Degradation Processes and Mechanisms of PV Wires and Connectors

9 Degradation Processes and Mechanisms of PV Wires and Connectors Sumanth Varma Lokanath 1, Bryan Skarbek 1 and Eric John Schindelholz 2 1 First Solar Inc, Mesa Arizona, United States; 2 Sandia National Laboratories, Albuquerque, New Mexico, United States

9.1 Introduction Photovoltaic (PV) power plants and their constituent components, by virtue of their application, are exposed to some of the harshest outdoor terrestrial environments. Most equipment is subject directly to the environment and myriad stresses (micro and macro environment). Other aspects including local site conditions, construction variability and quality, and maintenance practices also influence the likelihood of such hazards. Many discrete components, including PV modules, combiner boxes, protection devices, inverters, and transformers, make up the PV generation system. The connections between these discrete components are accomplished using PV connectors, PV cables (both above and below ground wires), and wire splices, as shown in Fig. 9.1A and B. Fig. 9.1A illustrates the DC side of a power plant and Fig. 9.1B illustrates the AC side of the power plant (commercial or utility scale). There can be a sizeable quantity (thousands to millions) of connectors or feet of cable that can scale according to the PV power plant size and layout. Wire management devices are yet another family of components used to secure these electrical cables and connectors to mechanical and structural members supporting the PV components. ]Failure of these interconnecting components can affect the generation capacity of the power plant by a factor of the load it is carrying. Of more serious concerns are the safety (e.g., fire and shock) hazards such failures can precipitate. Failures in such components can result in open circuits, short circuits, faults, and leakage driven impacts. Faults can further be classified as ground faults, line faults, and arc faults [1]. Table 9.1 illustrates the potential risk/impact of these failures. A sensitivity study of failure rates completed as part of this study on the DC segment of the system is quite revealing with respect to the impact of failure of

components such as PV connectors, harnesses, and fuses. A wire harness [2] is a preassembled connectorized parallel connection of PV cable lengths and splices. The connectors were segregated into their own category for this study. A change factor of 10 in the baseline assumed failure rate (Rank 5) and its resulting impact on system availability and cost of replacement parts were simulated for a 20 MW DC power plant over 30 years of operation, as illustrated in Table 9.2. The simulated data indicate that beyond the PV module, failure rates in cables, cable splices, fuses, and connectors can be significant destroyers of DC health in that order ranked on the Availability or Spare Part Costs. Assumptions for availability include corrective measures implemented on failure occurrence within a 30-day period. From field data, when partitioning causes of field replacements from the harness, w40% of harness replacements were from connector failures, ~25% from wire failures, ~20% from fuses, and balance w15% from installation issues, which also supports the above simulation findings. Wire management can also influence the failure rates and for simplicity is excluded from the analysis. The failure and remediation of these components can however incur a significant labor cost and expose the overall system to additional risks as discussed later in Section 9.7. Another challenging issue with failures of these components is that of fault localization. Depending on the failure location, only a single PV module string may be impacted, and such outages may not be readily detected and remediated with the monitoring infrastructure in place. The expected energy output of the plant (throughput) can be degraded due to component outages and can be expressed as a ratio of actual to expected, and is defined as the throughput capacity ratio. Fig. 9.2 illustrates the throughput capacity ratio impacted by detection and repair delay days.

Durability and Reliability of Polymers and Other Materials in Photovoltaic Modules. https://doi.org/10.1016/B978-0-12-811545-9.00009-4 Copyright © 2019 Sumanth Varma Lokanath & Bryan Skarbek. Published by Elsevier Inc. All rights reserved. Contribution by Eric John Schindelholz is in public domain.

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Figure 9.1 (A) Illustration of components in the DC side of a PV power plant. (B) Illustration of components in the AC side of a PV system.

Table 9.1 Potential Risk/Impact of Failures Risk Performance

Shock

Fire

System Shut Down

Open circuit

Yes

Possible

No

Possible

Short circuit

Yes

Possible

Possible

Possible

Ground fault

Yes

Possible

Possible

Possible

Line fault

Yes

Possible

Possible

Possible

Arc fault

Yes

Possible

Possible

Possible

Leakage

Possible

Possible

Possible

Possible

Failure

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Connector

Harness (Wire D Splice)

Fuse

Spare Part Costs ($)

Availability (%)

Rank

10

1

1

1

61,893,187

97.4

1

1

1

10

1

6,840,058

99.42

2

1

1

1

10

1,769,962

99.66

3

1

10

1

1

1,162,080

99.84

4

1

1

1

1

1,131,350

99.85

5

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1

0.1

1

1

1,115,021

99.86

6

1

1

1

0.1

1,087,949

99.87

7

0.1

1

1

1

677,779

99.92

8

1

1

0.1

1

643,421

99.89

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Bold highlights the top 3 riskiest components by Rank.

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Table 9.2 DC Field Sensitivity Study Failure Rates

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Figure 9.2 Throughput capacity ratio impact due to fault detection and repair delay times.

