PID in Offshore PV Modules: IEC 61643-11 Surge Protection and Quantitative Thermography

Introduction and Dynamics of Potential-Induced Degradation (PID) in Marine Environments

Potential-Induced Degradation (PID) is an electrochemical degradation phenomenon that severely compromises the performance and operational lifespan of crystalline silicon photovoltaic (PV) modules. In floating and offshore PV plants, this failure mechanism is exponentially accelerated due to extreme environmental conditions characterized by high salinity, persistent relative humidity, and cyclic thermal fluctuations. PID primarily manifests in two main variants: Shunting PID (PID-s), which degrades the fill factor (FF) and open-circuit voltage ($Voc$) through the formation of parasitic conduction pathways across the p-n junction; and Polarization PID (PID-p), which adversely impacts the short-circuit current ($Isc$) due to surface charge accumulation that alters the surface recombination velocity.

The physico-chemical mechanism of PID-s is initiated by the electric potential difference between the silicon solar cells and the module structural support frame, which is typically grounded for personnel safety and equipment protection. In modern systems operating at string voltages up to $1500 V _{DC}$, modules located at the negative polarity end of the string experience an intensely strong internal electric field. This high negative potential attracts sodium cations ($Na ^+$) present within the front cover glass matrix (typically soda-lime silicate glass).

Under the influence of this electrostatic field, $Na ^+$ ions migrate through the ethylene-vinyl acetate (EVA) encapsulant toward the front surface of the silicon cell. Accumulating sodium ions infiltrate structural crystal defects in the silicon lattice, specifically stacking faults, diffusing deeply until reaching the p-n junction interface. This process creates metallic micro-shunts that locally short-circuit the cell, causing a drastic reduction in the parallel shunt resistance ($Rsh$) of the device.

In offshore marine environments, this degradation process is heavily catalyzed by continuous marine aerosol deposition. Salt spray deposits a highly conductive aqueous film over the outer glass surface of the module. This saline layer functions as a continuously distributed electrode, reducing surface insulation resistance virtually to zero. Consequently, the entire system potential drop is concentrated perpendicularly across the combined thickness of the glass and encapsulant, maximizing the local electric field strength $E$.

The degradation kinetics of PID in marine environments can be modeled using a modified Arrhenius-type equation incorporating the non-linear dependencies on system operating voltage and relative humidity:

Where:

• $KPID$ is the module degradation rate (power loss per unit time).
• $A$ is a pre-exponential factor characteristic of the module design, bill of materials (BOM), and chemical composition.
• $VDC$ is the direct current string voltage relative to earth ground ($V$).
• $\gamma$ is the voltage exponent, typically ranging from $1.5$ to $2.0$.
• $RH$ is the local relative humidity (expressed as a decimal fraction or percentage).
• $\beta$ is the humidity sensitivity exponent, usually evaluated between $2.0$ and $4.0$ due to the non-linear increase in encapsulant conductivity resulting from moisture absorption.
• $E_a$ is the activation energy for the $Na ^+$ ion diffusion process, typically ranging between $0.7 eV$ and $0.95 eV$.
• $k_B$ is the Boltzmann constant ($8.617 \times 10^{-5} eV/K$).
• $T$ is the absolute junction temperature of the cell ($K$).

On floating platforms, relative humidity ($RH$) remains almost continuously above $85\%$, and surface salinity enhances the hygroscopicity of the glass, facilitating surface condensation even at temperatures exceeding the standard dew point. This dramatically lowers the electrical bulk resistance of the encapsulant ($REVA$), which varies exponentially with temperature and water absorption, allowing the $Na ^+$ ion leakage current to increase by several orders of magnitude compared to land-based installations in temperate or arid desert regions.

Electromagnetic and Grounding Interaction: The Role of the IEC 61643-11 Standard

Grounding system architecture in offshore PV plants presents unique engineering challenges due to the absence of a direct terrestrial earth mass reference and the highly conductive nature of seawater. Structural metallic support frames (whether high-density polyethylene HDPE floats with aluminum substructures or steel jackets/monopiles fixed to the seabed) must be equipotentially bonded to ensure safety and system protection. However, the overall grounding impedance of the system is dynamically influenced by water salinity, ambient sea temperature, and the galvanic corrosion of sacrificial anodes.

