Grid connected photovoltaic systems
Photovoltaic electricity generation offers the benefits of: clean, non-polluting energy generation, production of energy close to the consumer (in case of DPGS), the very little or no maintenance requirement, and of having a very long lifetime. Due to these advantages, today, the photovoltaic is one of the fastest growing market in the world. However, PV power is still considered to be expensive, and the cost reduction of PV systems is subject to extensive research. From the point of view of power electronics, this goal can be approached by maximising the energy output of a given PV array. The inverter should ensure the highest possible conversion efficiency, while the requirement for the MPPT control is to operate the
PV array at the optimum working point (MPP) in all environmental conditions. A considerable amount of PV capacity today is installed in temperate climate zones, i.e. Central and Northern Europe, where passing clouds are often present on the sky, producing varying irradiation conditions for PV installations. Although modern PV inverters’ MPPT efficiency is very high in stable conditions, further research is needed to achieve similar performance levels in variable conditions.
The 7.8GWinstalled capacity means an enormous number of photovoltaic panels, in the order of tens of millions, operating today. As the number of panels and operating time increases, the insuring of optimal operating conditions is becoming more crucial, in order to minimise production losses due to system failures or external reasons, such as dirt, or shadows. This creates the new challenge of performance monitoring and diagnostics for PV modules and arrays.
The terrestrial applications of photovoltaic systems are usually divided int four primary categories off-grid domestic systems, off-grid non-domestic installations, grid-connected distributed PV systems, and grid-connected centralised systems. When terrestrial applications began, the main market for PV was remote industrial and household applications. However the penetration of PV systems as Distributed Power Generation Systems (DPGS) , (i.e. power generators connected directly to the low-voltage grid of
the buildings) has increased dramatically in the last decades. The proportion of grid-connected PV systems installed - in the International Energy Agency-Photovoltaic Power Systems Programme (IEA-PVPS) reporting countries - rose from approximately 25% in 1992 to about 94% in 2007.
The efficiency of a grid-connected (residential) PV system is higher than that of a standalone one, as it is not limited by battery storage capacity, and surplus electricity generated can always be fed into the utility grid. This also saves the cost of battery storage. PV plants in grid connected applications are tied to the grid via power conditioning units (inverters) of several technological concepts. These inverters play a key role in energy efficiencysince their task is not only to convert the generated electricity to the desired frequency and voltage with the highest possible efficiency, but also to operate the PV array at the Maximum Power Point (MPP) .
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