Additionally, there could be an impact on module degradation during the time the string is in open circuit. This is seen in all curves except the 180-day curve where it is not a significant factor due to time. Fault detection and remediation should therefore occur in a timely manner to minimize losses. While there are abundant data that illustrate PV modules and PV inverters to be the major contributors of PV system failures, the mentioned data illustrate the importance in minimizing failures in the often ignored components such as PV connectors, PV wires (both above and below ground), wire splices, fuses, fuse holders, fuse holder enclosures, and wire management devices. With the exception of PV fuses, these components predominantly use polymeric materials. Therefore, it is crucial to understand the degradation processes and mechanisms leading to component failure and their impact on system performance or failure. The following sections treat each of these components independently and provide an insight into commonly found and emerging failure modes and mechanisms, considerations, and impacts on overall system health.

9.2 PV Connectors PV connectors are traditionally single pole locking connectors used between the DC carrying parts of the PV system, as shown in Fig. 9.3. These are high voltage, high current, and nonload breaking devices and often subjected to voltages as high as the system

voltages (600e1500 V). Recently with the introduction of tracking structures, other multipole connectors that are low voltage, high current, and nonload breaking devices are also finding use in PV systems. Regardless, all connectors in PV applications are outdoor, exposed over the lifetime of the power plant. There are other connectors used with inverters and enclosed assemblies that are not discussed here. The focus in this section pertains to single pole outdoor exposed connectors used to connect the DC energy within the PV system. Fig. 9.3 illustrates the typical construction of a common locking connector used in PV. The body and endcaps of locking connectors are typically constructed of a UV-resistant polycarbonate. Rubber seals are also common to minimize moisture and dirt intrusion. Pins of these connectors are typically copper with tin or silver plating. The plating provides corrosion protection and reduces contact resistance relative to bare, oxidized copper. Material choice is an important consideration. Typical thermoplastic materials used in the construction of PV connectors are polyamide (Nylons), Poly Vinyl Chloride (PVC), and ThermoPlastic Urethane (TPU). When exposed to the elements, thermoplastics are robust polymers but they remain vulnerable to environmental stressors. Although they are cheap and easy to manufacture, thermoplastics may decompose (or depolymerize) when exposed to heat, radiation, and/or oxidants. As with most polymers, thermoplastics may also experience side reactions between polymers and fillers, which degrade

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Figure 9.3 Typical female and male sides of a PV connector with exploded view of components (male and female).

the mechanical properties of thermoplastics over time. Additives and unreacted monomers in thermoplastics may also migrate or diffuse out of the bulk and surface to cause the loss of mechanical stability and integrity. The severity of environmental degradation of thermoplastic polymers is shown in the spider chart in Fig. 9.4 on a scale from zero to two: “0” signifies the polymer is not vulnerable, “1” indicates the polymer is vulnerable, and “2” indicates the polymer is extremely vulnerable and prone to degradation [3e6]. A collaborative field study conducted by First Solar, Case Western Reserve University and Underwriter laboratories (UL) subjected combinations of PV connectors and wires to accelerated indoor exposures of

cyclic and multistress factors, as well as real world outdoor exposures. A Fourier transform infrared spectroscopy (FTIR) analysis of the accelerated exposure samples indicated polymer degradation processes occurring due to UV exposure impacts. Additional data from this research are pending. Degradation of the connector housing and improper installation can lead to an increase in connector contact resistance. These events can increase transport of moisture and environmental contaminants to the metallic electrical contacts, in turn causing corrosion. Differential thermal expansion and contraction caused by diurnal temperature changes could additionally introduce fretting of the connector plating, accelerating the process [7]. Full or partial demating of the

Figure 9.4 Degradation aspects and severity of thermoplastics.

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contacts may result from mechanical damage as discussed in the following section. Beyond the obvious, though possibly acceptable, costs of ohmic power loss, connector contact degradation can also lead to arc fault events. There have been instances where arc faults related to PV connectors have been documented with their prevention being identified as a critical knowledge gap [8,9]. In one study, connector failure was attributed to causing 29% of fires in surveyed PV system fires, with module (34%) and other BOS components (37%) making up the remainder of fires [10]. Other than a possible root cause, the risk of arcfault from connector corrosion remains unclear. A recent study found commercial connectors to be relatively resilient to several types of laboratory corrosion tests, including damp heat (85  C/85% RH) with sea salt contaminants. The increase in contact resistance observed during testing was attributed to corrosion but was relatively insignificant with respect to arc fault risk, as shown in Fig. 9.5. The authors state that further work is needed to capture and accelerate contact degradation seen in the field and quantify its impact on arc faults [8].

