With solar power generation scaling up in India, there has also been a rise in demand for power generating and transmitting technologies and equipment. One such critical component of solar power generation is cables and connectors, which link the different parts of the solar system. The efficiency of the solar power system depends on the quality and performance of cables and conductors, which can also help avoid considerable losses to the system.
Solar cables
Cables are used to transmit power. There are two types of cables used in solar power plants – direct current (DC) and alternating current (AC). DC cables directly connect solar panels to junction boxes or to inverters. They can be of three types – earth wires, single-core wires and twin-core wires. Special extension cables are used to connect the positive and negative cables to the junction box (or directly to the solar power inverter). To avoid earth faults and short circuit, the positive and negative wires may not be laid together in the same cable. Single-wire cables with double insulation offer high reliability. DC cables that connect modules as well as the junction box and the solar power inverter are two-core cables comprise a current-carrying live wire and a negative wire, which are protected by an insulation layer.
AC cables connect inverters to the substation. In India, electricity is more commonly transmitted in AC form. Thus, inverters are used to convert DC power into AC. Solar systems with single-phase inverters require three-core AC cables and those with three-phase inverters require five-core AC cables.
Companies specialising in solar cables adhere to certain standards for E-beam cross-linking, special purpose compounding solutions (for sheathing and insulation), conducting material, testing and certifications. Since the solar plant, along with cables, has to function in an open environment over a long period, it should be able to withstand severities like UV radiation, rain, dust, temperature variations, humidity and insects. Further, these plants have to withstand mechanical stress due to pressure, bending or stretching as experienced during installations, as well as chemical stress caused by acids, alkaline solutions and salt water.
The frequent failure of solar cables decreases the overall project efficiency and therefore it is crucial to ensure its successful functionality over a desired life span. For the optimal performance of solar systems, solar cable accessories, connectors and crimping tools are used to allow fast and error-free installations. As solar projects have a minimum life of 25 years, the components used for solar projects are also expected to last as long.
Material and design for cables
It is important to use the right size of solar cable when connecting the various components of a solar photovoltaic system. This ensures that there is no overheating and it limits the loss of energy. Undersized cables can be a potential cause of fire. While choosing the size of the wire, the generation capacity of the solar panel and the distance between the panels and the load needs to be taken into account. Cable sizes will increase with the increase in generation capacity and the distance from the load to the panel. The risk of losses is greater in the case of AC cables as compared to DC cables. DC main cables are designed to ensure that the generation loss is lower than 1 per cent of the peak power output from the solar project. This requires cables to have a low ohmic resistance.
Besides length and cross-section area, this resistance depends on the material used in making these cables. Typically, aluminium and copper are used to make solar cables. Copper has a lower resistance compared to aluminium at a given temperature. Further, aluminium is a lighter and cheaper alternative for making cables as compared to copper. The insulation and sheath have to perform at a higher temperature range. They should have high mechanical stability and flame retardation, and should be free of halogens. To meet these requirements, cross-linked polyolefin copolymers can be used.
Solar cables are manufactured in accordance with various international standards. For instance, the international standard EN 50618 specifies the performance requirement of solar cables. It covers a range of applications involved in solar plants, such as providing the interconnection of solar panel arrays. These standards ensure that the cables are robust enough to withstand severe environmental conditions and degradation from UV light exposure.
Connectors
Connectors used in solar power plants play an important role in facilitating connectivity throughout the system and also prevent misconnection. This helps avoid loose cable ends, which can lead to energy losses and other performance issues. Thus, connectors provide secure and touch-proof connections between components. Many different versions of connectors or standard non-connector junction boxes are used in solar plants throughout the industry.
Similar to cables, connectors are exposed to harsh environmental conditions and mechanical stress. They should be able 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. They should have low contact resistance and firm locking mechanisms. One of the most common connectors globally is the MC4 product manufactured by Multi-Contact. This unit comprises a 4 mm single-contact cylindrical plug and socket shell design. MC3 is the 3 mm version of the MC4 device. MC4 components and compatible models account for the majority of solar panels in current use. The male and female pairs are snapped together by hand for easy assembly, but require a tool to detach them, making the link more reliable.
Every connector needs a cable coupler. Crimping the cable coupler is an important part of connecting the modules in a solar PV system. If crimping is not done properly, it would lead to higher resistance, which would significantly reduce the efficiency. Thus, crimping has emerged as a safe solution for attaching connectors to cables, and it is being used for on-field and pre-assembled connections. Connectors could either be pre-installed on solar panels or they can 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. 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
Solar power has been gaining traction on a large scale in India, and this trend is expected to continue. In order to set up a large number of solar plants, there is a need for cost-effective and reliable components such as cables and connectors. To avoid losses, cables should have good quality insulators. The availability of such insulating material at low costs will drive the growth of solar cables in the Indian market. Further, there is a need for clear guidelines and standards for components established for the Indian market. The import substitution of cross-linked polyolefin compound, which is used in insulation and jacket material for cables, will further drive growth in the Indian solar cable market.
In sum, there needs to be a continued focus on driving down the cost of these components and making them more competitive, while ensuring that proper quality is maintained. This, in turn, will improve the cost economics and efficiency of solar plants.
By Meghaa Gangahar