The interconnected grid in Germany and Europe
Operation of today's interconnected system in Germany is the result of historical evolution. More than a hundred years ago, the first regional service areas developed in German cities and agglomerations. Individual generating units supplied consumers with electric power via power lines. Very often only one power plant unit fed electricity into such a subsystem. Any failure of such power plant led to the immediate power blackout for all connected consumers. As more and more electricity was needed and improved supply security was demanded, all the electricity utilities operating in Germany gradually interconnected their extra-high-voltage subsystems forming today's "German interconnected system" in the course of decades.
Interconnection led to significant improvements. Several power plants operating in parallel could back each other up in the event of power plant failures. The interconnection of the grids also enabled power plants to be used more efficiently. The parallel connection of the subsystems also improved security at several interconnection points in the extra-high-voltage system. Ring-shaped or meshed circuits were formed. In the event of a particular line failing the electricity can be transmitted across the remaining line(s) to the consumer.
In Germany, there are currently four transmission grid operators that have interconnected their 380 kV and 220 kV systems via national interconnect lines to form the German interconnected system. The 380 kV system and the 220 kV system are electrically connected with each other through transformers, so-called system interconnectors.
However, the interconnected system does not end at the German border. International tie-lines from Germany to neighboring foreign countries as well as tie-lines between foreign partners link the subsystems to form a synchronous European extra-high-voltage system.
Today's extra-high-voltage system is heavily meshed. Electricity flows in the system based on the laws of physics described by Kirchhoff's or Ohm's Law. The current circuit arrangement in the system, also referred to as topology (lines switched on or off, transformer tap positions etc.), the local power output to subordinate distribution grids, delivery to large industrial customers as well as the current deployment of power plants determine how much electricity flows across individual lines. The transmission grid operator therefore only has limited control over physical load flows by changing the power plant deployment and by rearranging circuits in the system. This is why it is extremely important to monitor the flow of electricity as well as the loads on all the equipment in order to identify overloads and bottlenecks in the network soon enough and to be able to take corrective action. The systems of each energy utility were designed in such a way that demand in any utility's own control sphere can be met from its own power plant capacity without any bottlenecks arising in the system.
A major goal of interconnected operation in the electricity industry consists in exchanging electrical energy between the interconnected partners while maintaining security. If more energy flows across national or international interconnect lines into a control zone than flows out of it, this difference constitutes the import of electrical energy. Conversely, if more energy flows out of the control zone than into it, electrical energy is exported.
Each control zone is lined up to the program value by means of load frequency control in order to be able to specifically influence and control export/import even in a highly meshed system. In its interaction with the primary-controlled power plants, load frequency control also maintains the network frequency (typically 50 Hz).
The systems of the four German interconnected companies with parts of Denmark, Luxembourg and Austria together form the German control bloc. The load frequency controller for the German control unit is located in Brauweiler (Amprion GmbH, Transmission System Operation). It controls the exchange of electrical energy vis-à-vis the UCTE system, including the CENTREL system (Poland, Slovakia, Czech Republic, Hungary). Within the German control bloc, each interconnected company controls the import/export with the neighboring systems for his own control zone.