Solar photovoltaic (PV) projects require a high-quality cabling system that connects all the electrical components together with minimum loss in energy. Unlike cables for wind projects, solar cables have to be weather and ultraviolet resistant as they are usually installed outdoors and are subject to direct sun radiations and air humidity. Further, solar cables have to withstand both high and low temperatures and be usable within a temperature range of 40°C to 90°C. They also have to withstand mechanical stress from pressure, bending or stretching as experienced during installations as well as chemical stress in the form of acids, alkaline solutions and salt water.
In a typical solar PV set-up, direct current (DC) cabling is provided between solar PV panels and inverters (including junction boxes), and alternate current (AC) cabling between the inverter and the substation. Low-voltage power cables are used from the panel to the combiner box, from the combiner box to the inverter, and from the inverter to the transformer. Medium-voltage power cables are used for transferring electricity from the transformer to the substation, and high voltage and extra high voltage ones from the substation to the grid. There are three main types of DC cables and wires which are used in PV installations, namely earth wires, single core wires and twin core wires. Meanwhile, systems with single-phase inverters require the use of three-core AC cables and systems with three-phase inverters require the use of five-core AC cables.
The market for solar cables is being driven by the significant surge in solar PV installations. On a conservative side, a 200 MW solar project uses around 10,000 km of solar cable. Given the Indian government’s target of 100 GW of installed solar energy capacity by 2022, the solar cable requirement for India alone is in excess of 5 million km.
Key considerations for solar cables
The size of a solar cable to be used is directly dependent on the generating capacity of the solar module. Hence, the larger the capacity of the solar module, the bigger should be the cable size. The cable size also depends on the distance of the solar module from the load – the greater the distance, the bigger the size.
The type of components to be connected also influences the cable size. For instance, module-module connections are usually enabled by connecting the positive and negative plug connector terminals of the cables of preassembled junction boxes. For safety reasons, the positive and negative cables are laid separately. Further, if the length of the cables is not sufficient, then special extension cables are used. On the other hand, battery-inverter connections are enabled by using short and thick cables to reduce system losses, and improve efficiency and reliability. Meanwhile, battery-charge controller connections require cables whose cross-sectional area is in accordance with the maximum current output.
Choosing an optimum cable size helps avoid overheating and ensures very little energy loss. On the one hand, using an undersized cable not only carries the risk of fire due to overheating, but is also a code violation in most jurisdictions. On the other hand, with an oversized cable, money is wasted on extra and costly conductive material. However, as per industry experts, sizing wires to exactly match the current ratings of modules is not advisable. It is safer to allow for a slight over-sizing margin. This helps to not only address potential voltage drops due to overheating, but also cater to additional appliances that might be added to the system at a later point.
Further, it is important to ensure that solar cables are not bent outside their maximum permissible bend radius, which is usually stipulated in the cable specifications. A good practice to avoid faults and stress risks is to lay the cables into protected ducts. Meanwhile, for floating solar projects, although cables are not kept under water, properly rated cables and waterproof junction boxes are important.
Material and design trends
The two most common conductor materials used in residential and commercial solar installations are copper and aluminum. While copper is more expensive than aluminium, it is a better conductor and carries more current than a given aluminium wire of the same size. Further, aluminium partially loses its strength during bending.
The solar power cables can be solid or stranded, where stranded wires consist of many small wires that make the installation flexible. Further, since current tends to flow on the outside of the wire, stranded wires have slightly better conductivity as there is more wire surface. This type of arrangement is recommended for projects of larger sizes.
The conducting wire of a solar cable is typically insulated for protection from heat, moisture, ultraviolet light and chemicals. Traditionally, solar PV installers used neoprene for insulation. However, these neoprene-insulated cables were susceptible to cracks and damages in harsh environments. Currently, insulation made from electron-beam, cross-linked polymers that do not melt or flow even at high temperatures is gaining traction.
Meanwhile, several alternative solutions have come up in the market for trenching or burying of cables for ground-mounted projects. Burying power cables in restrictive underground conduit entails excessive expenditure on labour and runs the risk of water filling in the trenches. On the other hand, free air transit of cables is more cost efficient and eliminates the need to reduce the capacity of cables. To this end, solar hangers or hooks have been developed to hold solar cables above the ground. The hanger configuration to be used depends on the size of the cable. There are no weight limitations for the hangers.
Challenges and the way forward
Solar cable manufacturers face a daunting challenge of balancing upfront costs with long-term reliability. The manufacturers also have to constantly innovate their products in line with the evolving installation practices. Over the past few years, the industry has witnessed the introduction of several varieties of cable designs and practices, many of which may not necessarily support long-term solar needs. Moreover, often the codes and standards for solar cables fail to keep pace with new technologies and applications. Solar cable manufacturers must aim to develop application-specific cables and significantly invest in research and development activities to ensure the long-term performance and reliability of these cables.
Meanwhile, a key challenge for solar energy developers is that the cables for a project are ordered months ahead of the actual commencement of construction work. The cabling requirements, however, might change as the project progresses, especially in the case of larger capacity projects. In this regard, establishing an early and upfront relationship with a cabling manufacturer can help ensure the selection of the right cable for the job, as well as guarantee product availability and seamless logistics once the project commences.
Going forward, a collaborative approach involving all the stakeholders in the value chain, including cable manufacturers, standards bodies, utilities, regulators and project developers, is imperative for the sustainable growth of the solar cables industry.