Surge Protective Devices (SPDs) governed by the IEC 61643-11 standard (low-voltage surge protective devices for power distribution systems) play a critical and dual role in PID dynamics. These protective devices, designed to limit transient overvoltages resulting from atmospheric discharges or switching maneuvers, are connected between the active DC circuit conductors ($L+$, $L-$) and the protective earth ($PE$) busbar.

Leakage Currents and Offset Voltage

SPDs utilizing Metal Oxide Varistors (MOVs) exhibit a continuous internal residual/leakage current ($Ipe$) flowing to the grounding system. Under nominal operating conditions, this current is extremely low (typically less than $1 mA$). However, in marine operational environments, the accelerated thermal and chemical degradation of MOVs caused by ingress of high humidity and repetitive micro-surges elevates this leakage current in a non-linear manner.

When an imbalance occurs between the leakage currents of the positive and negative pole SPDs, the potential of the PV array's virtual neutral point shifts dynamically. In ungrounded or floating system configurations (IT systems), this voltage shift alters the entire string potential distribution relative to earth ground. If the positive pole SPD undergoes degradation that increases its conductivity, the positive pole potential drifts toward ground potential, forcing the negative pole to shift down to $-1500 V$ relative to earth ground. This voltage shift drastically increases the dielectric voltage stress at the negative end of the string, catastrophically accelerating PID-s kinetics.

The interaction between the parasitic capacitance of the PV module ($Cpara$), the SPD internal impedance, and the grounding system impedance is dynamically modeled using the following equivalent circuit formulation:

Where $R_{SPD+}$ and $R_{SPD-}$ represent the dynamic insulation resistances of the SPDs on each respective pole, and $R_{isol+}$ and $R_{isol-}$ represent the insulation resistances of the PV plant array itself. Under high marine humidity, if $R_{SPD-}$ decreases due to degradation governed by IEC 61643-11 testing profiles, the offset voltage ($Voffset$) shifts strongly positive, thereby driving the potential of the cells at the opposite end of the string to severe negative values, triggering sodium ion migration.

Switching Transients and High-Frequency Impact

High-frequency transients induced by lightning electromagnetic pulses (LEMP) or fast switching operations drive transient currents through the parasitic capacitances of the modules into the metallic support structure and seawater. The IEC 61643-11 standard defines rigorous testing procedures to verify the capability of SPDs to withstand impulse discharge currents ($Iimp$) characterized by a $10/350 \ \mu s$ waveform (Class I) and nominal discharge currents ($I_n$) characterized by an $8/20 \ \mu s$ waveform (Class II).

If the voltage protection level of the SPD ($U_p$) is excessive or if the device interconnecting lead inductance is too high, transient overvoltages will not be effectively clamped. The presence of fast-front voltage transients (high $dv/dt$) induces severe transient electric fields across the ultra-thin silicon dioxide ($SiO _2$) and aluminum oxide ($Al _2 O _3$) passivation layers used in high-efficiency solar cell architectures (such as PERC or TOPCon). These transient overvoltages can cause localized dielectric breakdown of these nanometric passivation layers, creating permanent defect nucleation sites that permanently lower local insulation resistance and serve as preferential conductive channels for subsequent PID degradation under static operational DC voltages.

Quantitative Infrared Thermography (QIRT) for Offshore PID Diagnostics

The field detection, identification, and quantitative evaluation of PID in offshore PV plants using Quantitative Infrared Thermography (QIRT) demands a rigorous analytical treatment of radiative heat transfer and the optical interference phenomena introduced by the marine boundary layer. Unlike land-based PV plants, floating platforms are subject to continuous wave-induced motion, necessitating active optical stabilization platforms and high-resolution thermal cameras operating at high frame rates.

