It follows from the Directive 2001/77/EC that the EU shall increase the generation of electrical energy from renewable sources to 20 % of total power consumption by 2020. For this reason the generation of electrical energy from renewable sources and especially photovoltaic sources is coming to the front.
In practice a single photovoltaic (PV) cell has only a small use. That is why individual cells are connected into bigger units, so called PV panels modules. A group of PV panels connected in series is called a string. Powerful photovoltaic sources are formed by big number of series-parallel connected panels, which form so called PV array. PV cells generate d.c. electrical energy, which is then transformed by an inverter to a.c. electrical energy of required parameters.
Specific characteristics of photovoltaic sources and design requirements arising from them:
High investment costs
The focus is on the return on investment. This of course depends on the design and efficiency as well as on service life of installations. That is why special attention is paid to reduction of risks and minimization of failures.
Photovoltaic panels, inverters and cables represent the most expensive part of the whole PV application, and thus protection is focused above all on these components. Failure or, in extreme case, destruction can be caused by atmospheric or switching overvoltages.
Another potential source of failure is shortcircuit of part of the electrical circuit, which can result in overload of certain parts up to their eventual destruction or possibly even in fire.
For this reason special attention is paid to the use of devices from a proven manufacturer.
Relatively high values of d.c. voltage
As it has already been mentioned, PV cells are source of direct current electrical energy. In practical applications direct current voltages are mostly up to 1000 V d.c. The values of voltage are given by the number of panels connected in series of individual PV arrays.
In ideal case the direct current is of a constant value, and does not come through zero as opposed to alternating current. For this reason it is evident that switching off the direct current, especially that of a higher voltage, is more difficult compared to alternating current, and therefore in d.c. applications it is necessary to use special protective and switching devices, which are designed for this purpose.
At the inverter‘s outlet there are alternating electrical quantities, for which it is possible to use appropriate elements for protection and switching of standard a.c. circuits.
High purchase price of electricity (high cost of power losses)
Thanks to generous state support PV applications are often considered above all from the financial point of view, with focus on return on investment. Project optimization can bring interesting savings already in the design stage, typically, for example in the design of a cable line. Total cost of the line can be minimized with use of an optimal ratio of cable purchase price and losses in the line, which will favourably show itself in the return on investment. For optimization of a.c. side of the cable line it is possible to use program Sichr version 10 advantageously.
All these specifics shall be considered in designing protective and switching elements of electrical installations.