The steady increase in the number of installed decentralized power generators presents new challenges to the network and system security. Frequency stability is a specific problem: if an overfrequency of 50.2 Hz or more is achieved, the generator that is connected to the low voltage network has to date shut down automatically. Because approximately 20 GW of power is already installed now just in the area of photovoltaics in Germany alone, such a sudden disconnect could seriously disrupt the system stability. VDE-AR-N 4105 is a technical regulation that exists for new generators that will make it possible to better integrate generator capacity. However, in terms of old systems, basic aspects of retrofitting measures need to be clarified first. That was the objective of a joint study of transmission system operators, the German Solar Industry Association and the VDE FNN under the umbrella of the Federal Ministry of Economics and Technology (BMWi) network platform with the Federal Ministry of Economics, Federal Environment Ministry and Federal Network Agency.
Results of the study
The core result of the study finds that around 315,000 medium to large photovoltaic systems that are connected to the low voltage distribution network should be technically converted between 2012 and 2014. The retrofitting is recommended for all photovoltaic systems put into operation after Sep. 1, 2005 with an installed power greater than 10 kWp (i.e. smaller rooftop systems on single family homes would, for example, be excluded from the retrofitting).
The recommendation of the study, under consideration of the specific technical conditions or respectively implementation options and with the goal of keeping scope and costs of a retrofitting as low as possible, basically envisions a frequency-dependent reduction of the active power fed into the network which is triggered at an overfrequency of 50.2 Hz until a frequency of 51.5 Hz is achieved. At 51.5 Hz the generators in the network are disconnected.
It is to be structured through the following partial solutions:
In the wide majority of the cases, this involves a software update or a change of parameter settings in the inverter. The specific structure of "Solution 3" requires the consideration of requirements for a safe and reliable operation of the distribution network, particularly in the case of an operation of standby systems.
- Requirement of VDE-AR-N 4105 (improved characteristic curve statics between 50.2 Hz and 51.5 Hz; reconnection frequency: 50.05 Hz)
- Requirement of the technical directive "Generators in medium voltage distribution networks", BDEW, June 2008 (characteristic curve statics between 50.2 Hz and 51.5 Hz, reconnection frequency: 50.05 Hz)
- Requirement of the FNN instruction "Temporary Arrangement for Frequency-Dependent Active Power Control of PV Systems in the LV Distribution Network", method a).
The measures to be developed are a precondition for the continued safe operation of the European network system. Moreover, they are necessary in order to reconfigure the electrical system with respect to the objectives expressed by the German federal government in terms of expanding renewable energies.
Further course of action
The basic results and courses of action recommended by the study should provide the basis for the implementation in the regulatory framework (Energy Management Act-EnWG Amendment, § 12 Para. 3a). The corresponding legal regulation is being developed under the direction of the BMWi in coordination with the BMU and should be available by Jan. 1, 2012.
A summary of the most important facts:
- Technical retrofitting of existing PV systems is necessary so that the stability of the German and European power grid is not jeopardized
- The study estimates there are 315,000 PV systems in Germany that need to be converted
- Estimated time period for the conversion: three to four years
- PV systems with smaller power levels (usually private households and home owners) are not affected