Managing Solar Cables and Connectors For Safety and Longevity of PV System
A significant amount of work goes into the complex process of designing and planning a photovoltaic (PV) power plant, whether on the rooftop of a building or ground-mounted on the field. This then translates into an efficient, working PV power plant in situ. With the rapid uptake of PV, it is also common to see homeowners who design and implement their own small PV rooftop projects. With so much already invested, it would be vexing if careless cable management after installation led to losses. And dangling, untidy cables are simply unaesthetic. Cables are subjected to thermal, mechanical, and external loads. Just like the rest of the system, cables need to last the stipulated 25 years or more. Being exposed to harsh environmental conditions like temperature fluctuations and direct ultraviolet (UV) rays can damage unprotected cables and in turn the wires in them that carry the power generated.
To connect the components of a solar energy system, you will need to use correct wire sizes to ensure low energy loss and to prevent overheating and possible damage or even fire.
There are four components to connect together: the solar panels, the charge controller, the batteries, and the inverter. The charge controller is used to prevent the batteries from overloading; the wires that connect the panel to the charge controlled should be correctly sized to minimize transmission power loss. Correspondingly, the further away the panels are, the larger the wire gauge should be. The inverter is used to convert the DC power collected by the panels into AC power, which is the most popular form of electricity accepted by appliances. These systems are typically outdoors, so any cable used for this type of application needs to be UV radiation resistant and suitable for wet locations. For solar tracking panels, the cables used need to be flexible as the panels will be moving along with the sun. Depending upon the system capacity cabling utility varies as follows:
- Small-scale systems with string inverters: A three-core AC cable is used for connection to the grid if a single-phase inverter is used, and a five-core cable is used for threephase feed-in.
- Large-scale system wiring with central inverters.
- Larger power collector cables are used to interconnect from the generator box referred to as the Main DC and DC combiner to the central inverter. These cables must be shielded when over 50 m in length (IEC62548).
Significance of DC and AC Cables
DC cables are used predominantly in solar projects and hence, issues around their usage are still not understood very well unlike AC cables, which are used extensively across the power sector. Moreover, intense commercial pressure is forcing project developers and contractors to reduce capital cost resulting in the selection of inferior products and/or sub-optimal design.
DC cables connect modules to inverters and are further segmented into two types.
String DC cables
These cables are used to interconnect solar modules and to connect modules with string combiner boxes or an array combiner boxes. Cables for interconnecting modules come preconnected with modules, whereas the cables required to interconnect strings and to connect with combiner boxes are procured separately. String DC cables carry current of only around 10 Ampere (A) and a small cross section (2.5 mm2 to 10 mm2) is sufficient for this purpose.
Main DC cables
These cables are used to connect array combiner boxes with inverters. These cables carry higher current of around 200–600 A in utility scale projects and require a larger cross section (95 mm2 to 400 mm2). DC cables, except for those preconnected with modules, account for only around 2 per cent of solar project cost, but can have a significant impact on the power output. Improper design and/or poor cable selection can lead to safety hazards, reduced power output, and other performance issues.
Experts believe that power output loss in DC cables can be as high as 15 per cent but it is time consuming and arduous to empirically isolate and quantify the role of DC cables in poor performance. Further, a higher voltage drop typically leads to heating up of cables and fire accidents. Power loss in DC cables is measured in terms of voltage drop from module to inverter. As current in the cables remains the same, voltage drop implies proportionate loss of power.
LT and HT Cables (AC Cables)
LT and HT cables are AC cables with a higher voltage rated capacity. These cables are used to connect inverters to transformer and transformer to the on-site substation. At present, cables of 1,000 V rating are typically used for this purpose but the trend is now shifting towards the use of 1,500 V cables. HT cables are used for power transmission at high voltage from on-site substation to transmission grid substation. Depending on the project capacity, voltage rating of these cables can range from 11,000 V to 33,000 V. LT and HT cables are widely used in the power sector including both conventional and renewable energy power generation plants. However, DC cables are used primarily in solar projects.
Aluminium is widely used in AC cables, which have a life of over 35 years and have been in wide operation throughout the world. In AC cables, flow of current is usually continuous, whereby the cable reaches steady state with minimal thermal stress. Operation in a solar plant is discontinuous because of ever changing irradiation. Figure 1 shows the type of cables used in a solar PV plant.
Economically generating electricity from renewable sources requires a cabling system engineered to optimize efficiency and minimize line losses. This allows more of the generated power to reach substations where it is transmitted to the grid. To optimize efficiency, cables used at the point of solar power generation offer a higher voltage range of up to 2,000 V versus the standard 600 V rating for conventional applications. Medium-voltage cables used between transformers and substations are being re-engineered to provide better efficiency over the life of the cable through cooler operation and lower line loss.
Solar cables, which are UV and weather resistant and can be used within a large temperature range, are laid outside. Single-core cables with a maximum permissible DC voltage of 1.8 kV and a temperature range from –40°C to +90°C are the norm here. A metal mesh encasing the cables improves shielding and overvoltage protection, and their insulation must not only be able to withstand thermal but also mechanical loads. The cross-section of the cables should be proportioned such that losses incurred in nominal operation do not exceed 1 per cent. String cables usually have a cross-section of 4–6 mm2.
