Increasing efficiency of PV systems

In today’s world, green means “the way in which we build, design, manufacture, work, and live”, writes the American best-selling author Thomas Friedman. “Green is becoming the most intelligent, efficient, inexpensive path to take care of things, provided all real costs are accounted for.”

Criticism of photovoltaic (PV) is usually based on the conflict between efficiency and subsidies. When evaluating efficiency, however, all factors have to be accounted for. The ‘true’ costs that Friedman is referring to are even more difficult to quantify in the post-Fukushima era. The costs caused by climate change are also rarely taken into account in efficiency calculations.

It is undisputed that maximising the yield and the availability of the system increases the overall cost-effectiveness of the PV system. However, a maximum yield is only possible with well-planned concepts and reliable components.

Surge protection

An unexpected defect in an inverter caused by a lightning strike can result in a reinvestment of up to 20%. Surge voltage protection as a preventative measure increases system availability and contributes to ensuring that the break-even point is reached as planned. To produce an effective protection concept, all of the devices and system components requiring protection have to be logged and the requisite protection level evaluated.

In developing efficient protection concepts for large PV systems, there are further aspects that come into play. This includes, for example, the correct integration of lightning conductors. On the AC side, single- and multiphase grid forms need to be taken into account when feeding into the grid. In PV systems with tracking and monitoring systems, further surge arresters are needed in addition to DC and AC protection on the voltage side. With extensive ground-mounted installations, there is an increased risk that surge voltages occur in the data cables and disrupt the electronics in the inverters and control systems. Particular attention must be paid here to protecting the control and communication technology.

Errors in PV systems can be caused by damage to modules, contamination, shading, output losses and theft. Therefore, in order to keep losses as low as possible, the operator should immediately respond to outages in individual strings. With Phoenix Contact’s Solarcheck string-current monitoring system, operators are constantly informed of the system’s performance and can implement countermeasures in a targeted manner in the event of an error.

Solarcheck consists of two components. The measurement module determines the ‘string current’ and ‘voltage’ key characteristics and transmits them to the communication module, which then forwards the data to the master computer based on the Modbus RTU protocol. The current is measured using Hall effect sensors to ensure that the cables are not interrupted during installation. The measuring module bundles the cables from up to eight PV strings into the smallest of spaces, whereby each communication module can be operated with up to eight measuring modules. The distance between the communication and measuring modules can be up to 500 m. The Modbus RTU bus line can be up to 1200 m long - this means that even larger distances in PV farms are no problem.

Reliable installation

Plug connectors ensure a quick and secure connection for all system components - from the PV module to the grid feed. In close cooperation with module manufacturers, Phoenix Contact has developed a method of ribbon contacting based on spring cage connections for PV junction boxes. The spring cage connection is used for both thin-film and crystalline modules. The modules are connected to one another using moulded Sunclix connectors.

Configurable Sunclix connectors are available for conductor cross-sections ranging from 2.5 to 16 mm² and 1500 V, and also facilitate simple and quick on-site connection without expensive special tools. By simply pressing down on the connection spring, the insulated conductor is contacted and permanently clamped. The plugs come premounted, so no additional parts are needed. With a ‘broad’ cross-section range and a nominal voltage of 1500 V, Sunclix has plenty of reserve for future installations. Sunclix wall ducts facilitate continuous pluggable system wiring. This applies not only to the generator junction boxes but also to the individual, preassembled surge voltage suppression sets from Phoenix Contact and to the inverters from SMA Solar Technology AG. For system wiring, there are also preassembled PV cables and Y-splitters with moulded plug connectors. The new STME 6 DIO compact spring cage diode terminal for 5 A and 1000 VDC was developed for use in thin-film modules. This hinders back currents in the modules caused by defects or shading. There are six ‘jumper slots’ to bridge the diode terminal blocks.