How to Prevent Potential Induced Degradation in Solar Modules: 5 Methods

Prevent PID by grounding module negatives (resistance <4Ω) to dissipate induced voltage, using silane-enhanced encapsulants (slowing ion migration), limiting system voltage to ≤1000V, monthly surface cleaning (reducing conductive paths), and weekly open-circuit voltage checks for early detection.

Use PID-Resistant Module Types

While a standard module might lose 15% to 30% of its power output within just 2-3 years due to severe PID, specially engineered modules can limit this degradation to less than 2% over the same period, effectively making the problem negligible. The global push for higher system voltages (1500V systems are now the norm) increases the stress that causes PID, making this foundational choice more critical than ever. Investing in inherently resistant modules isn't just a technical detail; it's a direct financial safeguard for the 25- to 30-year lifespan of your asset.

The most significant innovation is in the encapsulant, the plastic sheet (typically EVA - Ethylene Vinyl Acetate) that seals the cells. Standard EVA can break down and form acetic acid, which facilitates the ion migration that causes PID. PID-resistant versions use a much higher grade of EVA with additives that create a robust barrier against sodium ion movement. The key metric here is volume resistivity. While standard EVA may have a resistivity of 5.0×1014Ω·cm, advanced PID-resistant encapsulants can achieve values greater than 1.0×1016Ω·cm—a twenty-fold increase in resistance to leakage current.

Furthermore, the anti-reflective coating on the glass plays a role. Some manufacturers use a coating with a higher surface resistance, adding another layer of defense. But the most significant shift has been the move towards polyolefin elastomer (POE) encapsulants. POE films have an inherently different chemical structure that is non-polar and highly hydrophobic, meaning it naturally repels the moisture that accelerates PID. The cost of a module using a POE encapsulant might be 3-5% higher upfront compared to one using standard EVA, but the long-term benefit is dramatic. Testing under standardized conditions (like 85°C and 85% relative humidity with -1000V applied for 96 hours) shows POE-based modules experiencing power losses of less than 1%, whereas standard modules can degrade by over 20%.

Request the certified results from the IEC TS 62804-1 test standard. A credible manufacturer will provide a report showing performance after the rigorous damp heat and high-voltage bias test, with a maximum power degradation guarantee of less than 2%. This isn't an area for assumptions. The small upfront cost premium of 0.02 to 0.05 per watt for a truly PID-resistant module is a calculated investment that protects the entire value of your system.

Connect Panel Negative Side to Ground

While this design enhances safety by limiting fault currents in some scenarios, it creates the primary condition for PID. In a typical 1500V system with a string voltage of, for example, 1000V, the negative pole can sit at -500V relative to the grounded module frames.

Transformerless inverters, which are popular for their high peak efficiency (often 99.0% or above), are incapable of providing a direct current (DC) grounding point. Their design inherently creates a floating array. To ground the negative pole, you must use an inverter with an internal or external isolation transformer. This transformer electrically separates the DC side from the alternating current (AC) side, allowing you to establish a stable ground reference on the negative DC terminal. While these inverters have a slightly lower peak efficiency—typically around 98.5% to 98.8% due to transformer losses—this difference of 0.2% to 0.5% is almost insignificant when weighed against preventing annual power losses of 2% to 5% or more from PID. The transformer itself adds weight (approximately 15-30 kg for a central inverter) and cost, increasing the initial price by about 0.02 to 0.04 per watt for the system.

By grounding the negative pole, the voltage difference between the cells and the grounded frame is reduced from, for instance, -1000V to just a few tens of volts. This lowers the electrical "pressure" causing ion migration to a level where it becomes virtually harmless. Field studies on utility-scale projects have demonstrated that grounded systems show PID-related degradation rates of less than 0.5% per year, compared to ungrounded systems that can exhibit 3% to 5% losses in the first year alone.

Keep System Dry and Well-Grounded

The relationship is quantifiable; for every 10°C increase in ambient temperature at a constant relative humidity, the rate of chemical reactions approximately doubles, accelerating PID. In humid coastal environments where the average relative humidity consistently exceeds 70-80%, PID can degrade a system's output by 4% or more in a single year, whereas a similar system in an arid climate might show only 1% loss.

The National Electrical Code (NEC) typically specifies a maximum resistance of 25 ohms for a single grounding electrode. For a commercial solar farm, you should be targeting a much more robust 5 ohms or less for the entire array. Achieving this requires proper installation: ground rods must be driven to a depth that reaches permanently damp soil, often 2.5 to 3 meters (8-10 feet) deep, and using multiple rods spaced at least twice their length apart (e.g., 3 meters apart for 1.5-meter rods) to effectively lower the overall resistance. The connection point is critical; exothermic welds or listed irreversible compression connectors create a permanent bond with a resistance of less than 0.001 ohms, ensuring the connection will not degrade over the system's 25-year lifespan, unlike mechanical clamps that can loosen with temperature cycles.