9.2.1 New Failure ModesdWithdrawal Force The cable to connector interface is subjected to multiple mechanical stresses during the lifetime of 4.5 Type 2 Type 3 Type 1

Resistance (mΩ)

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the product. One major failure mode of this interface is the cable pulling out of the connector body, which could potentially lead to a safety hazard (shock or fire). The force at which this occurs is named the withdrawal force. Steps should be taken to ensure that the withdrawal force is high enough to withstand the stress that the connector could see in the field. Some possible causes that may result in the cable being extracted from the connector body result from higher than normal forces due to the added weight of the bundled harnesses. Examples include: 1. cable ties or other means of supporting large bundles of cable failing with the connector taking this additional load; 2. snow/icedif the cable is locked in place by snow or ice and the tracker starts to track, this could put high stress at the connector to cable interface; and 3. improper wire managementdif the cable is not properly routed, it can get hung up on moving parts. The locked connector release force test ensures that in the event of a wire management failure, the PV connector will release at the male to female interface instead of the cable pulling out of the body of the connector. Ideally, the locked connector release force should be greater than 89 N (20 lbf) but less than the withdrawal force. If samples are successfully tested (tensile testing) to these criteria, this will lower the risk of having exposed conductors in the field.

9.2.2 Considerations for PV Connector Factory versus Field Assembly

3.5 3.0 2.5 2.0

AND

0

2000

4000 6000 8000 Elapsed Time (hours)

10000

Figure 9.5 Resistance measured across three models of mated commercial quick connectors during damp heat (85  C/85% RH) testing. Connector pins were contaminated with sea salt (dotted line), simulated desert dust (dashed line), or left in asreceived condition prior to testing. From Schindelholz (2014).

Field failure data from several installations for w1.9 GW DC for 1 year are plotted in Fig. 9.6. These data are within the initial life period of the system (1e3 years). As seen, connector failures are the second item on the Pareto chart after fuses, with an order of magnitude lower. The failure rate is in the 0.01%e0.1% order of magnitude. These low numbers are attributed to controlled factory assembly of connectors. The assembly of connectors onto cables occurs in a factory setting under controlled conditions with a formal and robust quality assurance program.

M ECHANISMS

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6

3

2

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1

1

r H C B fu se ar ne Fu ss se ba H d ol de r D H is C co B N nn ot ec Te tio rm n in at ed in ... Fe ed er

e ul

ba

od

H

M

C

p W

hi

d Ba

14

d

4

20

O th e

33

M

450 400 350 300 250 200 150 100 50 0

AND

Pareto of “Harness and HCB” Failures over 1 Year

417

IL F

Quantity

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Figure 9.6 Pareto of “harness and HCB” failures over 1 year.

Table 9.3 illustrates the critical considerations for quality control in the connector assembly process, and its impact on failure modes. Such considerations can be significant control factors/variables. There are human factors issues to consider in a field installed environment that introduce a wider variance of noise into control factors/variables as illustrated in Table 9.3. For example, manual installation of connectors is repetitive (hand fatigue) and error prone (wasted and faulty parts). Crimp quality variation is difficult to control and dust and debris can influence connection quality or reliability. Further, detection of bad quality assemblies is not possible in a field setting. Therefore field assembled connectors are inherently expected to demonstrate higher failure rates and can potentially lead to increased open circuit, exposed live parts (shock hazard), excessive leakage current (shock hazard),

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and high resistance/arcing (fire hazard). The authors of this chapter estimate the field assembled PV connector failure rate in the order of approximately 2%e5% as surveyed by O&M practitioners. Therefore, factory assembly of PV connectors (PV source and branch circuits) is strongly recommended. Alternatively, strict control of assembly in the field is necessary, such as proper use of tools, calibration of equipment, and training certification. If field replacement of factory-assembled connectors is necessary, the authors suggest considering field assembly of preassembled connectorized ends with butt splices (see Fig. 9.7). Crimping a butt splice is simpler (see Fig. 9.8) and reliable than crimping and assembling PV connectors with multiple parts in the field. Further, an approved insulation material can be easily applied to insulate the splice. The long-term durability of heat shrink (see Fig. 9.9) is an important consideration. Alternatively, an additional cover may be considered in addition or in lieu of heat shrink tubing. Finally, proper wire management and strain relief for spliced joints is another aspect to address in order to ensure reliability.

9.3 PV Wire (Above Ground) Chlorinated Polyethylene (CPE) and HighDensity Polyethylene (HDPE) are thermoplastic materials whereas Ethylene Propylene Diene Terpolymer (EPDM) and Ethylene Propylene Rubber (EPR) are elastomers; Cross-Linked Polyethylene

Table 9.3 Summary of Critical Considerations in Connector Assembly Process Requiring Quality Control Measures Result Steps in the Assembly Process that Need Special Quality Control

Too Short/Low

Too Long/High

Strip length

Pins cannot be fully inserted into the body

Exposed conductor/Excessive leakage current

Crimp height

Deformation of metal components

Low withdrawal force

Fire risk/high resistance; low withdrawal force

NA

Fire risk/high resistance

NA

Excessive leakage current

Excessive leakage current

Dirt and moisture can lead to corrosion on internal parts

NA

Pin fully inserted into the body Mating area across pins Torque on cap nut Cleanliness of components

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Figure 9.7 Illustration of a preassembled connectorized end with butt splice for field use. A single crimp in field using standard tools.

Figure 9.8 Illustration of a butt splice connector with one end crimped in factory.