Physics of Infrared Radiation in Marine Environments

To accurately compute the actual surface temperature of a PV module ($Tobj$) from the total spectral radiance recorded by the uncooled microbolometer sensor array of the thermal camera ($Wtot$), the radiative transfer equation must be solved explicitly by accounting for target emissivity ($\epsilon$), atmospheric transmittance ($\tau$), and background reflected temperature ($Trefl$):

Solving explicitly for the true object surface temperature:

Where:

• $\sigma$ is the Stefan-Boltzmann constant ($5.670374 \times 10^{-8} W/m ^2 K ^4$).
• $\epsilon$ is the emissivity of the module's front glass surface. Uncoated soda-lime glass exhibits an emissivity of $\approx 0.85 - 0.90$ within the long-wave infrared (LWIR) spectral band ($8 \ \mu m$ to $14 \ \mu m$). However, the buildup of crystallized salt crusts significantly alters this value, decreasing effective spectral emissivity while increasing specular reflectivity, introducing temperature measurement errors of up to $\pm 5 °C$ if not dynamically compensated.
• $\tau$ is the atmospheric transmittance. In marine environments, high ambient concentrations of water vapor and suspended brine aerosols induce substantial absorption and scattering of LWIR radiation. Transmittance must be dynamically calculated as a function of optical path distance $d$ and absolute atmospheric humidity.
• $Trefl$ is the background reflected apparent temperature. In offshore environments, the surrounding seawater and clear sky act as intense background thermal sources. Seawater exhibits high emissivity, but its high reflectivity at shallow viewing angles can reflect the cold sky radiance onto the module surface, masking genuine thermal hot spots.

Thermal Signatures of PID

Shunting PID (PID-s) displays a distinct and diagnostic thermal signature. Because local micro-shunts severely reduce the cell parallel resistance, affected cells operate at an operating point where they dissipate electrical energy as heat rather than delivering power to the string. The photocurrent generated by unaffected, healthy cells within the same string is forced through the PID-degraded cells, which act as localized resistive loads.

The string-level thermal signature exhibits a characteristic temperature gradient that increases progressively along the string toward the negative potential terminal (in floating or positive-grounded systems). At the individual module level, PID-s manifests initially as a distinctive "checkerboard" thermal pattern, wherein individual cells or groups of cells immediately adjacent to the grounded aluminum frame exhibit substantially elevated operational temperatures ($\Delta T$ ranging from $5 °C$ to over $25 °C$ above healthy cells). This occurs because the shortest physical path for ionic leakage current is the minimum spatial clearance distance to the grounded metallic frame.

The power loss of a degraded cell can be quantitatively derived from its thermal signature by applying local heat balance equations:

Where:

• $Ueff$ is the overall effective heat transfer coefficient (combining convective and radiative components) under offshore boundary conditions, heavily dictated by local wind speed ($v_w$): $Ueff = a + b \cdot v_w^c$.
• $Acell$ is the surface area of the solar cell ($m ^2$).
• $G$ is the total incident global solar irradiance ($W/m ^2$).
• $\eta_{ref}$ is the nominal electrical conversion efficiency of the cell at standard reference temperature $Tref$ ($25 °C$).
• $\alpha_T$ is the temperature coefficient of power ($\%/ °C$).

Cell Technology	Dominant PID Mechanism	Critical Electrical Parameters	Limits according to IEC 61215 / IEC 62804 / IEC 61643-11	Offshore Dielectric and Operational Impact
p-PERC (Passivated Emitter and Rear Cell)	PID-s (Shunting caused by $Na ^+$ diffusion into p-n junction dislocations).	Severe drop in $Rsh$, degradation of $Voc$ and Fill Factor (FF).	Power loss $< 5\%$ after IEC 62804-1 test ($85 °C /85\% RH$, $96 h$, $\pm 1000 / -1500 V$).	Surface saline film drastically lowers insulation resistance, accelerating dielectric breakdown rates by a factor of $5\times$.
n-TOPCon (Tunnel Oxide Passivated Contact)	PID-p (Surface polarization and structural damage to the ultra-thin silicon dioxide $SiO _2$ tunnel layer).	Reduction in $Isc$ and severe degradation of surface passivation quality.	Strict leakage current limits on SPDs (IEC 61643-11) to avoid transient voltage shifts.	Moisture ingress into the $1.5 nm$ tunnel oxide layer induces parasitic tunneling currents and loss of surface passivation.
HJT (Silicon Heterojunction)	Reversible PID-p caused by charge accumulation within the Transparent Conductive Oxide (TCO) layer.	Temporary loss of optical transmittance and heightened surface recombination.	Requires strict insulation coordination according to IEC 62109-1 and IEC 61643-32.	High electrical conductivity of TCO minimizes PID-s, but electrostatic polarization is exacerbated by accumulated surface salt charge.

