INDUSTRY I GRID
T
he announcement of Germany’s revised Renewable Energy Sources Act (EEG 2012) in February may have been one of the reasons why the operators of distribution grids have been flooded with applications for the connection of new solar PV farms this year. While feed-in tariffs have been further reduced, these cuts have been less extensive than originally planned. And since prices of PV modules are going down while the efficiency of the modules is going up, solar PV plants remain an interesting option for investors and home owners. Last year alone, the operators of distribution grids paid around 16 billion euros in feed-in tariffs to owners of solar PV plants.
However, these new power generating plants also pose a challenge for grid stability. The power output of the PV modules is highest between 11 am and 5 pm, i.e. the period with the most intense solar radiation. While this – except at weekends – admittedly often coincides with high energy demand, the power output is affected by weather fluctuations and therefore not always reliable. To ensure that solar PV plants will contribute to the stability of our electricity grids, we need to test their electrical characteristics and the components that make up these plants. Once connected to the grid, solar PV plants must perform similarly to conventional, industrial-scale power stations and offer the same fault ride-through performance.
What needs to be certified? Basically, the relevant guidelines and directives demand that all technical components of the generating plant contribute actively to maintaining stability of the grid voltage and frequency. However, power generating units and plants are subject to different directives and guidelines depending on the grid into which the electricity is fed. At present, photovoltaic plants are connected either to the low- or medium-voltage grid depending on their capacity. They do not yet play a role for high-voltage grids.
Criteria for connection to the medium- voltage grid
Connection to the medium-voltage grid is governed by the Medium Voltage Directive of the German Association of Energy and Water Industries e.V. (Bundesverband der Energie- und Wasserwirtschaft, BDEW). Photovoltaic plants in this high- capacity range must undergo grid compatibility certification if their apparent power exceeds the threshold of one megavolt- ampere. Apparent power comprises the active power fed into the grid and the ‘reactive power’ of the plant. While reactive power is needed for grid operation, it is not subject to remuneration.
The second criterion for determining whether a plant is subject to certification is the length of the cable leading from the installation to the point of common coupling (PCC). Long cables can significantly impact on the plant’s behaviour at the PCC. Given this, the applicable regulation requires certification if the
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