Jatorrizko komen eragina saihesten

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.

Assembly of a junction box to a flexible solar module

PHOTO: TOM BAERWALD/GLOBALSOLAR

Safe and weatherproof connections A PV plant’s electrics consist of the DC cables between modules, generator junc-tion 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 exclu-sively 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 ca-ble is used for three-phase feed-in.

Cables connect individual modules to the PV generator. The module cables are con-nected 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 insula-tion, need to be laid separately. The sharp edges must be fitted with edge protec-tors. The minimum bend radius must be taken into account when laying the ca-bles 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 neigh-boring 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 in-clude 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. Sin-gle-core cables with a maximum permis-sible DC voltage of 1.8 kV and a tempera-ture range from –40 °C to +90 °C are the norm here. A metal mesh encasing the cables improves shielding and overvolt-age 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 ca-bles 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 mm^2.

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 under-ground cables at low and medium volt-age levels. However, their poor conduc-tivity 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, pos-sibly 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 elec-tric 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 accumu-late. 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 con-nectors (e.g. in the module junction boxes and for connection to the inverter) are gradually being replaced by special, shock-proof plug connectors, which sim-plify connection between modules and with the string cables.

Crimp connection (crimping) has proven itself to be a safe alternative for attach-ing 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 deforma-tion creating a durable connecdeforma-tion.

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. Subse-quently, 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 con-nectors, several modules can easily be connected to form a string. Connecting these strings to the inverter or genera-tor junction box, on the other hand, is not always straightforward. A variety of dif-ferent cable connectors are available on the market, and as yet no standards have been established for these interconnec-tion systems.

Plug connectors from different manu-facturers 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 crosso-ver 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 connec-tion 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 con-nections which have been tested in ac-cordance 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 manu-facturer B combined with plug manufac-turer A).

A standard for photovoltaic plug connec-tors, which should be as international and uniform as possible and is similar to that for domestic Schuko plugs, is de-sirable and necessary to ensure reliable connections between products from dif-ferent manufacturers. If such a standard were to be introduced, manufacturers would be in a position to offer reciprocal warranties for specific crossover connec-tions.

Example of strings connected in parallel

PHOTO: MULTI-CONTACT AG