Forensic Failure Analysis of Offshore Electrical Infrastructure

Potential-Induced Degradation and severe marine atmospheric exposures do not limit their destructive degradation effects to photovoltaic solar cells alone. The plant's balance of system (BOS) electrical power infrastructure—including step-up power transformers, submarine array cables, medium-voltage switchgear, and protective relays—experiences severe synergistic degradation that threatens electrical safety and operational continuity.

Step-Up Transformers (Inverter to Medium Voltage)

Coupling transformers serving centralized or string solar inverters suffer severe dielectric and thermal stress resulting from common-mode currents and DC voltage components induced by PID asymmetry and SPD residual leakage currents. When multiple PV strings suffer heavy PID degradation, the aggregated DC leakage current to ground ($\sum Ileak$) returns to the inverter through the protective earth ($PE$) conductor system.

This direct current component flows into the low-voltage (LV) windings of the transformer if the inverter topology lacks active galvanic isolation (transformerless inverter designs). The injection of a continuous DC component into the transformer LV winding leads to asymmetric magnetic core saturation. Core saturation results in:

• A massive increase in transformer magnetizing current, which exhibits a highly distorted waveform characterized by sharp current spikes.
• Severe localized hysteresis and eddy current losses in core laminations and structural tank walls, elevating dielectric oil temperatures beyond allowable thermal limits (per IEC 60076-14).
• Accelerated generation of dissolved fault gases (primarily hydrogen, $H _2$, and methane, $CH _4$) driven by thermal breakdown of cellulosic paper insulation and liquid dielectric, detectable via Dissolved Gas Analysis (DGA).

Direct Current Cables and Interconnectors

High-voltage DC array cables (typically stranded copper conductors insulated with flame-retardant XLPE or Low-Smoke Zero-Halogen LSZH compounds) and field quick-connectors (such as MC4 or MC4-Evo2 types) are continuously exposed to saltwater capillary ingress. Under a continuous DC potential stress of up to $1500 V$ relative to ground, accelerated electrolysis and electrochemical corrosion are triggered at insulation discontinuities or improperly crimped connector interfaces.

The simultaneous presence of marine humidity, aggressive chloride ions ($Cl ^-$), and negative electrical potential initiates "water treeing" phenomena within the bulk XLPE insulation polymer. These microscopic water trees, filled with conductive salt solution, gradually propagate under high electric field stress until evolving into "electrical trees." Electrical treeing causes catastrophic dielectric breakdown of the cable insulation, initiating sustained high-energy DC arc faults that are extremely difficult to clear and represent an extreme fire hazard on floating platforms.

Medium-Voltage Switchgear and Switching Devices

Medium-voltage switchgear assemblies (typically operating from $33 kV$ to $66 kV$) housed inside offshore substations suffer dielectric insulation failures caused by conductive salt deposition onto support insulators and primary bushings. High-frequency current harmonics generated by inverter switching actions, combined with unattenuated switching surges resulting from SPDs degraded under IEC 61643-11 stress profiles, trigger surface partial discharge (PD) activity. Surface partial discharges erode the organic polymeric or ceramic insulator surfaces, creating permanent carbonized conductive tracks (tracking) that lead directly to catastrophic phase-to-phase or phase-to-ground short circuits.

Advanced Mitigation Strategies and Engineering Design

Effective mitigation of PID and robust protection of power infrastructure in offshore PV power plants demands a comprehensive engineering methodology spanning material selection, grounding system topology optimization, and active potential compensation technology.

Active Regeneration Systems (Offset Boxes)

The most effective active mitigation approach to reverse and prevent PID-s is the installation of nocturnal potential-shifting generators (commonly designated as Offset Boxes). These active devices continuously monitor system array voltage and, during nighttime non-generating hours (when solar inverters are disconnected and string open-circuit voltage collapses to zero), apply a high positive DC potential (typically between $+1000 V _{DC}$ and $+1500 V _{DC}$) between the DC conductors and the system grounding grid.

This inverted electric field reverses the electrostatic potential gradient across the module layers, forcing sodium cations ($Na ^+$) that drifted into the p-n junction during daytime operation to migrate in the reverse direction, returning back to the EVA matrix and front cover glass. The kinetics of nocturnal ionic recovery can be mathematically expressed using the inverse electro-diffusion transport equation:

Where:

• $JNa$ is the sodium ion flux density.
• $D(T)$ is the temperature-dependent diffusion coefficient of sodium in EVA at night ambient conditions.
• $C$ is the localized concentration of sodium ions.
• $\mu_{\text{Na}}(T)$ is the ionic mobility of sodium, which exhibits an exponential dependence on temperature.
• $Erev$ is the reverse electric field imposed by the active regeneration system.

