Solar power systems are required to perform in the outside environment for over 25 years. Three often overlooked but crucial elements of these systems are cables, connectors and junction boxes. Although they make up a small share in the total project cost, they perform crucial functions to ensure high efficiency and system longevity. A look at the role of these three components and the emerging technology trends in this space…
Cables and connectors
A solar array is all about connections, wherein panels are connected to each other, as well as to the charge controller, the battery and the inverter. A charge controller is used to prevent the battery from overloading; the wires that connect the panels to the charge controller should be correctly sized to minimise transmission power losses and prevent overheating. The further away the panels are, the larger the wire gauge should be. The inverter is used to convert the direct current (DC) power collected by the panels into alternating current (AC) power, which is typically required to run appliances.
All these components need to be connected properly by deploying the right size of wires and using the right set of connectors to avoid possible damages.
Given that these systems are typically outdoors, the cables used in solar generation must be designed to withstand long-term exposure to sunlight. They must be resistant to ultraviolet (UV) radiation and be suitable for wet locations. For solar tracking panels, the cables should be flexible as the panels move along with the sun.
Although connectors appear to be a standard component in a solar power system, they are very crucial. They require adequate engineering consideration to meet the project requirements. In addition to surviving extreme swings in outside temperatures, direct sun, snow and rain, connectors have to support increasing voltages. One of the biggest issues that prevents connectors from moving electricity from point A to point B is that not all of them can function together efficiently.
On DC-based projects, panels must be connected in strings. Typically, the connectors are installed on the panels though they can also be field-installed. End-panels have to be then connected to an inverter or a combiner box, and these connections are often carried out on the field. On AC/ microinverter projects, connections are made between the panel and the microinverter. These connections are usually pre-installed with cabling, especially on plug-and-play integrated solar modules.
Apart from their role in enabling electrical connections in photovoltaic (PV) solar arrays, connectors must meet the voltage and current requirements for their service while providing a low resistance point of contact.
Mostly pre-installed on the back side of a solar module, the PV junction box has a simple, but important role: housing all the electric components on a solar panel and protecting them from the environment. Wires connect to diodes inside the junction box, providing an easy way to link panels together. However, solar developers and owners do not get to state their choice regarding the junction box type as module manufacturers work out these contracts during manufacturing.
There are two different junction box production techniques — soldering/potting and clamping. With the soldering and potting method, foils coming out of the solar panel are soldered to the diodes in the junction box. The junction box then has to be potted or filled with a type of sticky material to allow thermal transfer of heat, keep the solder joint in place and prevent it from failing.
Once enough time has passed for sufficient curing of the potting material, the panel is ready for installation. With clamping production, a simple clamping mechanism attaches the foil to the wires. There are no fumes and no major clean-up is required as with the soldering/potting method. The prices of both methods are fairly equal in terms of material and labour costs. The clamping box may be more expensive, but the labour needed to solder and pot the other boxes is often higher.
Although there are differing opinions on the best way to produce a junction box, there has been little discussion over the main role of this often-ignored product.
Innovations and emerging practices
The industry has witnessed several innovations in cables, connectors and junction boxes, which have helped improve efficiency and ease of deployment.
Given the often extreme environments, and the need to save time and ensure reliability, pre-connectorised cable solutions have been developed for solar power systems. Ideal for utility-scale generation systems, these solutions enable fast and easy connections, simplifying installation while removing the inconsistencies associated with field termination. Along the same lines, DC feeder cables for connecting combiner boxes to inverters are now offered as all-in-one metal-clad cables that increase reliability and eliminate the need to install a conduit. PV cables are now also being engineered in a full array of colours to easily identify source, output and inverter circuits without the need for a time-consuming marking tape or tagging cables.
Screw terminals and spring clamp connectors (used in module junction boxes and for connection to the inverter) are gradually being replaced by special, shock-proof plug connectors, which simplify connection between modules and string cables. Further, crimp connection (crimping) has proven to be a safe alternative for attaching connectors and bushes to cables. It is used both in the work carried out by fitters on the roof and in the production of preassembled cables in the factory. An alternative plug connector design has been developed to allow the connection to be fixed without the need for special tools. In this instance, the stripped conductor is fed through the cable gland in the spring-loaded connector. Subsequently, the spring leg is pushed down by a thumb until it locks into place. The locked cable gland thus secures the connection permanently. Plug connectors and sockets with welded cables are also available in the market. However, such connections cannot be used during installation work on the roof, but only during production in the factory. Another development is preassembled circular connection systems for the AC range. These are intended to reduce the high level of installation work required when several inverters are used in one plant.
