A joint initiative of grid operators and the solar industry is recommending that the Federal Government take measures to ensure supply security in the German and European power grid. According to a scientific study, a retrofitting of some solar power systems is necessary to solve the so-called 50.2-hertz problem. This measure is a response to the increasing role of solar power and the other renewable energy sources in the power supply of the future.
In the first six months of 2011, renewable energy sources supplied approx. 20 percent of the total amount of electricity generated in Germany. Photovoltaics had a share of over 3.5 percent on their own. The technical guidelines for operating decentralized power generating systems were established at a time when these systems played only a minor role. For this reason, the FNN (Forum network technology / network operation in the Association of Electrical, Electronic and Information Technologies – VDE) has now revised the guidelines for new systems. Since May of 2011, grid operators and manufacturers of inverters have begun to apply these guidelines on a voluntary basis, so that in the future, problems with frequency stability in the European grid can be avoided. The rules apply to all decentralized power generating systems, including photovoltaic generators. The objective is to achieve a “gentle” disconnect of systems from the grid when overfrequencies occur. Overfrequencies, however, are extremely rare. Power reduction measures take effect in the frequency range of 50.2 Hz and 51.5 Hz.
Since the large number of already installed systems has a significant influence on network stability in the German and European electricity grids, there must be a regulation of retrofitting. In Germany, these modifications are recommended for all photovoltaic systems that became operational after 1 September 2005 and that have a capacity of over 10 kWp peak power. Technical implementation is to be carried out between 2012 and 2014 on around 315,000 mid- to large-sized solar power systems that are connected to the low-voltage grid. This means that small roof-mounted systems on single-family homes would not be required to undergo retrofitting. Although solar inverters were always installed and set up properly and according to current guidelines, necessary progress in shutdown specifications requires that changes be made to existing systems. According to experts, the retrofit recommendation is the simplest and most cost-efficient solution. In the vast majority of cases, the installer must only update the software or change the parameter settings of the solar inverter.
Until the introduction of a temporary arrangement in April 2011, generators connected to the low-voltage grid – including solar power systems – were required to disconnect from the public grid as soon as the grid frequency exceeded 50.2 hertz. In normal operations (50.0 hertz), this level has so far not been reached, and is not likely to be reached. At any given moment, the balance between power generation and consumption must be ensured. However, given today’s installed PV capacity, in the rare case that an overfrequency occured on a sunny day with a high level of infeed from solar power systems, the infeed from those systems would, at that moment, suddenly be lost. The recommendation to retrofit older PV systems that are connected to the low-voltage grid is meant as a precautionary measure for such a case, and will optimize approximately 9 GW of installed PV capacity. Much as conventional generating plants do, the inverters will lower the power infeed as the frequency increases. Should older inverters not support such a frequency-based power reduction, the shutdown frequencies of the systems will be spread in such a way that virtually the same result will be attained on the whole. Another component of the recommended solution is the system-compatible re-connection to the grid following the normalization of the frequency.
The study also provides the first far-reaching analysis of the financial impacts of such retrofit measures. The findings form the basis for corresponding measures in the regulatory framework, which are currently being developed by the Federal Ministry of Economics.
Initial estimates of the costs of updating the PV systems range between 65 and 175 million euros. Additional costs of up to 2 million euros would be incurred for the adjustment of emergency standby power systems, as well as for administrational costs for manufacturers and grid operators. Such alternate power systems ensure that the supply of power to grid customers is maintained at times when the public grid is not available, for example when a transformer is replaced. Given a pace of approx. 8,500 to 11,000 PV system updates per month, the authors of the study estimate that the measures will take three to four years to complete.
The study, which is entitled “Effects of a high proportion of decentralized power generators on system/network stability in the event of overfrequency, and the development of proposals to overcome those effects” was carried out by the environmental consultancy firm Ecofys and the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart. It was commissioned by the four German transmission grid operators, represented by EnBW Transportnetze AG, the German Solar Industry Association (BSW-Solar) and the Forum network technology/ network operation in the VDE (FNN). On 1 September 2011, the recommendations were presented to the Federal Ministry for the Environment and the Federal Ministry of Economics. “The presentation of the study’s findings made it possible, within a relatively short period of time, to reach a common understanding that serves as the basis for solving the 50.2-hertz question,” according to Heike Kerber, Managing Director of the Forum network technology/ network operation in the VDE (FNN).
Photo credit: Matt Montagne