To optimize power consumption and preserve dielectric integrity, the control loop of the Offset Box must continuously measure the array's overall insulation resistance. Under heavy salt fog conditions, insulation resistance drops dramatically, increasing the current required to maintain the reverse regeneration potential. The control unit must dynamically regulate the output voltage to prevent overcurrent conditions that could overload module bypass diodes or trip protective relays.

Material Selection and Anti-PID Module Design

In the engineering specification stage of offshore PV projects, module bill of materials (BOM) selection constitutes the primary physical line of defense against PID:

• Polyolefin Elastomer (POE) Encapsulant: Standard EVA encapsulant must be replaced with high-grade POE. POE exhibits a Water Vapor Transmission Rate (WVTR) up to two orders of magnitude lower than EVA ($< 0.1 g/m ^2 /day$ compared to $> 10 g/m ^2 /day$ for EVA) alongside a significantly higher volume resistivity ($> 10^{16} \ \Omega\cdot cm$ compared to $10^{14} \ \Omega\cdot cm$ for EVA). This prevents moisture ingress and drastically restricts sodium ion transport, eliminating PID-s susceptibility.
• Sodium-Free Glass or Diffusion Barriers: Engineering specifications should mandate modules constructed with quartz or sodium-free borosilicate glass, or modules utilizing advanced physical barrier layers such as silicon nitride ($Si _3 N _4$) or silicon oxide ($SiO _x$) deposited via Plasma-Enhanced Chemical Vapor Deposition (PECVD) onto the front solar cell surface to block ionic penetration.

Protection Coordination and SPDs According to IEC 61643-11 and IEC 61643-32

To prevent SPDs from contributing to potential shifts or accelerating PID through continuous leakage currents, hybrid surge protection topologies must be specified. Rather than employing SPDs based solely on MOVs—which exhibit inherent continuous residual leakage currents—engineers must specify hybrid SPDs combining a MOV connected in series with a Gas Discharge Tube (GDT) or an encapsulated spark gap, per the application rules set forth in IEC 61643-32 (selection and application principles for SPDs in photovoltaic installations).

The insertion of a GDT in series with the MOV completely interrupts galvanic leakage currents during nominal operating conditions, as the GDT acts as an open circuit with extremely high insulation impedance ($> 10 G \Omega$) until its spark-over voltage is reached. This design stabilizes the DC array virtual neutral point, preventing unwanted voltage shifts that induce PID while providing robust, high-energy surge protection compliant with Class I and Class II requirements under IEC 61643-11.

Power System Simulation and Validation Using Vexten Suite

Analyzing dielectric integrity, PID susceptibility, and short-circuit fault stability across an offshore PV power plant requires sophisticated, high-precision power system simulation software. The following sections detail the comprehensive system modeling and validation performed using specialized modules of the Vexten Suite software package.

Short-Circuit Module (IEC 60909 / IEEE 141)

To evaluate the breaking capacity of offshore substation switchgear and perform insulation coordination, the $66 kV$ AC transmission interface and the $33 kV$ collector network were modeled within the Vexten Suite short-circuit calculation engine adhering strictly to the IEC 60909 standard formulation.

Inverter-based PV sources were modeled as controlled current sources with fault current injection capabilities limited by power electronics controls (typically $1.1$ to $1.5$ times rated nominal current $I_n$). The mathematical model evaluates the worst-case operating condition under three-phase symmetrical faults and single-phase-to-ground faults at the medium-voltage collector busbars.

The short-circuit impedance of the submarine collector network was calculated by modeling positive-sequence resistance ($R_1$) and reactance ($X_1$) parameters at fundamental power frequency ($50 Hz$). The short-circuit calculation results executed in Vexten Suite for a single-phase-to-ground fault on the $33 kV$ floating substation busbar yield the following governing parameters:

Where:

• $c = 1.1$ (voltage factor for maximum short-circuit current calculations).
• $U_n = 33 kV$.
• $R_Q, X_Q$ are the positive-sequence resistance and reactance of the upstream utility grid system.
• $R_T, X_T$ are the impedance parameters of the main $100 MVA$, $220/33 kV$ offshore step-up power transformer.
• $R_C, X_C$ are the impedance values of the export submarine power cable.