With increased power outputs and voltages, improvements are being made in junction boxes to protect connections. As the module output gets higher, the bypass diodes have to do more work and they absorb this energy by shedding heat. Junction boxes have to handle the heat of the diodes. In order to mitigate the excessive heat generated by higher module outputs, cool bypass switches are being deployed in some junction boxes to replace traditional diodes. A traditional diode prevents shaded panels from consuming power, but heat is generated in the process. A cool bypass switch has an on/off feature, which opens the circuit when panels are trying to pull energy, preventing heat from building up.
Another trend is that of system owners turning to bifacial panels. Energy is still fed through one junction box even though power is being produced on both the front and back of the module. Therefore, junction box manufacturers have started introducing innovative panel designs. For instance, TE Connectivity offers three small SOLARLOK PV Edge junction boxes for bifacial modules, one each for the left, middle and right corners of the module, effectively working as one large rectangular box. Another manufacturer, Stäubli’s PV-JB/MF multifunction junction box is customisable with an open format, so it is ready for any future updates pertaining to optimisers or microinverters. Junction box manufacturers are also looking at adding inverter technology to their future models.
Quality certification and standards
A significant amount of work goes into the complex process of designing and planning a solar PV power plant, whether it is on the rooftop of a building or ground mounted on the field. Therefore, ensuring best quality standards is a must for the system, which has a life of over 25 years. Cables, connectors and junction boxes are subjected to thermal, mechanical and external loads. Being exposed to harsh environmental conditions like temperature fluctuations and direct UV rays can damage the equipment and in turn impact the efficiency of the entire system.
Thus, testing and certification of equipment, especially for rooftop solar projects, is essential. Rooftops are a dangerous place for wires and cables due to a variety of conditions, such as high heat, extreme cold, wind, snow and rain. The potential for damage is multiplied when wires are not well secured. The Ministry of New and Renewable Energy (MNRE) has defined a set of quality certification and standards for all equipment used in grid-connected rooftop solar PV systems. These include a set of guidelines to be followed while choosing and installing cables for a rooftop solar system. The best practices are as follows:
- For DC cabling, XLPE, or XLPO insulated and sheathed, UV-stabilised single core flexible copper cables should be used; multicore cables should not be used.
- For AC cabling, polyvinyl chloride (PVC), or XLPE insulated and PVC sheathed single or, multicore flexible copper cables should be used; outdoor AC cables should have a UV-stabilised outer sheath.
- The total voltage drop on the cable segments from the solar PV modules to the solar grid inverter should not exceed 2 per cent.
- The total voltage drop on the cable segments from the solar grid inverter to the building distribution board should not exceed 2 per cent.
- The DC cables from the solar PV module array should run through a UV-stabilised PVC conduit pipe of adequate diameter with a minimum wall thickness of 1.5 mm.
- Cables and wires used for the interconnection of solar PV modules should be provided with solar PV connectors (MC4) and couplers.
- All cables and conduit pipes should be clamped to the rooftop, walls and ceilings with thermoplastic clamps at intervals not exceeding 50 cm; the minimum DC cable size should be 4 square mm copper; the minimum AC cable size should be 4 square mm copper. In three-phase systems, the size of the neutral wire should be equal to that of the phase wires.
Codes and standards are struggling to keep pace with new technologies and applications, while a relatively new contractor base is in need of continuous training to stay one step ahead of the evolving installation practices. As such, the industry has seen a variety of cable wiring designs and junction box practices, many of which may not necessarily support the long-term solar needs.
Wires, in particular, have no standard warranties and are mostly the first component of the system to have problems or fail completely. When this happens, it is generally the solar provider’s responsibility to fix the wire related issue. Poor wire management could lead to service calls every couple of years, entailing significant costs. An integrated solution is needed for wire management, and it is up to racking manufacturers to design new solutions that can be integrated into existing system components.