PREMIERE: The first high-frequency filter for the Offenbach wind farm provides clean power grids in the frequency range up to 9 kHz

The Offenbach an der Queich II wind farm is an excellent example of the regional energy revolution in Germany. In the southern part of Rhineland-Palatinate, 13,800 households receive clean wind power from six power plants that produce more than 40 million kilowatt-hours of power annually. The wind farm has a total power output of 15.8 megawatts, featuring onshore wind turbines with a hub height of 139 meters and a rotor diameter of 120 meters. The project was implemented by EnergieSüdpfalz and project developer juwi. The installation of the high-frequency filter system in a wind farm that ensures clean power grids in the frequency range up to 9 kHz was a first. The Power Quality experts from Maschinenfabrik Reinhausen (MR) were responsible for providing a turn-key solution for this complex task, from measurement and simulation to implementation and plant commissioning.

It became evident even in the wind farm's planning stage that measures would be needed to manage the harmonics. The state-of-the-art wind power plants used in Offenbach work with IGBT-based power converter technology, which offers key benefits for applications that require high dynamics as well as high efficiency and power density, in particular. The harmonic currents generated by the IGBTs in this case occur in a frequency range up to 9 kHz and lead to limit value violations at the point where the wind farm connects to the utility grid. Exceeding the limit value by 100% repeatedly for individual frequencies has a negative effect on grid quality. A method for limiting harmonics was essential due to the need to meet standard specifications. Otherwise, the situation could result in the wind farm being disconnected from the grid intermittently or even having its operating license revoked. In the future, due to increased general grid loading and the new generation of rectifier transformers, it must be assumed that harmonics up to 9 kHz will become ever more relevant.

Confronted by this exceptionally complex problem that had never been seen before, juwi turned to the Power Quality experts. Employees started by taking detailed measurements at several points within the wind farm using precise measurement equipment to develop the most accurate picture possible of the actual condition. A mathematical simulation in conjunction with the collected real-world measurement data provided the best possible results. "A combination of measurement and mathematical modeling is crucial for getting an optimal solution," said Dr. Thomas Schlegel, Head of Engineering at Power Quality. The insights from the simulation and the resulting solution recommendation were discussed in detail with juwi, the FGH certification office and the Pfalzwerke utility company, especially since handling harmonics in the frequency range up to 9 kHz was a first for all of the parties involved. The step-by-step approach taken by Power Quality was very important for the customer juwi and for FGH in achieving maximum transparency at every level. "As an independent certification office, we were deeply involved in the processes and were finally able to conclude the last pending conformity test relating to the harmonics, completing the certification of the system," said Christoph Lütke-Lengerich, Head of the specialized PGU department of the FGH certification office.

As a solution, Power Quality developed a world-first filter concept that drops the harmonic level in the frequency range up to 9 kHz over a broad spectrum, significantly reducing individual harmonic frequencies as a result. This was a real challenge. After all, it meant covering new ground with respect to the inductance and capacitance of not only the HF filter circuits themselves, but also of other primary components such as circuit breakers, fuses or special current and voltage transformers. Power Quality coordinated with suppliers to qualify all of the equipment in the filter circuit system for use at frequencies up to 9 kHz. Three filter circuits with tuning frequencies of 189/3800/5600 Hz and a total power of 2800 kvar installed in a concrete station now ensure cleaner operation. The filter circuit system is the first of its kind and was successfully put into operation at the start of 2017. The assured performance of the filter circuit system was confirmed as part of the verification measurement. "For us as customer, it was very important to receive everything from a single source. This highly complex turn-key project was crucial and was carried out by Power Quality professionally and with transparency for everyone involved. This solution has ensured operation of the wind farm without restrictions," says Michael Link, Team Leader of Network Construction, Planning and Implementation at juwi Energieprojekte GmbH. "That's why we've already commissioned the next project."

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