Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 282

6.2.4 Grid-connected systems

Photovoltaic systems feed the generated electric energy into the power grid using
inverters to adapt the direct current from the photovoltaic system to the character-
istic of the mains (for examples see Table 6.2). The basic structure of such sys-
tems is illustrated in Fig. 6.27.

Photovoltaic

modules

=

Meter box
with fuses

Inverter

House distribution system

Fig. 6.27 Roof mounted photovoltaic generator directly feeding into the public grid

In terms of grid-connection, the following concepts of photovoltaic systems are
available (Fig. 6.28).
− For so-called decentralised systems, that most commonly have the photovoltaic
module installed on house roofs, relatively small photovoltaic generators of
only a few kW are connected to the mains via an inverter adapted to the photo-
voltaic generator capacity. They most commonly feed into the low voltage grid.
The difference between photovoltaic generator energy provision and the cur-
rent energy demand of the respective household is balanced by the grid.
− "Quasi centralized" systems (Fig. 6.28) are a very rare mixture of small scale
systems and large scale photovoltaic power plants. Within such a system con-
figuration the photovoltaic modules can also be mounted on available support
structures (e.g. roofs). Yet, unlike decentralised systems, the individual solar
generators are combined to larger units on the direct current (DC) side with an
electrical capacity ranging between some 100 kW up to several MW. The sys-
tems are then connected to the respective electricity supply grid by larger in-
verters. For realising such systems a technical-economic optimum between the
distance to be covered and the related transportation losses, on the one hand,
and the lower inverter losses associated with higher installed capacities, on the
other hand, needs to be found. As the electric energy is fed into the medium-
voltage power grid additionally a transformer is needed. However, quasi cen-
tralised systems have not yet been put into practice on a large scale.