(XLPE) is a thermoset. Polymers used in the construction of PV wires typically use EPDM, XLPE, or EPR/EPDM composite materials for the insulation and CPE or composite HDPE/XLPE for the sheathing or jacket. Thermoplastics are already described in Section 9.2. Elastomers may depolymerize when exposed to heat, radiation, and/or oxidants. Elastomers and especially natural rubbers are more prone to corrosion than any other class of polymers and they are particularly vulnerable to ozone, moisture, bacteria, and fungi. Rubbers are also extremely flammable. As

Figure 9.9 Heat shrink applied to cover the butt splice connector in field.

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with most polymers, elastomers may age mechanically over long periods and degrade by experiencing phase segregation or side reactions that are caused by polymer chain scission or crosslinking [11,12]. Thermosets are relatively more resilient to the elements when compared to thermoplastics and elastomers. Thanks to their three-dimensional network of covalent bonds, thermosets exhibit great mechanical and thermal stability when exposed to the elements. Despite their thermo-mechanical stability, thermosets are extremely prone to oxidation, photolysis (radiation-induced depolymerization), and side reactions with fillers and additives. Thermosets are prone to physical and mechanical aging over long periods and may experience phase segregation to some degree as mass diffusion paths may be hindered by crosslinks [13,14]. Polymer composites are mixtures of different polymers in a single-phase material. Composites typically experience hybridized properties that originate from their constituent polymers. Blending polymers may not only be used to enhance their mechanical and chemical stability but they may also be used to reduce the cost of polymer components, as is the case with EPR/EPDM. They both have a similar structure but different properties. EPDM is more durable against heat, oxidation, and mechanical aging than EPR, thanks to its saturated bonds, but EPDM is also much more expensive than EPR. Hence by mixing EPDM and EPR, a trade-off is made between enhanced properties and reduced costs. With the HDPE/XLPE combination, a thermoset and thermoplastic give the best of both worlds, and mixing low-cost HDPE with high-cost XLPE balances the price of components. Although composites and nanocomposites are typically better performers than their constituent polymers, they are still vulnerable to environmental degradation and may depolymerize when exposed to heat, radiation, or an oxidizing environment. Composites are especially vulnerable to moisture, bacteria, mechanical aging, phase segregation, and side reactions between the different constituent polymers, additives, fillers, and substrates. The severity of environmental degradation of elastomeric polymers is shown in Fig. 9.10 on a scale from zero to two, from least to most severe [15e20]. There is no right or universal solution in selecting polymers for PV components as the decision depends mainly on the physical and chemical composition of the environment at hand. Rubber composites such as

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Figure 9.10 Degradation aspects and severity of polymers used in PV cables.

EPR/EPDM may be a cost-effective solution in dryer environments with high UV radiation such as the Arizona desert. Other composites such as HDPE/ XLPE may be a great choice for components in humid environments, with slightly less UV radiation such as the Dubai desert. Polymer material selection must be designed around the presence of UV photons (primarily), oxidants, heat, and long-term mechanical aging, which are typically the main modes of polymer degradation.

9.3.1 New Failure ModesdUV Robustness UV robustness in sunlight-exposed cables is commonly achieved utilizing additives of carbon black and this provides the black color for the cable. However, there is also a trend where the UV robustness is alternately provided by addition of light stabilizers such as Tinuvin and a coloring agent is added to give the black color or any alternative color desired by the customer. There are limited field reported data that the latter form of cables sees premature cracking failures in the field. The dosing quantity of light stabilizers may be a contributor. A lower flexibility conductor may further exacerbate the issue.

9.3.2 New Failure ModesdSlip Force Cables from end of PV module strings or a parallel combination of strings leading to combiner boxes are called as PV subarray cables. PV subarray cables are often terminated in fuse holders within combiner boxes. Such PV subarray cables may also transition from above ground to below ground using connectorized distinct sections with the other end terminated directly at a fuse terminal in a combiner box. Thermal cycling stress from diurnal temperature changes may result in the shrink-back of the insulation, which has been found on some PV cables. This shrink-back is the result of the cable insulation “walking” over the cable due to these thermal cycles. Shrink-back can potentially be a safety hazard or can lead to a shock or thermal event if the cable is terminated to a fuse holder or terminal block and the insulation creeps back far enough to expose a large area of the conductor as shown in Fig. 9.11. If the exposed conductor makes contact with grounded metal or conductors of opposite polarity, this could result in arcing. Preliminary data indicate that some insulation materials, such as XLPE, are more susceptible to shrink-back than other materials when

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Figure 9.11 Cable shrink-back exposing bare conductor on fuse terminals.

exposed to thermal cycling. Further research is necessary to determine if this failure requires to be alleviated by altering material properties, but at this time there has been no major field failures reported due to shrink-back.

9.3.3 New Failure ModesdRoundness of Cables (Filled vs. Unfilled) Multiconductor cables are commonly used for communication signals on tracker systems in PV plants. One of the biggest challenges is preventing moisture and dust ingress at the connector to cable interface on these harnesses due to the roundness of the cable. Connectors typically use a gland with a back-nut that is torqued to the manufacturer’s specifications. The proper torque value is critical to ensure that a uniform clamping force is applied to the gland. If an unfilled cable is used, the shape of the outer diameter is more of an ellipse shape instead of it being round. This allows voids between the gland and the insulation of the cable that can allow moisture ingress. The fix is to use a filled cable, which maintains a symmetric cable shape and prevents voids between the gland and the insulation of the cable.