Module-Level Sealing: The junction box on the back of each module is a critical failure point. High-quality boxes use IP67 or IP68-rated seals (capable of withstanding immersion in 1 meter of water for 30 minutes). The potting material inside must remain pliable across a temperature range of -40°C to 90°C to prevent cracking. A compromised seal allows humidity to condense on the terminals, creating a leakage path that can bypass over 50% of the module's surface area.

For large-scale installations in high-humidity regions, consider specifying modules with butyl rubber or silicone-based edge seals instead of standard polyisobutylene (PIB), as they offer superior resistance to moisture vapor transmission. On the system level, ensure there is a minimum air gap of 150mm (6 inches) beneath the array to promote airflow, which helps to reduce the relative humidity at the module surface by 15-20% compared to a flush-mounted system.

Control System Voltage Appropriately

While the industry's shift to 1500V architectures reduces balance-of-system costs by up to 15% through thinner cables and fewer combiners, it simultaneously intensifies the PID stressor. The critical factor isn't the inverter's maximum input voltage (1500V), but the actual operating voltage of the string, which constantly fluctuates. On a cold, sunny morning, a string's voltage can spike 20% above its typical operating point due to the negative temperature coefficient of silicon (approximately -0.3% per °C). A string designed for 1000V at 25°C can easily exceed 1150V at 0°C.

The relationship between voltage and PID is not linear; it's often exponential. Research indicates that increasing the voltage stress from 600V to 1000V can increase the rate of power degradation by a factor of 3 to 5, all other conditions being equal.

The most impactful strategy involves managing the voltage-to-ground during non-production hours, particularly at night. When the sun goes down, the inverter stops converting power, but if the DC disconnect is closed, the entire string remains at its maximum voltage potential relative to the grounded frames. This means PID can actively occur for 12-14 hours a day without any energy generation to offset the damage. A simple yet highly effective mitigation technique is to implement a nighttime disconnection strategy. This can be achieved by using an externally controlled DC disconnect or an inverter with a built-in contactor that physically opens the circuit when the generation power drops below a certain threshold (e.g., 10 watts for 5 consecutive minutes). This action collapses the voltage across the strings to near zero, effectively halting the PID process. The energy loss is negligible, as it only shaves off a few minutes of low-light production at dawn and dusk, typically amounting to less than 0.1% of annual yield.

While the MPPT normally seeks the highest possible power point, you can set a soft limit that prevents the string voltage from operating above a user-defined threshold, such as 1000V, even on the coldest days. This sacrifices a tiny amount of energy during a few peak cold-weather hours—perhaps 0.3% of the annual production—in exchange for dramatically lower year-round electrical stress on the modules. This proactive voltage management, combined with the other methods, creates a robust, multi-layered defense system that directly addresses the root cause of PID.

Select Inverters with Negative Grounding

Transformerless inverters, which dominate the market with peak efficiencies of over 99%, create a significant challenge: their "floating" DC side inherently generates the high voltage potential to ground that drives PID. Specifying an inverter designed for negative grounding is a proactive engineering choice that addresses the root cause. This typically means selecting a model with an internal isolation transformer. Field data from multi-megawatt installations shows that systems with properly implemented negative grounding can limit PID-related degradation to less than 0.5% per year, compared to floating systems in similar environments that can suffer 3-5% annual losses. The decision essentially trades a minimal, upfront cost increment for long-term performance security over the system's 25-year lifespan.

The fundamental advantage of a transformer-based inverter with negative grounding is that it clamps the voltage of the entire PV array's negative pole to within a few volts of ground potential. This action reduces the primary driving force for PID by over 95%, effectively neutralizing the threat.

A high-quality, modern transformer-based inverter will have a peak efficiency of 98.5% to 98.8%, which is only about 0.3% to 0.5% lower than a comparable transformerless model. Over a year, this difference might result in a 0.2% to 0.4% energy production loss. However, this is a fraction of the 3% or more annual energy loss you are preventing from PID. The transformer itself adds weight—approximately 20-40 kg for a central inverter in the 250-500 kW range—and contributes to a system cost increase of roughly 0.02to0.04 per watt. This investment often pays for itself within the first 18-30 months of operation through preserved energy output.

When specifying these inverters, precision is critical. You must confirm the exact configuration procedure with the manufacturer. The process generally involves:

l A specific hardware terminal: A dedicated grounding lug or terminal on the DC side, often labeled "NEG GND," with a required conductor size of at least 6 AWG copper.

l A firmware setting: A software switch within the inverter's configuration menu that must be explicitly enabled to activate the negative grounding function.

l Compatibility verification: Confirm with your module manufacturer that their products are certified for use in a negatively grounded system, as some thin-film technologies may have restrictions.

The slightly higher initial cost of 15,000 to 30,000 for a 1 MW system is not merely an expense; it is an insurance premium with a demonstrably positive return on investment. By stabilizing the array's voltage relative to ground, you are ensuring that the capital invested in the modules themselves is protected, guaranteeing they will deliver their promised energy yield for decades.