The simulation output calculated an initial symmetrical short-circuit current ($I_k''$) of $18.4 kA$ with an aperiodic DC current component ($idc$) exhibiting a decay time constant ($\tau$) of $45 ms$. This necessitates vacuum circuit breakers specified with a minimum rated short-circuit breaking current of $25 kA$ and an appropriate DC asymmetry factor.

Cable Sizing Module (IEC 60287 / NEC 310) with Harmonic Derating

The presence of ground leakage currents driven by PID alongside elevated harmonic currents generated by high-frequency switching inverters mandates rigorous thermal sizing of submarine array cables. Utilizing the cable sizing module of Vexten Suite governed by IEC 60287 standards, the continuous current-carrying capacity (ampacity) of a three-core copper XLPE-insulated steel-wire-armored (SWA) submarine cable buried at a depth of $1.5 m$ below the seabed was evaluated.

The thermal resistivity of the wet seabed sediment was set to $0.8 K \cdot m/W$, and the design ambient seawater temperature was set to $20 °C$. However, due to current Total Harmonic Distortion ($THD_i$) induced by inverter Pulse Width Modulation (PWM), the cable suffers additional Joule losses caused by high-frequency skin and proximity effects.

Vexten Suite applies a dynamic harmonic derating factor ($F_H$) computed via the following formulation:

Where:

• $I_h/I_1$ is the relative current amplitude of the $h$-th harmonic relative to the fundamental component.
• $R_h/R_1$ is the AC resistance ratio of the conductor at the $h$-th harmonic frequency relative to power frequency, computed via Bessel functions integrated within the Vexten Suite numerical engine:

Where $y_s(h)$ and $y_p(h)$ are the skin and proximity effect factors computed for each harmonic frequency. For a typical central inverter harmonic spectrum exhibiting significant sideband switching harmonics ($2.5 kHz$ to $5 kHz$), Vexten Suite computed a harmonic derating factor of $F_H = 0.912$. This required upsizing the conductor cross-sectional area from $185 mm ^2$ to $240 mm ^2$ to ensure steady-state conductor operating temperatures remain safely below the $90 °C$ limit under peak generation, preventing accelerated thermal aging of the XLPE insulation and subsequent water treeing breakdown.

Resonance Mitigation and Frequency Impedance Module

The interaction between the distributed parasitic capacitance of long medium-voltage submarine cables and the inductive impedance of step-up transformers and inverter LCL filters creates a complex, multi-node resonant circuit. If an inverter switching frequency or characteristic harmonic coincides with a system natural resonant frequency, severe harmonic overvoltages and overcurrents will develop, destroying SPDs compliant with IEC 61643-11 and accelerating module insulation degradation.

Using the harmonic analysis and frequency scan module of Vexten Suite, the driving-point frequency impedance response of the $33 kV$ collector network was simulated as viewed from the inverter terminals. The calculated impedance spectrum revealed a sharp parallel resonance peak at $1750 Hz$ (35th harmonic order), where equivalent driving-point impedance exceeded $2500 \ \Omega$.

To eliminate this dangerous parallel resonance, a C-type passive damped filter was designed and simulated within Vexten Suite for installation on the $33 kV$ substation busbar. The optimized filter parameters derived by the Vexten Suite tuning algorithm are:

• Main Capacitance ($C_1$): $1.85 \ \mu F$.
• Damping Resistor ($R$): $120 \ \Omega$.
• Tuning Inductance ($L$): $45 mH$ connected in parallel with an auxiliary capacitor ($C_2$) of $2.15 \ \mu F$.

Post-mitigation frequency scan simulations demonstrated a collapse of the resonant impedance peak to values below $85 \ \Omega$ across the entire spectrum of interest ($50 Hz$ to $5 kHz$). This stabilizes overall operating voltages, reduces background voltage distortion to acceptable levels ($THD_v < 1.5\%$), and completely prevents thermal overloading of SPDs and transient dielectric stress on PV modules, ensuring complete immunity against PID and transient overvoltage failures in harsh offshore marine environments.