05-03-2016 - A municipality disconnected from the interconnected network. For the residents of Wildpoldsried, this is a further step in the direction of a successful project – and thereby the success of the research project IREN2. The municipality in the Allgäu has the declared goal of being able to supply itself with energy. With the installation of the converter coupling, researchers at Kempten University of Applied Sciences and RWTH Aachen, the grid operators AllgäuNetz and industrial partners Siemens and ID.KOM are now on the brink of completing the first phase of the isolated grid test.
The currently existing grid of Wildpoldsried features a high number of wind turbines and photovoltaic systems. Although renewable energy sources already generate 4.5 times the amount of electricity actually required every year: temporally speaking, the locality draws power from the grid for 60 percent of the year. However, a reliable, independent isolated grid must be able to supply the required energy at all times. During strong wind and sunshine, the excess has to be stored and made available as required. The systems must also offer ancillary services, however.
To test an isolated grid, the engineers used an upgraded battery of the previous project IRENE. The 16-tonne energy storage system with the dimensions of an overseas shipping container helps balance loads and stabilise the grid. It has an output of 300 kVA and can be operated in all four quadrants. This flexible mode of operation can receive and supply both reactive and active power. Furthermore, the battery can be used as a grid regulator to ensure a stable frequency and voltage.
To test its regulation ability, an ohmic load of 150 kW was installed that can produce both balanced and unbalanced loads. The project partners then installed two generators, one of which is operated with vegetable oil.
New converter coupling installed
"To further exploit and test this, a DC short coupling with 500 kVA will be installed that feeds directly into the isolated grid," explained Andreas Armstorfer from Kempten University of Applied Sciences. It is an inverter-based coupling between the interconnected network and the tested isolated grid. Using the converter, the engineers deliberately introduce disturbances into the isolated grid in order to reproduce, for example, controllable photovoltaic systems. In addition, the converter coupling can be used as a 'virtual storage system' with an almost unlimited capacity. It draws the required energy from the interconnected network.
Multi-stage test sequences
To test the stability of the micro-grid, the developers deliberately introduce balanced and unbalanced sudden load variations into the grid. To do this, they use the converter coupling and the ohmic load. In addition to that, it is crucial to still be able to resynchronise the isolated grid with the interconnected network. In the final step, they aim to show that the isolated grid along with the selected components and regulations is black start-capable – that is, able to resume operation independently after a power outage.
The first trials have already taken place in an isolated test area. In the next step, several loads will be incorporated before the entire local network 'Wildpoldsried Salzstrasse' is made available in the final step. In this final stage of the project, the residential households will also be supplied by the isolated grid. If the plan works, the entire power supply system in Wildpoldsried could become independent from the European interconnected network. The residents will like it – as it is yet another step towards the successful completion of their project.