9.3.4 New Failure ModesdCable Subjected to Flexing Degradation of the cable and conductor insulation can occur due to cyclic bending or flexing in the field especially when installed on PV trackers. PV trackers go through cyclic movement every day and can

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further be impacted by random cyclic movements from high wind effects. Cables and assemblies should be carefully chosen to be robust to such stresses as polymeric materials are impacted and may possibly result in leakage, shock, and fire hazards. Further laying cable on sharp edges, pinch points, and introducing twists in cable leads to the fatigue of the conductor and shield in the flex area. Fatigue of the conductor and shield at the point of termination (stress at the termination point) may lead to individual stands of the cable failing, or the grounded shield becoming isolated from the ground. The appropriate flexibility classification should be considered when installing cable sections on tracker systems or other systems with moving part as described in the next section. The choice of material type in the application also matters. EPR insulation may be suited better than XLPE for hot environments, and where flexibility is critical. Further, additional steps can be taken to address this issue, such as conducting mockups and detailed reviews prior to the field rollout, through proper training during first installations, and verified by evaluation through compressed time Accelerated Life Testing (ALT). Finally, all the learning is to be documented in detailed wire management drawings and installation instructions.

9.3.5 Approach for Determining Bend RadiusdVarious Competing Sources A key failure mode for wires is the insulation degradation from stress caused due to wire management or the lack of it. Wire management requires that best practices and measures for installation be mandated, such as avoiding: needless connections, sharp twists in cable, transitioning/laying the cable over rough/sharp edges, tight spaces, thermal expansion, and choosing cables with the appropriate flexibility class. The flexibility of the cable is a critical consideration depending on the application scenario as illustrated in IEC 62440 [21]. There are differences in the minimum bend radius criteria which lead to the dilemma of which one to choose. The cable manufacturers may have their own compliance requirements for bend radius to assure warranty and service life. Table 9.4 provides a rational waterfall-based approach to use for demonstrating compliance. It is important to note that PV connector manufacturers may also impose bend radius criteria on

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Table 9.4 Summary of Wire Management Guidance and Condition Based Priority Priority

Bend Radius Governance

Guidance

Reference

1

Use manufacturer guidance

Use this, especially if contingent to secure warranty/service life per manufacturer

Use dynamic for tracker use

1

International Electrotechnical Commission

IEC 62548:2016 Section 7.3.7.3 [21]

Manufacturer guidance

2

National Electric Code

NEC 338.24 for USE-2 cable type

5  OD

2

International Electrotechnical Commission

IEC 62440 Section 5.6 Table 9.3 a [21] 4  OD (free movementdmodule to module) 6  OD (mechanical loaddend of module string)

4  OD 6  OD

a

Application should also consider a fixed tilt use versus a tracker installation. While IEC 62440 [21] applies for lower voltage cables (450e750V), a similar approach of implementation is reasonable for higher voltage PV cables.

cable transitions terminating inside connectors. It is also equally important to consider maintaining the minimum bend radius of Outer Diameter (OD) of cables in packaging, shipment, and handling of cables or premanufactured harness assemblies, from the supplier all the way to the installation site. Flexibility class is interpreted differently in various regions. The IEC 60228 [21] standard defines flexibility class by the cross section of the conductors, whereas within the United States flexibility class is governed by the strand count. The vast majority of systems in the United States use a 19-strand cable and there have been no reported gross failures with this cable. US installations also use a single insulated cable whereas a double insulated cable is required within the IEC countries. There has been recent trend to require flexibility class 5 for PV cables. While the authors believe there is justified use for special requirements, over generalization of such requirements will needlessly increase the costs of PV cables. Fixed installations and even flexible areas where the cables are managed and supported do not require Class 5 cables. Class 5 cables are recommended in areas subjected to unrestrained loading/twisting or movement such as tracker to fixed transitions as described in IEC 62738 Section 7.3.4.4 [21].

9.4 PV Wire (Below Ground) Deterioration or damage to wire insulation of buried cabling could lead to leakage current through the soil that is capable of corroding metallic PV module mounting systems and nearby third-party infrastructure (e.g., pipelines). This risk, especially for large PV plants with kilometers of buried cable, has recently been brought to attention [9,22]. In PV installations, the leakage current loop will flow through the ground via parallel paths of least resistance. Metallic structures buried or in contact with the ground can serve as preferential least resistance paths, given their conductive nature, and conduct this current. At each point where current leaves the metallic structure to earth on this path enhanced corrosion can occur (Fig. 9.12). Although PV systems incorporate ground fault detection, Charalambous and coworkers argue this leakage current could go undetected [22]. For example, in a 300 kW grounded PV system, the fault current detection threshold to prevent catastrophic failures could be as high as 5 A. Although documented cases of PV stray current corrosion are lacking in the literature, calculations by Charalambous and coworkers suggest that it should be taken into consideration during design and operation over 25 year

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Figure 9.12 Hypothetical schematic of leakage current from cabling causing stray current corrosion in metallic pipeline [23].

lifetimes of commercial PV systems [22]. Furthermore, stray current corrosion is a common degradation mode of buried infrastructures, such as pipelines, especially in the area of linear electrified rail lines [23].