Cables used in solar generation must be designed to withstand long-term exposure to sunlight. To maintain long-term performance and reliability, solar cables have been developed to resistant UV, ozone, and water absorption, as well as provide excellent flexibility for subzero conditions and deformation resistance during prolonged exposure at high temperatures. Given the oftenextreme installation environments for solar power systems, coupled with the need to save time and ensure reliability, pre-connectorized cable solutions have been developed. Ideal for utility-scale generation systems, these solutions enable fast, easy connections, simplifying installation while removing the inconsistencies associated with field termination. Along those 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 conduit. PV cables are also being engineered in a full array of colours to easily identify source, output, and inverter circuits without the need for time-consuming marking tape or tagging cables.
There was a need to develop connection technology rapidly over the last few years, as inadequate contacting can cause electric arcs. Secure connections are required that will conduct current fault-free for as long as 20 years. The contacts must also show permanently low contact resistance. Since many plug connectors are required in order to cable a PV plant, every single connection should cause as little loss as possible, so that losses do not accumulate. Given the precious nature of the solar power acquired from the PV plant, as little energy as possible should be lost.
Screw terminals and spring clamp connectors (e.g. in the 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.
Crimp connection (crimping) has proven itself to be a safe alternative for attaching connectors and bushes to the 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 in place 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 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. Such connections cannot, however, 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 levels of installation work required when several inverters are used within one plant. Owing to the sharp increase in copper prices, aluminium has recently gained significance as an electrical conductor. It is possible to save around 50 per cent by using aluminium cables, particularly for underground cables at low- and medium-voltage levels. However, their poor conductivity means that they are thicker than copper cables. Careful attention must be paid to the default breakaway torque of their screw connections, as, in comparison to copper, aluminium tends to creep under roofs which are very heavy. If the screw connections are too tight, the cable loosens over time, possibly resulting in an electric arc, not to mention the associated risk of fire and all the consequential damage.
Standards for Plug Connectors
Since PV modules generally come equipped with preassembled plug connectors, several modules can easily be connected to form a string. Connecting these strings to the inverter, on the other hand, is not always straightforward. A variety of different cable connectors are available in the market, and as yet no standards have been established for these interconnection systems. Plug connectors from different manufacturers are usually either completely incompatible or they fail to provide a connection that will remain permanently snug. If the connector fits too tightly, this can cause the insulating plastic parts to break. A loose fit, on the other hand, poses the risk of creating high-contact resistance. This leads to yield losses and the areas around the connection heating up, even causing an electric arc and the connector to melt. When connecting a plug with a socket from a different manufacturer, a cross-over connection is created, which can generally only be proved to be reliable if complex, expensive tests are performed. In addition to measuring the contact resistance and determining the connection strength, accelerating aging tests and weather exposure tests must also be carried out. Such tests will make it clear whether or not the different materials are compatible. This concerns both the metals used to manufacture the contacts and the plastic materials employed. There are currently no cross-over connections which have been tested in accordance with DIN EN 50521 VDE 0126- 3:2009-10: ‘Connectors for photovoltaic systems; safety requirements and tests’ and approved by both manufacturers (socket manufacturer A combined with plug manufacturer B or socket manufacturer B combined with plug manufacturer A). A standard for photovoltaic plug connectors, which should be as international and uniform as possible and is similar to that for domestic Schuko plugs, is desirable and necessary to ensure reliable connections between products from different manufacturers. If such a standard were to be introduced, manufacturers would be in a position to offer reciprocal warranties for specific cross-over connections.
Despite the promising growth of solar power and related cable developments aimed at ensuring its economic viability, this emerging market is not without challenge. Codes and standards are struggling to keep pace with new technologies and applications, while a relatively new contractor base is in need of continuous on-going training to stay one step ahead of evolving installation practices. As such, the industry has seen a variety of cable designs and practices, many of which may not necessarily support long-term solar needs. Application-specific cables and contractor certification are paramount to ensuring the economic viability of solar power systems. Cable manufacturers are challenged with balancing up-front costs with long-term reliability while continually meeting evolving requirements and trends, from developing cables for new micro inverter technology where DC power is converted to AC at the panel, to meeting more stringent fire ratings, test methods, UL and CSA standards, National Electric Code requirements, and global standards for halogen-free, fire-retardant, and low-corrosive gas emissions. To meet these advancing trends and standards, develop applicationspecific cables, and ensure the performance and reliability to support the long-term needs of solar applications, manufacturers must be committed to solar energy with significant investment in R&D efforts and a strong presence in the market through continued participation with standards bodies, utility regulators, and renewable developers.Consumer demand for distributed solar energy systems is rapidly growing, and smallto medium-scale solar photovoltaic (PV) systems are turning up in any location with available space and abundance of sunlight—from rooftops and parking lots, to brown fields and highways. No matter what the size of the system, all PV applications require high-quality cabling that provides excellent mechanical properties and superior sunlight resistance for outdoor installations, flame-resistance for added safety, and flexibility for easy handling.
Contributed by Er. Khyati Vyas, BE, MTech, Chemtrols Solar Pvt Ltd, Mumbai, India.