Cables and Connectors
The electrical connections in a system may be inconspicuous, but their effects should not be underestimated. As a relatively large number of electrical connections are required in order to connect the modules of a PV plant to the inverter, the losses at contact points can add up. Long-lasting, secure cable connections with low contact resistances are necessary to avoid defects, losses and accidents.
Safe and weatherproof connections
A PV plant’s electrics consist of the DC cables between modules, generator junction box and inverter, and the AC cable running from inverter to grid. The cables and wires must be laid in such a way to ensure that they are ground-fault and short-circuit proof. To achieve this, the DC installation is made up of two single-core, double-insulated cables that should be tested in accordance with the PV1-F standard. As the cables are almost exclusively laid outside, the insulation must be weatherproof. 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 three-phase feed-in.
Cables connect individual modules to the PV generator. The module cables are connected into a string which leads into the generator junction box and a main DC cable connects the GJB to the inverter. In order to eliminate the risk of ground faults and short circuits, the positive and negative cables, each with double insulation, need to be laid separately. The sharp edges must be fitted with edge protectors. The minimum bend radius must be taken into account when laying the cables and wires, and it is important that they are fixed in a durable and sufficient manner.
To avoid them acting like a burning fuse, which could cause fire to spread to neighboring houses, solar cables must not pass over or through firewalls unprotected. If laying the cables in this way cannot be avoided, they must be protected with a fire-resistant sheath. Further options include laying them in fire-resistant ducts or using a fireproof bulkhead.
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. As a consequence, plastics which have been cross-linked using an electron beam are increasingly used today. The cross-section of the cables should be proportioned such that losses incurred in nominal operation do not exceed 1%. String cables usually have a cross-section of 4 to 6 mm2.
Owing to the sharp increase in copper prices, aluminum has recently gained significance as an electrical conductor. It is possible to save around 50% by using aluminum 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 also be paid to the default breakaway torque of their screw connections, as, in comparison to copper, aluminum tends to creep under roofs which are (too) 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.
Losses add up
Connection technology has needed to develop 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 with the 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. Here, litz wire is pressure bonded with a contact using a crimping tool, which causes both to undergo plastic deformation creating a durable connection.
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 on the market. Such connections cannot, however, be used during installation work on the roof, but only during production in the factory.
Another development are 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.
Standards for plug connectors
Since PV modules generally come equipped with pre-assembled plug connectors, several modules can easily be connected to form a string. Connecting these strings to the inverter or generator junction box, on the other hand, is not always straightforward. A variety of different cable connectors are available on 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 crossover 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 crossover 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 crossover connections.