Solar power cables and connectors are among of the most important components of a solar project. Being the lifelines of solar power plants, they transmit generated solar power from modules to inverters and finally to the grid. However, the quality of cables and connectors is often ignored to save costs, although their counterparts (that is, modules and inverters) continue to garner attention. Solar plants use large quantities of connecting equipment that are exposed to various environmental hazards and any fault in these impacts the overall plant performance and security. Thus, albeit they account for a small share in the total capital cost of a solar project, improper selection and sizing of cables and connectors can have huge cost implications for the project life cycle costs.
Solar power plants have two kinds of cables – direct current (DC) and alternating current (AC). DC cables connect solar panels to junction boxes or to inverters directly, and can be of three types – earth wires, single-core wires and twin-core wires. AC cables, meanwhile, connect inverters to the substation. Solar systems with single-phase inverters require the use of three-core AC cables and those with three-phase inverters require five-core AC cables. Cable size is usually determined on the basis of the generation capacity of a solar panel. Thus, the higher the generation capacity of a solar panel, the bigger is the cable size. Optimum cable size is very important to avoid energy losses and overheating while connecting solar power components. Undersized cables can cause fire due to overheating, while using an oversized cable can result in additional material expenses. However, most installers keep a slight margin to address potential overheating and voltage drop issues, and allow for additional appliances to be added in the future.
Since solar panels are connected in series, inverters should be installed close to the feed-in counter to minimise losses, which are higher on the AC side than on the DC side. Thus, the DC generated from solar panels should reach the solar inverter with minimum power loss. Power loss is determined by voltage drop which, in turn, depends on cable resistance as the current remains constant. Resistance is a factor of cable length, cross-section area and material resistivity. An optimum-sized cable should ideally have a short cable length and a larger cross-section area. Since cable length depends on the distance among various components and resistivity is an inherent property of the constituent material, the design focus is on increasing the thickness or cross-section area of the cable as long as it is economically feasible. The DC main cables are designed to ensure that generation loss is lower than 1 per cent of the peak power output from the solar project.
Copper and aluminium are the two most common conductor materials used in solar power plants. Of the two, copper has greater conductivity. Thus, a copper cable can carry more current than an aluminium one of the same size. However, aluminium wires are less expensive than copper wires. Cable conductivity also depends on whether it is solid or stranded. Stranded cables are typically suitable for larger sizes as they consist of many small wires for higher flexibility. Thus, when using stranded cables the surface area increases as current tends to flow on the outside of the wire, thereby offering slightly better conductivity.
Since solar cables are placed outdoors and are always exposed, they need to have a high resistance to environmental factors. At higher temperatures, material resistance increases and conductivity decreases. So, cables should have proper insulation to protect them not only from moisture, but also from heat, ultraviolet radiation and chemicals. Traditionally, neoprene-insulated cables were used in solar power plants; however, these have been replaced by ones with insulation made from electron beam, cross-linked polymers, as the former were susceptible to cracks and damage under harsh conditions. In contrast, the newer polymer-based insulation does not melt or bend even at high temperatures and is more durable. Trenching and airborne cables in ground-mounted projects have also gained traction. But burying cables in underground conduits entails additional labour costs. Moreover, there is a huge risk of water filling in the trenches and messing up the wiring as well as causing accidents. Thus, airborne cables held above the ground by solar hangers or hooks are preferred more. Hangers and hooks can be designed and customised as per the cable size and project requirements.
Connectors are crucial components of a solar power plant as they provide secure and touch-proof connections between components. They prevent loose cable ends, which can lead to energy losses and other performance issues. Like cables, connectors are always exposed to harsh environmental conditions and mechanical stress. Therefore, they need to be designed with adequate engineering consideration in mind so as to withstand adverse conditions without getting disconnected. Hence, secure connections that can conduct current fault-free for 25 years are required. These connectors should be able to meet the voltage and current requirements along with low contact resistance, and should have firm locking mechanisms.
Connectors can be pre-installed on solar panels or they can also be installed on site. If they are installed on field, end panels are connected to an inverter or a combiner box. In microinverter projects, connections are made between solar panels and the microinverter. They usually have pre-installed cabling and there are no field connections. Traditionally, screw terminals and spring clamp connectors have been used in solar applications. However, simple shock-proof plug connectors are fast taking their place. In addition, crimping has emerged as a safe solution for attaching connectors to cables, and it is being used for on-field and pre-assembled connections. Plug connecters and sockets with welded cables and pre-assembled circular connection systems are also being used to save time and labour costs.
The way forward
With the demand for solar power increasing on a massive scale, solar projects need to be set up in a cost-effective and timely manner. This need for decreasing costs and time duration in project implementation has led to the emergence of a variety of pre-assembled solutions for solar power plants. For instance, cable solutions with pre-installed connectors have been developed for utility-scale solar plants. These solutions simplify installation by enabling fast and easy connections, and doing away with the complexities in field installations. Similarly, all-in-one metal clad DC feeder cables are available for connecting combiner boxes to inverters. These increase the longevity and reliability of cables, and save significant project execution effort by eliminating the need to install conduits. Colour-coded cables are also being used in solar power projects for easier identification of specific cables and wires during installation and operation without any need for tagging them.
The rapid evolution of the solar power industry has led to an increased demand for cheaper and better quality components. Thus, to stay afloat in this intensely competitive space, manufacturers need to keep innovating new designs and concepts. While many recent technology developments in the cables and connectors space might become obsolete in the future, the present trend of simplifying installation through pre-assembly is here to stay.