation material of the junction box.
3, System level anomaly: from string mismatch to power generation loss
The impact of diode aging on photovoltaic systems will be amplified through cascading effects:

String mismatch loss: Aging diodes result in missing open circuit voltage (Voc) of component substrings, causing a "step like" distortion in the I-V curve of the string. A simulation of a 1MW photovoltaic power station shows that when 5% of the bypass diodes age, the maximum power point (MPP) power loss of the string reaches 3.2%, and the annual power generation decreases by about 28000 kWh.
Decreased inverter efficiency: Fluctuations in the output voltage of the series will force the inverter to frequently adjust its operating point, reducing conversion efficiency. Experimental data shows that when the voltage fluctuation range expands from ± 2% to ± 5%, the inverter efficiency decreases from 98.5% to 97.2%.
DC side safety hazard: Aging diodes may pose a risk of DC arcing. When the diode is open circuited, the string current is forced to pass through other paths (such as metal brackets), forming an arc discharge. An investigation into a fire accident found that the open circuit of the diode in the junction box was the direct cause of the DC side arc.
4, Detection and Diagnosis: From Manual Inspection to Intelligent Monitoring
To address the issue of diode aging, a multi-level detection system needs to be constructed:

Infrared thermal imaging detection: By using a high-precision thermal imaging device mounted on a drone (such as the Zenith H30T, with a resolution of 1280 × 1024), abnormal temperature in the junction box can be identified. The actual measurement of a certain power station shows that the normal diode temperature is 10-15 ℃ higher than the environment, while the aging diode temperature may be higher by more than 30 ℃.
Electrical performance parameter testing: Use an IV curve tester to collect component I-V data and locate faulty diodes by analyzing the "step" feature. For example, a diode short circuit can cause a loss of sub string Voc, while aging diodes can cause abnormal step slopes.
Online monitoring system: Deploy intelligent junction boxes (such as integrated MSOP8 controller type ideal diodes) to monitor parameters such as Vf, Ir, Tc (junction temperature) in real-time. A demonstration project has reduced the detection time of diode faults from a monthly level to an hourly level by using threshold alarms (such as Vf>0.45V or Ir>5 μ A).
5, Response strategy: From passive replacement to proactive prevention
Material and process optimization: wide bandgap materials (such as SiC Schottky diodes) are selected, with a Vf as low as 0.2V and a temperature resistance of up to 175 ℃; Using laser welding technology to reduce contact resistance, experiments have shown that laser welding can reduce Rc by 80%.
Redundant design: Parallel backup diodes are connected in the junction box, which automatically switch when the main diode fails. A certain manufacturer's product reduces the failure rate from 0.5%/year to 0.1%/year through a dual diode design.
Intelligent operation and maintenance system: Establish a diode life prediction model, and calculate the remaining life based on operational data such as current flow time and junction temperature history. A certain power station has extended the diode replacement cycle from 5 years to 7 years through big data analysis, reducing operation and maintenance costs by 30%.