9.4.1 New Failure ModesdTermite Resistant Cables A termite resistant cable is required in some jurisdictions for any direct burial cable. The most common methods used to achieve “Termite Resistant” classification are to either chemically treat the insulation of the cable or to add a Teflon or Nylon material between the conductor and the insulation of the cable. There are concerns to address with both methods. The chemically treated cable requires wearing gloves when handling the product and can put off a strong odor when storing the cable indoors or in shipping containers. Airing of this material is recommended to give the odor time to dissipate and not affect the installation crews. On the other hand, the Teflon or Nylon material can reduce the insulation slip force and allow the insulation of the cable to slide on the conductor. This could potentially be a safety hazard if the cable is terminated to a fuse holder or terminal block and the insulation creeps back far enough to expose the conductor. If the exposed conductor is in close proximity to the grounded metal or conductors of opposite polarity, this could result in arcing and possibly a thermal event. An approach to arrest the cable from the slip force is to utilize the preassembled connectorized

ends with butt splices as described earlier in Section 9.2.2.

9.5 Wire Splices and In-Line Fuse Holders A wire harness is a factory assembly component that aggregates the output of multiple PV string conductors along a single main conductor. Wire splices and in-line fuses are components of wire harnesses; they are typically constructed of an overmold and an under-mold material. The over-mold material must be robust enough to withstand the environmental stress that it will be exposed to for 25 years. Cold impact and UV exposure tests are commonly used to determine if the polymer can withstand the time. As for the under-mold, it is critical that the material makes a good bond to the insulation of the cable to ensure there is no moisture or dust ingress into the component. A presence of moisture within the joint or in-line fuse can lead to corrosion or excessive leakage current. Currently, the UL9703 standard covers such assemblies. The authors found that the IEC 61215 [21] damp heat exposure test was found to identify field correlated failure modes that the UL standard does not address. Below are further recommendations for inclusion in the technical specification requirements for wire splices and in-line fuse holders:

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1. A force of greater than 200 N is needed to disconnect cables from wire splices. 2. It should be ensured that dissimilar galvanic metals are not used in wire splices that would form a galvanic couple. 3. The crimp should be rated for both aluminum and copper conductors when splices interface the two. 4. The choice of splice design and construction must consider typical stresses in packaging, shipment, installation, and operation, such as torsional bending, strain from load, and current ampacity. Solder or weld by itself must not form the primary mechanical connection; an alternate mechanical securement should be provided in addition to solder. 5. Heat shrink tubing alone must not be used as primary insulation due to the multicable construction unless appropriately chosen for the design and end use environment. 6. The over-molds should be capable of accepting the cable dimensions and durometers used for the cables to which they are fitted. 7. Cavities should be avoided in splice overmolds. All cavities (e.g., pin location points and windows) must be potted with insulation rated for the environment and additionally form a rigid seal preventing pollution ingress paths. 8. The minimum over-mold coverage length for each wire in splice must be dimensioned to protect insulation damage against the expected bend radius range (temporary and sustained) in normal application. 9. The splice insulation must have a voltage rating equal to, or greater than, the PV system voltage and have a dielectric withstand voltage rating equal to, or greater than, twice the PV system voltage plus 1000 V. 10. The splice insulating material must have a temperature rating of 40 to 90 C. 11. The wire splices, if exposed to the environment, must be rated for outdoor use, be UVresistant, and have a minimum rating of IP65 (IP68 is recommended). 12. Wire splices and other components of a wire harness must prevent moisture ingress.

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In addition to the product specifications for the construction of components within wire harnesses, there should also be installation instructions that provide guidance on 1. the maximum/minimum bend radius of cables going into wire splices during packaging/ handling, 2. recommended best practices for unpacking and handling during installation, 3. the maximum/minimum bend radius of cables going into wire splices during site installation, and 4. wire management recommendations to prevent damage to wire harness.

9.6 Combiner Box A combiner box is generally a metal enclosure that houses components such as terminals, fuse holders, disconnect switches, surge suppression, metrology, etc. Polymeric materials are often used in these components, and are subject to degradation mechanisms resulting in failures. Doors of combiner box enclosures use rubber gaskets. The gasket materials are prone to mechanical and thermal degradation. Aging allows water ingress into the junction box and can significantly affect the ingress properties of the box. Multiple replacements may be necessary over the lifetime of the plant. Material selection and service lifetime prediction are important considerations. Combiner boxes utilize disconnect switches which expose the polymeric handle to the elements outside of the box. Such handles are prone to UV, humidity, and thermal degradation. Again, material selection and service lifetime prediction are important considerations. Combiner boxes have multiple termination points that are prone to thermal events if the hardware is not properly torqued or the equipment used in the design is not rated for its intended use. Equipment such as fuse holders, terminal blocks, and disconnect switches must be rated for use with other components in the design, and tested as such. Using products outside of their intended use may result in field failures even if they passed testing during the initial qualification. An example of this is using a fuse holder rated for use with PV cables,

