General connection diagram of photovoltaic source
The general connection diagram (Fig. 1) shows an example of PV source working in parallel with the distribution network. The source of electrical energy is a photovoltaic array, which is connected by a line wth the inverter.
In case of a higher number of strings connected in parallel it is necessary to ensure protection of PV panels against reverse currents, and overcurrent protection of cables of PV array. At the same time overvoltage protection is provided there (1). In case of a longer line between the PV array and the inverter it is appropriate to use surge voltage arresters both at the inverter and close to PV arrays. To ensure maintenance of the inverter, it is necessary to meet the requirement for its possible disconnection from both a.c. and d.c. side; therefore d.c. disconnector (2) and a.c. disconnector (3) are installed at the inverter. In case that it is functionally ensured that switching the d.c. side off/on always takes place without load, i.e. the a.c. side will be switched off sooner and switched on later, it is possible to use also a disconnector on the d.c. side.
Downstream of the a.c. disconnector there is a surge voltage arrester (4) installed; it is above all recommended if a long line is after it. Furthermore, it is possible to connect local meter of electrical energy generated by PV source, which is connected via a protective device to the switchboard (5).
In case of high-capacity PV source individual parallel line of the PV source is connected via protective devices to the switchboard. The switchboard and downstream wiring is protected by a surge voltage arrester (6) on the side of connection to the distribution network.
The meter of the supplied and consumed energy (generation and consumption on site – green bonus) or only of supplied energy (only generation without consumption) is preceded by the main disconnector (7) of the switchboard. The switchboard, disconnector and the line to the distribution system are protected against overload and short-circuit by the main protective device (8).
In case of a higher number of strings connected in parallel in the photovoltaic array (Fig. 2) it is necessary to ensure protection of PV panels against reverse currents and overcurrent protection of cables.
In case of application with maximum 3 strings there is no risk of panel damage by reverse current induced by short-circuit.
The risk of thermal overload of cables due to the short-circuit can be dealt with by their appropriate overrating. In a higher number of parallel strings it is necessary to take into account the value of possible reverse current with regard to maximum allowable reverse current of the PV panel.