Before photovoltaic (PV) systems can be connected to the power grid they must pass a comprehensive certification process. Time and again important documents are missing, causing delays in the certification process. What aspects are important for the owners of PV systems and units to ensure grid connection compatibility?
PV systems adding up to a total capacity of 3,000 MW were connected to the German power grid last December alone. The owners must obtain third-party confirmation of the grid connection compatibility of their systems, for example by TÜV SÜD. System- and type-specific unit certificates are important elements of this process. They document that the unit has the required electrical characteristics and performance in the power distribution grid. The design of new units must ensure that they contribute actively to voltage and frequency stability in the electricity grid. The rationale behind this is that integration of a growing number of decentralised, weather-dependent power generation units which do not feed consistent amounts of electricity into the grid will place power grids under even greater strain in the future.
Detailed requirements
Solar power is generally fed into the medium or low-voltage distribution grid. Verification of grid compatibility is carried out in accordance with the Medium Voltage Directive of the German Association of Energy and Water Industries (Bundesverband der Energie- und Wasserwirtschaft e.V., BDEW). The BDEW directive defines basic requirements regarding short-circuit withstand capability, continuous current-carrying capacity and the active and reactive power supplied and its control. In addition, it must be ensured that the power generation unit (PGU) or Plant satisfies fault ride-through requirements, for example in case of a sudden drop in voltage.
Certification is mandatory from a capacity of one megavolt ampere (MVA) upwards. A second, critical criterion is the length of the cable leading from the installation to the point of common coupling. Long cables impact considerably on the system’s performance in the power grid. Given this, a certificate is also required if cable length exceeds two kilometres – even if the power generation units and the line leading to the point of common coupling are located on company premises of, say, large-sized industrial facilities or airport grounds.
Owners must also note that almost all European countries have different regulations and directives that they must observe when connecting their PV systems to the grid. In Germany, the key documents of grid integration are also required to be eligible for public funding or tax privileges under Germany’s Renewable Energy Sources Act (EEG). When the assessment is completed with a positive result, a unit or plant certificate will be issued.