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but in the application, a buss bar is used to collect the power. The profiles of the terminals on the fuse holder are designed to increase the contact area of the round cable, but the comb profile has square contacts that engage in the fuse holder terminal. This decreases the surface area of the contacts. The stress from thermal cycling in the field may allow the torque on the hardware to back off over time and increase the contact resistance, eventually leading to a thermal event as shown in Fig. 9.13. Loosening of fasteners over time can lead to catastrophic failures, and the choice of insulation materials can impact a terminal’s thermal robustness to torque loosening issues. A thorough design review and lab testing can catch these failures during the qualification stage. It is recommended to subject samples of combiner boxes to an Accelerated Life Testing (ALT) of thermal cycling test from 40 to 55 C for 1000 cycles. In addition to the thermal cycling preconditioning, power cycling the unit during the high and low temperature dwells will give a true representation of field conditions as illustrated from an experiment conducted by authors. Post thermographic imaging and resistance measurement tests will identify any hot spots that may result in an increase in contact resistance. Fig. 9.14 shows the temperature rise of terminals within a combiner box that were not torqued to the correct value with temperature and power cycling.

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9.7 Wire Management Devices Millions of feet of wires can span the length and width of the structures supporting the PV modules in a power plant. Wire management of such cables becomes important and in most cases it is also governed by requirements in codes and standards. Polymeric materials of the Nylon type are commonly used for wire management products such as wire ties, clips, and tethers. Nylon 6 and specialized blends of Nylon 6 called Nylon 6.6 are typically used for cost effectiveness. Such materials are susceptible to stress corrosion cracking (SCC) when applied on galvanized steel structures. Run off Zn from the galvanized steel [24] is one of the leading drivers of salt embrittlement failure mode in Nylon 6. Results indicate that a critical Zn ion concentration window of 0e2000 ppm of Zn is required to facilitate this failure. A screening test utilizing ZnCl was developed [24]. Additionally, due to regulatory policies, site preparation and site management chemicals such as surfactants, dust inhibitors, and weed inhibitors are often utilized. The polymeric material to be used on a PV site should be verified to be robust against the exposure to such site chemicals at the concentrations typically used on site. Lastly, acid rain and naturally occurring salts are additional considerations. First Solar utilizes a custom test profile called as Universal Chemical Exposure Test (UCET) developed by its

Figure 9.13 Fuse holder overheating on a bus bar interface.

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Thermal Measurements vs. Power / Temperature Cycles 400

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Cycle Temperature C

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1 191 381 571 761 951 1141 1331 1521 1711 1901 2091 2281 2471 2661 2851 3041 3231 3421 3611 3801 3991 4181 4371 4561 4751 4941 5131 5321 5511 5701 5891 6081 6271 6461 6651 6841 7031 7221 7411 7601 7791 7981 8171 8361 8551 8741 8931

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Figure 9.14 Temperature rise of a terminal increases after multiple temperature/power cycles.

Reliability Engineering team to validate polymers against such field occurring chemicals. Fig. 9.15 illustrates that the Nylon 12 material has a lower variability response to the max failure loads in tensile testing post environmental exposures as identified by sequences. Also all failures for Nylon 12 material were attributed to Tensile Load (TL) failures in contract to Nylon 6.6 where failures from Chemical Exposures (CE) were also observed. Subsequently, based on testing as illustrated in Fig. 9.15, Nylon 12 has a better performance to chemical robustness, temperature variations, and high humidity conditions [24]. The initial upfront cost is a major decision factor when procuring the millions of cable ties that are

installed in PV plants. However, the authors contest that with labor accounting for more than 97% of the cost to install or replace cable ties in the field, it is more cost effective to pay the higher upfront cost for a product with a longer lifespan. When the authors compared the total cost of ownership over the 25year plant life, paying the higher upfront cost for a more robust cable tie pays off. The cost of Nylon 12 cable ties is three times higher than that of Nylon 6.6 ties but Nylon 12 are expected to last twice the life expectancy than Nylon 6 versions. Therefore, a single replacement cycle can satisfy the design lifetime versus three necessary replacements for Nylon 6 variants. First Solar has switched over to using the superior Nylon 12 as the specified material on its sites because of these findings. The authors strongly urge the PV industry to use Nylon 12 over Nylon 6.6 for PV wire management applications for lifetime cost, performance, and sustainability. PVDF wire ties are currently under development, which may only require a single installation to meet the 25-year design lifetimes of PV power plants.

9.8 Conclusion

Figure 9.15 Variability plot of wire tie tensile load performance post exposure to test sequence.

In summary, this chapter describes the typical materials used in components, the observed degradation processes and mechanisms leading to component failure, and its impact on system performance or failures. It further provides some practical considerations, approaches, and methods in

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addressing the problems with practical solutions in the design to assure the performance of the PV plant over the intended design lifetime.

