Cables, connectors and junction boxes are among the most critical components of a solar power project. Involved in transporting solar power from modules to inverters and finally to the grid, cables are the lifelines of solar projects. Meanwhile, connectors provide secure and touch-proof connections between components. They prevent loose cable ends, which can lead to energy losses and other performance issues. Similarly, simple in design yet critical to solar power operations, the junction box is a small enclosure that houses the electrical parts of solar panels and protects them from external harm.
Solar plants use vast quantities of connecting equipment that is exposed to various environmental hazards, and any fault in this impacts overall plant performance and security. Even so, its quality is often ignored to save costs. Meanwhile, other components such as modules and inverters continue to attract attention. Although cables, connectors and junction boxes account for a small share in the total capital cost of a solar project, their proper functioning is important to save project life cycle costs.
The quality of solar cables depends on several factors, including:
- Environmental resistance: Since solar projects are located outdoors and remain exposed to natural elements such as air, moisture and radiation, solar power cables need to be resistant to different climates, ultraviolet (UV) radiation and ozone. DC cables are coated with polymer mixed with about 2.5 per cent finely dispersed carbon black to reflect UV radiation. Further, cross-linking of polymer coating is recommended for protection against water, and electron-beam cross-linking is one of the most popular techniques used today. Moreover, cables should be tested via exposure to ozone concentration as per relevant standards to check their resistance to ozone.
- Heat-bearing capacity: Solar cables should be able to withstand high and low temperatures as well as the extreme temperature fluctuations that are common in deserts. Sustained exposure to high temperature causes thermal ageing of DC cables. The thermal ageing rate doubles for every 10 °C increase in temperature according to the Arrhenius law. Technical standards specify the testing of cables at 120 °C, requiring them to sustain for 20,000 hours, equivalent to 160,000 hours at 90 °C as per the Arrhenius law.
- Malleability and dielectric strength: Cables should be malleable and ductile to be able to withstand high mechanical stresses such as tension, shear load and bending. They should also have a high dielectric constant.
- Non-flammability: Solar plants are exposed to many fire risks. Thus, solar cables should be flame retardant with low smoke and halogen-free insulation. Such cables have flame-retardant additives consisting of inorganic minerals. However, they quickly absorb moisture and become ineffective. Thus, separators between insulation and the sheath are recommended to prevent moisture absorption and improve their flame-retardant capabilities.
- Design-related features: The outside diameter of cables should be small to provide protection against termites, pests and rodents. Further, the rated voltage capacity of DC cables should be equal to or greater than the rated voltage of the system. The cables should be thick, with a voltage drop of not more than 2 per cent.
Faults and failures in cables can result in plant downtime, performance loss or even fire. A key cause of cable failure is ageing, which leads to embrittlement, cracking and damage of the insulating and sheathing materials, thereby exposing the conductor and increasing the possibility of a short circuit. Increased electromagnetic interference due to breakage of the shield on continuous bending and abrasions can lead to damaged cables. To preserve cable life, it is important to install the cable in the environment it was designed for. Further, it should not be damaged during installation.
Connectors, like cables, should be designed to withstand adverse conditions as they are exposed to harsh environments and mechanical stress. Thus, they need to be designed with adequate engineering considerations such that they remain secure enough to conduct current fault-free without getting disconnected. They should have firm locking mechanisms and should be selected according to the voltage and current requirements of the solar plant.
In the case of junction boxes, causes of primary failure include burnt bypass diodes, low power and high energy stress on the system. Further, the melting of plastic components, broken latches, separation of external connectors, and external factors such as extreme heat and water seepage can cause faults. These components should be designed in accordance with the relevant standards to minimise, or even totally prevent, such faults in the future.
Standards and certifications
Given the critical functions of these components, their quality is paramount, and they must pass quality inspection checks by various international and national testing companies. In the case of cables, US-based Underwriters Laboratories (UL) published the first edition of DC cable standards in 2005, which was named UL4703. UL has since amended the standards from time to time, making them some of the most stringent globally. European Standards (EN), published in 2014 and named EN50618, are relatively less rigorous than UL4703. They specify that DC cables must be flexible, low smoke and halogen-free, and must have cross-linked insulation and sheathing.
The Technischer Uberwachungs Verein (TUV) standard, TUV 2Pfg1990/05.2012, was published in 2012 and requires the cables not only to be halogen-free, but also to pass fire protection tests as per the UL standard without any use of halogenated compounds. The International Electrotechnical Commission (IEC) standard, IEC 62548:2016, specifies design requirements for solar arrays, including DC array wiring, electrical protection devices, and switching and earthing provisions with requirements for construction, materials and testing.
There are also various Indian standards used for testing of cables, which include specifications for halogen-free compounds, flame-retardant properties of cables, etc. Some of the important standards are IS 10810 (various parts) and IS 8130. Further, public sector companies such as NTPC Limited and NLC India Limited have detailed requirements for DC cables for their own projects. For instance, NLC India Limited has mandated a voltage drop of not more than 1 per cent in string DC cables, which is a stringent requirement. Further, it requires greater insulation and sheath thickness for the cables to make them sustainable in tougher operating conditions on site. In the case of junction boxes, testing companies have various certification tests to assess their quality. Quality audits and checks based on international norms, guidelines and quality standards such as IEC 60529 are needed to be carried out by manufacturers.
The Indian solar market is extremely price-sensitive. There have been instances of tenders being cancelled and delayed approvals for power purchase agreements in expectation of lower tariffs. Further, cut-throat competition has forced developers and EPC contractors to lower their costs and take a hit on their profit margins. Thus, shrinking margins have, in some cases, led these players to cut costs, raising concerns regarding the use of lower quality products, and improper design and operating practices.
Cables, connectors and junction boxes are often the most ignored components and thus their quality is compromised in many cases. However, each part of a solar project has its own unique function, which is essential for the smooth running of the plant and ensuring revenues throughout the project lifetime of 25 years. Thus, product quality is critical, and developers and contractors should opt for components that meet the relevant standards. Further, standards and testing procedures should be customised according to the environment. Moreover, attention should be paid to the design and actual execution of projects, with contractors being trained to stay updated on the evolving installation practices. Finally, the health of these cables, connectors and junction boxes should be closely monitored through rigorous operations and maintenance activities to prevent faults and downtime and to ensure better performance.
By Khushboo Goyal