Connecting Decentralized Renewable Energy Sources

Author: Oliver Janke, OMICRON electronics, Germany

However, if the generators stay connected, it must be ensured that they are not receiving reactive power, as this would lead to a collapse of the grid.
In Germany, a number of legal requirements and grid codes are regulating the connection of such generators.
These documents also stipulate the use of Directional Reactive Power Undervoltage Protection (Q->& U< protection) that would disconnect such energy sources if they received reactive power during faults on the network.
This article gives an overview of the legal documents and an introduction to the protection function mentioned above. The basic principle of the Q->& U< protection is explained by means of its requirement specifications.
Furthermore, the article will show up standardized test methods evaluating the Q->& U< protection. It will describe how to test the releasing functions, the reactive power direction determination and also all binary inputs and outputs that are necessary for the protection function.
Decentralized and renewable energy sources are contributing an increasing amount to the electrical power mix in many regions of the world. Figure1 shows the growth of installed power from Wind Energy stations worldwide. During the past five years there has been an enormous rise in many countries while Germany observed a slight increase from a relatively high value. In many cases, this development is driven by society and subsidized by the state.
Those small or medium sized generators are often connected to the medium voltage network. Figure2 shows a typical example of a wind farm with multiple generators that are connected to a 20kV medium voltage busbar.
In the past, it was sufficient to disconnect such decentralized energy sources during a fault on the electrical network. This ensured that the fault was not feed from the distribution network and could be cleared by tripping the corresponding feeder.
With growing numbers, it is now important to use those suppliers to support the stabilization of the system voltage after short voltage dropouts. Otherwise, the short circuit power would decrease suddenly which would cause problems for many protection devices (such as decreasing the fault current). Loss of decentralized in-fed power should be avoided as the remaining conventional or nuclear power plants might not be sufficient to supply the whole network. A decrease of the network voltage followed by load shedding and blackouts would be the consequence.
With the high percentage of decentralized regenerative energy sources, the German grid operators had to find solutions to avoid these problems. A number of grid codes specify the requirements for connecting those power plants to the network and are also describing a protection function called Directional Reactive Power Undervoltage Protection (Q->& U<). This function disconnects the decentralized generators in the event that they receive reactive power during or after a network fault . As long as they are supplying reactive power to the network (overexcited operation or capacitive behavior) they stay connected to stabilize the network voltage.
The German regulations and the Q->& U< Protection could serve as an example for other countries or regions.

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