Acknowledgment The authors (Sumanth Lokanath, Bryan Skarbek, and Eric Schindelholz) would like to acknowledge the valuable contributions of Mr. Rajan Bedi, Mr. Sundar Subramanian, Mr. John Pickens, and Mr. Paul Williams from First Solar for their contributions with root cause of field findings; Dr. Mounir El Asmar and Mr. Georges Nassif from Arizona State University for their contributions in the review of various polymers and corresponding failure mechanisms and developing the spider charts; Dr. Roger French and Mr. Timothy Peshak of Case Western Research University and Mr. Liang Ji from Underwriter Laboratories for their ongoing work on PV connectors and wires. This work was in part supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DENA0003525.

References [1] A. Chokor, M.E. Asmar, S.V. Lokanath, A Review of Photovoltaic DC Systems Prognostics and Health Management: Challenges and Opportunities - Annual Conference of the Prognostics and Health Management Society, 2016. [2] Harness Assembly as Defined in Standards UL9703, IEC 62780 [12]. [3] Kambour, A review of crazing and fracture in thermoplastics, Journal of Polymer Science Macromolecular Reviews 7 (1973) 154. [4] Allen, Chirinis-Padron, Henman, The photostabilisation of polypropylene: a review, Polymer Degradation and Stability 13 (1) (1985) 31e76. [5] D. Gavrila, E. Gosse, Post-irradiation degradation of polypropylene, Journal of Radioanalytical and Nuclear Chemistry 185 (2) (1994) 311e317.

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[6] S. Satoto, T. Yusiasih, Watanabe, Hatakeyama, Weathering of high-density polyethylene in different latitudes, Polymer Degradation and Stability 56 (3) (1997) 275e279. [7] Y.W. Park, T. Sankara Narayanan, K.Y. Lee, Fretting corrosion of tin-plated contacts, Tribology International 41 (7) (2008) 612e628. [8] E. Schindelholz, et al., Characterization of fire hazards of aged photovoltaic balance-of-systems connectors, in: Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd, IEEE, 2015. [9] A. Demetriou, et al., Stray current DC corrosion blind spots inherent to large PV systems fault detection mechanisms: elaboration of a novel concept, IEEE Transactions on Power Delivery (2016), 2016. [10] M. Kontges, S. Kurtz, C. Packard, U. Jahn, K.A. Berger, K. Kato, T. Friesen, H. Liu, M. Van Iseghem, Review of Failures of Photovoltaic Modules, International Energy Agency, 2014. [11] A.K. Bhowmick, H.L. Stephens, Handbook of Elastomers, second ed., M. Dekker, New York, 2001. Print. “Plastics Engineering” (Marcel Dekker, Inc.) ; 61. [12] S. Deuri, Bhowmick, Ageing of rocket insulator compound based on EPDM, Polymer Degradation and Stability 16 (3) (1986) 221e239. [13] J.-P. Pascault, Thermosetting Polymers, Marcel Dekker, New York, 2002. Print. Plastics Engineering (Marcel Dekker, Inc.) ; 64. [14] S. Mitra, Ghanbari-Siahkali, Kingshott, Rehmeier, Abildgaard, Almdal, Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part I. Effect on accelerated sulphur crosslinks, Polymer Degradation and Stability 91 (1) (2006) 69e80. [15] N. Petchwattana, S. Covavisaruch, S. Chanakul, Mechanical properties, thermal degradation and natural weathering of high density polyethylene/ rice hull composites compatibilized with maleic anhydride grafted polyethylene, Journal of Polymer Research 19 (7) (2012) 1e9. [16] R. Rothon, R.N. Rothon, Rapra Technology Limited Content Provider, Particulate-filled polymer composites, second ed., iSmithers Rapra, Shrewsbury, 2003. [17] Osawa, Sunakami, Fukuda, Photodegradation of blends of poly(vinyl Chloride) and polyurethane, Polymer Degradation and Stability 43 (1) (1994) 61e66.

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[18] O. Haillant, Spectroscopic characterization of the stabilising activity of migrating HALS in a pigmented PP/EPR blend, Polymer Degradation and Stability 93 (10) (2008) 1793e1798. [19] Choudhury, Bhowmick, Ageing of natural rubber-polyethylene thermoplastic elastomeric composites, Polymer Degradation and Stability 25 (1) (1989) 39e47. [20] R.M. Jones, C.W. Bert, Mechanics of composite materials, Journal of Applied Mechanics 42 (3) (1975) 748. [21] Standard from the International Electrotechnical Commission [IEC].

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[22] C.A. Charalambous, et al., Impact of photovoltaic-oriented DC stray current corrosion on large-scale solar farms’ grounding and third-party infrastructure: modeling and assessment, IEEE Transactions on Industry Applications 51 (6) (2015) 5421e5430. [23] I. Cotton, et al., Stray current control in DC mass transit systems, IEEE Transactions on Vehicular Technology 54 (2) (2005) 722e730. [24] Lokanath, Skarbek, Ramanathan and Seidel. “Considerations and structured approach for selecting and deploying climate specific polymeric wire management means” IEEE PVSEC 44.