All solar cells generate direct current (DC) and require conversion to alternating current (AC). A photovoltaic power source of a certain capacity requires thousands of unit modules, each connected in a regular manner; similarly, the structure of a PV cable includes many insulated wires. Early research on photovoltaic cells focused primarily on components and their combinations; currently, research on PV cables has also been prioritized. Over the past decade, the global photovoltaic power generation market has experienced rapid growth, but there are still no international standards for photovoltaic cables. Therefore, existing standards and tests need to be adjusted, such as life cycle testing, and particular attention should be paid to the aging problem of cables exposed to strong ultraviolet radiation for extended periods.
Photovoltaic cells range in size from small to large. Small PV systems for residential rooftops typically have a peak capacity of several kilowatts (kWp - Kilowatt peak), medium-sized industrial PV systems have a peak capacity of several megawatts (MWp), and large-scale power plant PV systems reach a peak capacity of 2 gigawatts (2GWp). In 2008, the global market for newly installed PV systems was 5.6 GW, and most predict an annual capacity of approximately 8-10 GW. PV cables are typically considered only for the DC portion of the system, excluding grid connections, system control cables, and other connecting wires. The relationship between PV cable and PV installation capacity is roughly as follows: 1 MW is approximately equivalent to 40-60 km of PV cable. This is directly related to the arrangement and number of crystal and thin-film modules. The connection principle is shown in Figure 1, where the converter is not shown.
PV cables are exposed to sunlight for extended periods, are constantly submerged in water, and are subject to rapid temperature fluctuations, making cable lifespan a primary concern for users. Because there are no international standards (such as IEC standards), manufacturers feel they lack a basis for action, leading to debates about the quality and lifespan of PV cables during their development. However, most experts agree on these points. UL-4703, "Investigation Outline for Photovoltaic Cables" (and related standards and benchmarks), published and distributed by UL, and 2Pfg-1169, "Requirements for Cables Used in Photovoltaic Systems" (and related standards and benchmarks), printed by TÜV (German Bureau of Technical Inspection), are currently the main reference documents and may become the universally accepted standards in the future.
The cable conductors are made of very fine tin plating, conforming to IEC 60228 class, with cross-sections mostly of 4.00 mm² and 6.00 mm², consisting of 56 and 84 single wires respectively; the insulation sheath conforms to TÜV and UL requirements, and thermoplastic or cross-linked insulation can be used, which are standard specifications, with an insulation thickness of 0.5 mm.
Cable sheaths need sufficient mechanical strength to prevent damage during installation. They also need to be resistant to rodents and termites, sometimes necessitating the addition of a steel wire braid layer (as armor). The DC portion of a PV system is ungrounded, and some people object to metal braiding, which also presents corrosion problems.
Regarding the electrical performance of photovoltaic cables, the author believes that high requirements are not necessary. Cables used in the 1990s, after a short period of use, developed ring-shaped cracks in the sheath, with the cracks being more pronounced near the junction box. This suggests that the cables at that time were of poor quality, perhaps due to the high temperature around the junction box, possibly caused by thermal stress.
The protective fluid should be resistant to various chemicals, such as photovoltaic cell cleaning fluid, lubricating oil, acidic gases, acid rain, salt spray, mold, and other grease and fumes.
From an investment and return perspective, a 25-year lifespan for PV cables is considered a reasonable requirement. However, using existing testing standards may not yield such a precise conclusion. As PV cables are becoming increasingly common in households, their quality presents certain uncertainties compared to other industrial cables. To date, the exact guaranteed lifespan remains a subject of debate in both design and manufacturing. The cable sheath needs sufficient mechanical strength to prevent damage during installation. Additionally, protection against rodents and termites is necessary, sometimes necessitating the addition of a steel wire braid layer (as armor). The DC portion of a PV system is ungrounded, and some object to metal braiding, which also presents corrosion challenges.
Regarding the electrical performance of photovoltaic cables, the author believes that high requirements are unnecessary. Cables used in the 1990s, after a short period of use, developed annular cracks in the sheath, with more pronounced cracks near the junction boxes, suggesting that the cables at that time were of poor quality. This may be due to the high temperature around the junction boxes, possibly caused by thermal stress. The sheathing fluid should be resistant to various chemicals, such as photovoltaic cell cleaning fluid, lubricating oil, acidic gases, acid rain, salt spray, mold, and other grease fumes. Considering investment and return, a 25-year lifespan for PV cables should be considered a reasonable requirement. However, using existing testing standards may not necessarily yield such a precise conclusion.
As PV cables are becoming household everyday items, there are certain uncertainties in their quality compared to other industrial cables. To date, the debate over the guaranteed service life remains unresolved in both design and manufacturing.
